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Proceedings of the Symposium/Workshop on Parity Violation in Hadronic Systems, Vancouver, May 28-29,… Page, S. A.; Ramsay, W. D.; van Oers, W. T. H. 1987

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TRIUMFP R O C E E D I N G SOF TH ES Y M P O S I U M / W O R K S H O P  O N  P A R I T Y  V I O L A T I O N  IN H A D R O N I C  S Y S T E M SVANCOUVER MAY 28-29, 1987Editors: S.A. Page, W.D. Ramsay, and W.T.H. van OersCANADA’S NATIONAL MESON FACILITY OPERATED AS A JOINT VENTURE BY:UNIVERSITY OF ALBERTA SIMON FRASER UNIVERSITY UNIVERSITY OF VICTORIA UNIVERSITY OF BRITISH COLUMBIAUNDER A CONTRIBUTION FROM THE NATIONAL RESEARCH COUNCIL OF CANADA TRI-87-3TR I-87-3P R O C E E D I N G SOF TH ES Y M P O S I U M / W O R K S H O P  O N  P A R I T Y  V I O L A T I O N  IN H A D R O N I C  S Y S T E M SVANCOUVER MAY 28-29, 1987Editors: S.A. Page, W.D. Ramsay, and W.T.H. van OersPostal address:T R IU M F4004 W esbrook  Mall Vancouver, B .C .Canada V 6T  2A3 D ecem ber 1987PREFACEThe Symposium/Workshop on Parity Violation in Hadronic systems was held on Thursday, May 28 and Friday May 29, 1987 at TRIUMF. The Workshop was organized by S.A. Page, W.D. Ramsay and W.T.H. van Oers of the University of Manitoba. The organizers attemped to contact most physicists, whether experimen­talists or theorists, active in this subdiscipline. In all, 41 people representing 16 different institutions attended.The progam consisted of nine major presentations plus four short papers. In addition, Friday afternoon was devoted to Theoretical and Experimental discussion sessions. This report has been prepared directly from camera-ready copy supplied by the participants. The discussion following each paper and the text of the discussion sessions was taken from transcripts prepared by the Scientific Secretaries using audio and video tapes made during the Workshop.I would like to thank Pat Stewart, who handled much of the detailed organization of registration, accommodation and meals, Maureen Iqbal for assistance with the registration, Maureen White and Margaret Lear for preparing the camera-ready copy of the discussions and Michael LaBrooy of the TRIUMF information office for taking care of our audio-visual needs. Thanks also to Dev Saberwal and Kevin Ruth for their help, particularly in taping all the sessions and to the Scientific Secretaries (indicated on the Agenda) for transcribing the discussion tapes. Finally, I would like to thank the TRIUMF management and the TRIUMF Users’ Executive Committee (TUEC) for financial support, without which the Workshop would not have been possible.W.D. RamsayCONTENTSOpening remarksW.T.H. van Oers .............................................Parity Violation in Proton Scattering: Low Energy RegionW. Haeberli .................................................Parity Nonconservation in Proton Scattering at Higher EnergiesR.E. Mischke ................................................Parity Violation in Nuclear Systems, Scattering and Decay ExperimentsE.G. Adelberger .............................................Parity Nonconservation in the NN System: Nuclear StructureIssuesW.C. Haxton .................................................Weak NNM Couplings and Nuclear Parity ViolationB.R. Holstein ...............................................Measurement of the Parity Volation in the Quasi-Elastic Scattering of Polarized Electrons from 9BeR. Neuhausen ................................................Bates Parity Violation ExperimentS. Kowalski .................................................AS=0 Weak Interactions at the Quark LevelB.H.J. McKellar .............................................Noise Factors for Parallel Plate Ionization ChambersS.A. Page ...................................................Reduction of the Effects of Transverse Polarization in a Measurement of Parity Violation in p-p Scattering at 230 MevJ. Birchall .................................................Measurement of Parity Violation in the Photodisintegration of Deuterium and in the Production of Bremsstrahlung on TantalumA.B. McDonald .............................................Studies of Parity Violation using Polarized Slow Neutron Beams R. Wilson .................Page123050678096113129135143156176vOn the Analysis of Parity Violation in Proton Scattering ExperimentsM. Simonius ....................................... 189DISCUSSION SESSIONSSet of Questions as Basis for the Two Discussion Sessions .........  209Discussion I - ExperimentalLeader: J.D. Bowman ..........................................  210Discussion II - TheoreticalLeader: E.M. Henley ..........................................  219Summary of Theoretical DiscussionE.M. Henley ...................................................  224CONCLUDING SECTIONConclusions and Recommendations ...................................  228Agenda of the Symposium/Workshop ..................................  231List of Participants ................   233viOPENING REMARKSWelcome to all of you at the start of this Symposium/Workshop! We are in anticipation of two days of very frank and open discussions about the status of parity violation in hadronic systems. We would like to come to an assessment of this sub-discipline of nuclear science: Whathas been learned so far, which are the open questions facing theory and experiment, what is the comparison of theory and experiment telling us? And then with a look into the future, we should try to focus on challenges for this sub-discipline of nuclear science. Can we bring perspective into further theoretical development and into new experiments?Ample time has been set aside for discussions because they are most important to the symposium/workshop. Tomorrow afternoon there will be two discussion sessions which have as objectives to focus on the most important questions and challenges. A list of questions and comments has been generated which may form the basis of these discussions. The list of questions and comments has been given to the session chairpersons and discussion leaders.Finally we would like to arrive at a formulation of the recommen­dations stemming from this symposium/workshop.Willem T.H. van Oers2PARITY VIOLATION IN PROTON SCATTERING: LOW ENERGY REGIONW. HaeberliUniversity of Wisconsin, Madison, Wisconsin 53706ABSTRACTMeasurements of the parity violating longitudinal analyzing power, Az, in the scattering of low energy protons (Ep < 50 MeV) are reviewed. The experiments are based on the determination of the relative scattering cross section a+ and a~ for incident protons of positive and negative helicity. The first results were reported from Los Alamos, where a significant analyzing power [Az = (-1.7±0.8)xl0-7] was found for pp scattering at 15 MeV. All other results are for a proton energy near 45 MeV, primarily by the group working at SIN, which recently reported a new very accurate result for pp scattering of Az = (-1.50±0.22)xlO-7. A review is presented of the methods that have been developed by this group over the last 10 years to reduce systematic errors in the determination of Az to a few times 1CT9.1. INTRODUCTIONParity conservation requires that the presence of a longitudinal beam polarization (pz) have no effect on the observed scattering or reaction cross section when an unpolarized target is bombarded with a beam of polarized spin V2 particles. Therefore, one way to detect parity nonconservation (PNC), e.g. in pp scattering, is to measure the longitudinal analyzing power Az, which is defined asAz = - t -  2— 1-L , (1) ^Pz I o+ + 0Here, a1 denotes the cross section for incident beam polarization along (ct+) and opposite (cr~) to the momentum vector of the incident beam.The present paper concerns primarily measurements of Az in the pp interaction. This type of experiment was first suggested by Simonius, who proposed the use of a cylindrically symmetric detector coaxial with the incident beam (Fig. 1). The cross sections ct+ , a~ are deduced from the ratio of scattered particles Ng, detected in the annular detector, to incident particles Np, detected in the Faraday cup. The symmetry of the arrangement has the advantage that certain systematic errors areFig. 1. Measurement of Az by determination of the ratio of scattered number of particles, N , in a detector, compared to the incident number of particles, Np, measured in the Faraday cup. The arrangement has axial symmetry about the incident beam axis.reduced or eliminated, and that the detector covers a relatively large solid angle. In practice, the detector is chosen to average over a wide range of angles in order to improve counting statistics since the effects to be detected are very small (Az = 10-7 for pp scattering).It is not feasible to count individual scattered particles at a sufficiently high rate to reach the desired statistical accuracy in a reasonable length of time. Thus one integrates the dc current caused by the scattered particles in the detector.The quantity Az (Eq. 1) is often equated with the helicity dependence of the total cross section. This causes considerable confusion. For pp scattering at low energies (Ep < 50 MeV), where essentially only S and P-waves contribute, Az(0) is (nearly) isotropic, so that <AZ(6)> a Azot irrespective of the range of detector acceptance angle. However, for pd or pa scattering, this assumption is completely unjustified, even at low energies. Thus it is a poor practice to compare calculations of Azot, e.g. for pd scattering, with measurements of <AZ(0)>.The first measurements of Az were made at 15 MeV proton energy, using the polarized beam of the tandem accelerator at Los Alamos. ?3 Protons scattered by a 6 bar H2 target were detected in scintillation detectors. The beam helicity was reversed at a rate of 1 kHz. The currents from the Faraday cup and the photomultipliers were subtracted from one another electrically (analog subtraction), where the gain was adjusted such that the two currents were nearly the same. The resulting difference signal, divided by the beam current, was detected in an amplifier, phase-locked with the 1 kHz polarization reversal signal. A first result for pp scattering,2 Az = (l±4)xlCT7, was reported as a Letter in 1974. The final result3 was presented at the Symposium on High Energy Physics with Polarized Beams and Polarized Targets in 1978:| |d3ETECfOR : }4Az(15 MeV) = (-1.7+0.8)xl0-7 (2)The same paper contains a result for pd scattering at 15 MeV, but in view of the fact that the angular acceptance function of the apparatus is not known, this measurement is difficult to interpret.The present paper is primarily a report of the most recent measurement^ of Az in pp scattering at 45 MeV at SIN, and a review of a decade of work that led to the new result:-iAz(45 MeV) = (-1.50±0.22)xlO-7 (3)for the angular range 23°-52°. For these measurements, the energy was chosen to correspond roughly to the maximum in Az(Ep). Compared to 15 MeV, the higher energy has the advantage that the analyzing power is known to be nearly twice as large,5,6 but it has the disadvantage that the regular pp analyzing power A , which determines the sensitivity to unwanted transverse polarization components, is larger by a factor ten. The pp results by our collaboration (spokesperson M. Simonius) have been described in a number of publications. ,6’^  We also reported measurements8-10 of <AZ(0)> for pa scattering at 46 MeV for laboratory angles between 31° and 80°. More details can be found in the Ph.D. dissertations of Ch. Jacquemart11 and F. Nessi-Tedaldi (pp scattering) and Th. Roser13 (pa scattering). An experiment on pd scattering is presently in progress.The above mentioned efforts, together with preliminary results on pp s c a t t e r in g  at 46 MeV and 47 MeV from Berkeley ^ and Texas A&M, constitute the only published data on Az at low energies (< 100 MeV). The limited amount of data reflects the difficulty of these experiments. A measurement of Az in p+13C elastic scattering, where mixing of two states of opposite parity in 14N considerably enhances the magnitude of the effect, will be discussed by Adelberger.For proton energies of several hundred MeV and above, Coulomb scattering is essentially negligible, so that the helicity dependence of the total cross section, Azot, can be measured in an attenuation experiment (transmission experiment). The techniques and systematic errors in this type of experiment are sufficiently different from those in scattering experiments that high energy measurements are reviewed separately in a paper by R.E. Mischke.1^52. PRINCIPLE OF THE EXPERIMENTThe principle has been described in recent p a p e r s . ^  Briefly, the polarized beam is produced by an atomic beam polarized ion source in which the proton polarization is reversed by switching between two sets of RF transitions (weak-field w, strong-field s), which act on the hyperfine states of the neutral atomic beam prior to ionization. Until 1978, the polarized beam intensity on target was about 0.1 pA. A new ionizer increased the intensity to slightly more than 1 pA. In 1985, the atomic beam source was replaced by a new design based on a cooled dissociator nozzle, which now brings 3-4 pA on target. The average beam polarization is P = 0.83±0.02.Longitudinal polarization is obtained with a precession solenoid (solenoid 1 in Fig. 2) and a deflection magnet, which precesses the transverse polarization by 90° with respect to the beam momentum. Solenoid 2 is used, with solenoid 1 turned off, for tests which require polarization Px at the target (see coordinate system, Fig. 2).The scattering chamber is preceded by two special beam scanners (HI and H2), which measure the distribution of intensity and of transverse polarization (px,Py) as a function of x and y in two planes 85 cm apart. The scanners consist of wheels with narrow strips of graphite which are moved through the beam with stepping motors. InFig. 2. Schematic diagram of the arrangement used in the experiments at SIN. The situation shown corresponds to state s of the ion source RF transitions and to positive sign of the current in solenoid 1. The beam line components are described in the text.6each scanner, scattered protons are detected by four scintillation counters (Fig. 3). For accurate information on the polarization distributions, clean separation between elastic scattering and inelastic processes is important, as is a high count rate capability without counting loses or other distortions of the profiles. Possible hysteresis (e.g. photomultiplier gain shift during the ~2 msec when the target passes through the beam) is checked by taking measurements with reversed sense of rotation. The scanners are described in Ref. 18 with improvements mentioned in Ref. 6.For recent measurements the data acquisition rate was increased and the width of the graphite strips in scanner H2 was reduced from 0.5 mm to 0.25 mm to reduce the energy spread of scattered protons. In addition, with increasing beam current, the detector apertures have been reduced to limit the elastic count rate to about 0.2 MHz. The spectra are digitized into 128 channels each, where the channel widths are 0.31 mm and 0.1 mm for scanners HI and H2, respectively.Fig. 2 also shows other beam line elements immediately before the scattering chamber. For test purposes, two laminated deflectors Wl, W2 allow small changes in beam position and angle at the same rate at which the polarization reversal is normally made. A small quadrupole Q allows corresponding investigations with focus modulation (emittance modulation) of the beam.Scattered particles are detected in an annular ionization chamber of 20 cm diameter, concentric with the 100 bar gas target (Fig. 4). Integration of the currents in ionization chamber and Faraday cup over 20 msec (one period of the ac line voltage) yield numbers Ns and Np respectively. The ratios (Ns/Np)s and (Ng/Np)w are proportional to theBTFig. 3. Simplified drawing of the beam profile monitor H2. The incident beam is labeledB. The targets T are 0.25 mm wide graphite strips. Protons scattered by 51° are detected byscintillators Sc coupled to light pipes L.7Fig. 4. Scale drawing of scattering chamber and Faraday cup FC with graphite beam stop K.The active part of the ionization chamber IC is defined by a foil F and collector CO. T is the target and S an electron supressor.scattering cross section. The upper script s (strong field transition), corresponds to the polarization direction prior to solenoid 1 shown in Fig. 2 (out of the plane), and correspondingly into the plane for upper script w (weak field transition). Note that a deliberate reversal of the polarization direction was made after the measurements reported in Ref. 6. The change was made since we preferred to have the field in the ionizer of the ion source parallel to the cyclotron main field. The label of the current direction in solenoid 1 is such that for positive solenoid (Sol+) polarization out of the plane (state s) corresponds to positive helicity at the target (Fig. 2).The primary measured quantity of the experiment, R, is defined asR± = (Ns/Np)S - (Ns/Np)* ^W S + (Ns/Np>V where the upper script on R labels the polarity of solenoid 1.In practice, R not only contains the PNC signal, |pz |Az, we are interested in, but it also contains contributions Rj from unwanted instrumental effects:R~ = ±IPz lAz + RI ’ (5)where the sign on the right hand side indicates the sign of the solenoid current. Since the solenoid reverses the phase of the beam helicity with respect to the phase of the polarization switching (s,w) at the ion source, certain systematic errors (e.g., effect of beam intensity modulation associated with switching between w and s at the ion source) will be eliminated by averaging Az measured with positive and negative sign of solenoid 1.CO8After each 20.0 msec measurement, a -10 msec dead time is introduced to read and reset the integrators, to reverse the beam polarization, and to take one scan of a beam profile monitor. The dead time is adjusted to introduce a controlled phase drift between the sequence of 20 msec measurements and the line voltage, so that the results can be inspected for line frequency related effects. After 8x30 msec (one complete revolution of the beam scanners), the phase of the polarization switching pattern is reversed, and after 16x30 msec, the initial sign of helicity for the following group of 16 is selected in a quasi random way. The most recent value of Az for pp scattering is the result of 7xl06 individual measurements of R. With a beam current of 3 yA each measurement of R, lasting 60 msec, has a statistical error of +3.5xl0-5, as determined from the variance. Correspondingly, a 20 min run has an uncertainty of ±2.5xl0-7. With the increased beam current, the binning error of the ADC's became noticeable. For some of the most recent measurements a dc current from a constant current source was fed into the integrators, in parallel with the currents from Faraday cup and ionization chamber. This in effect subtracts a known, constant amount of charge from the integrated charge and thus reduces binning errors. All along, the starting voltage of the integration was varied from one measurement to the next with a white noise source because the resolution of the 16-bit ADC's is too coarse to detect effects of 1CT7.3. OVERVIEW OF SYSTEMATIC ERRORSThe most difficult aspect of a measurement of Az is that there is a large number of possible sources of small systematic error. If the final result is to achieve an accuracy of ~10-8, most error sources must be reduced to below ±lxlCT9. We distinguish three classes of instrumental effects:A. Ion source effects, i.e., changes in the properties of the ion beam entering the scattering chamber which are associated with reversal of the polarization at the ion source. In the following, these changes will be referred to as "coherent" modulations, to distinguish them from random fluctuations. The following beam properties are relevant:a) intensityb) position and anglec) emittanced) energye) transverse polarization9B. Helicity dependent background, i.e., contribution to the currents in the Faraday cup and the ionization chamber which change with beam helicity and are thus indistinguishable from the true PNC effect:a) P-activation of chamber and Faraday cupb) PNC in neutron and gamma ray background from the beam stopc) double-scatteringC. Helicity uncorrelated effects:a) electronic cross talkb) incomplete averaging of noise and fluctuationsThe effects will be reviewed in the following three sections. For additional discussion, see Refs. 6, 10.We believe that the above list of effects is complete, but of course there is no proof. On the other hand, one may ask, whether some of the effects cannot be eliminated as implausible, for lack of a known physical cause (e.g., energy modulation, emittance modulation) or because already a rough estimate shows the effect to be negligible (e.g., PNC in production of background in beam stop). Within the group, we initially talked about some of the less plausible effects as "absurd effects". However, we think it essential to work on the assumption that every effect exists. We therefore quote a value and an uncertainty (or an upper limit) for each effect, and add the systematic errors in quadrature with the statistical error.4. ION SOURCE EFFECTSOur treatment of ion source effects is based on the continuous on-line measurement of beam parameters to determine the magnitude of the modulation, and periodic determination of the sensitivity of the apparatus to each type of beam modulation.Some experimenters (see e.g. Ref. 14) propose to reduce coherent beam modulations by feedback systems to such an extent that the remaining effects are not influencing the parity measurements significantly. We do not consider this an effective approach.Depending on the control mechanism, a feedback system may well compensate for instance the coherent intensity modulation, which is easy to detect, but in the process introduce e.g. a small emittance modulation, which is very difficult to detect with the required accuracy. In either case, one needs to measure the remaining10modulation, and the uncertainty in the final result is limited by the accuracy with which the modulation can be determined. Furthermore, feedback requires a control mechanism. Unfortunately, for the most important ion source effect (nonuniform transverse polarization) no control mechanism is known.4.1 Intensity ModulationIt is found that the beam intensity changes by a small fraction when the hyperfine population of the neutral atomic beam changes, i.e., when one switches between weak field and strong field RF transitions. Explanations of this effect have been offered in Ref. 6 and Ref. 19.The magnitude of the modulation Mfl is known from the integrated charges in the Faraday cup, N^, N®:Mo = <Np - NpVCNp + Np) (6)The effect on R is proportional to MQ:where the "sensitivity" KQ is measured with artifically enhanced modulations (see Ref. 6). Typical results for one series of runs (Series 4) are: KQ = (-528±162)xl0~7, M* = (-406+8)xl0~7, M~ = (-399+7)xl0-7, where the uncertainties indicate observed variances of measurements during the series. The correction to R+ or R~ is0.02xl0-7. In the final result, the correction is at least 40 timessmaller yet, because the effective modulation relevant for the determination of |pz |A (Eq. 5), MQ = j(M* - M~) = (4±5)xl0-7 is muchsmaller than either MQ or M r_.With the cooled atomic beam, increased by a factor 10, but even in this case the net correction is well below 10-9 .4.2 Position ModulationSome experimenters stabilize the beam position by a feedback system from a pair of slits to a steering magnet some distance upstream. We found that we can avoid feedback systems by fixing the cause of the beam instabilities at the source (e.g., bad power supplies), and by measuring the actual beam position continuously during the experiment. The reservations about slits is that they center the tail of the intensity distribution, and not the centroid.In addition, they produce a halo of scattered particles. The11reservation about feedback is that there may be a danger of introducing other fluctuations (e.g., coherent intensity modulation coupled with different gain characteristics of the feedback amplifiers introduces coherent position modulations).The coherent position modulations are measured with the beam scanners. With the improved data acquisition rate, the uncertainty in the modulation is less than +0.2ym and ±0.1ym in the first and second scanner, respectively, when results are averaged over a series of about 60 runs. The sensitivity to position modulation is mapped as described in Ref. 6 by measuring the response of the chamber to artificial modulation for some 20 different beam coordinates in scanners 1 and 2. Throughout the experiment, no statistically significant position modulation has been detected in parity runs. Nevertheless, we apply a correction and assign an uncertainty which includes the uncertainty in the sensitivities. This correction is made separately for the two signs of solenoid current, since there is no reason to believe that solenoid reversal cancels a possible position modulation. In recent measurements, the correction for each series had an uncertainty of about +0.03xl0~7, including sources of systematic errors, which are discussed in detail in Refs. 11, 12.Tests with artificial beam position modulation provide a very simple and useful tool not only to determine the proper beam position relative to the scattering chamber, but to monitor the symmetry properties of the chamber. It is important to check the chamber symmetry repeatedly, because asymmetry can creep in, e.g., through temperature gradients in the ionization chamber on a cold day, or through gradients in gas impurities (which we avoid by circulation of high purity H2). Changes in chamber symmetry are very undesirable because they affect the sensitivity to higher moments of transverse polarization (Sec. 4.5) and double scattering effects (Sec. 5.3).The effect of temperature gradients in the 100 bar H2 target was discovered early in the experiment, when the higher beam currents became available for the first time. Fig. 5 shows the effect on R when the beam position in scanner 2 is modulated horizontally (x) and vertically (y), with the beam near the geometric axis of the chamber.The increasing sensitivity to vertical motion with beam current is caused by a vertical temperature gradient of 0.3 K/mm-yA in the target gas (solid line). The problem was solved by installing a blower system to circulate and cool the target gas.124.3 Emittance ModulationCoherent modulation of the beam diameter ("beam breathing" or modulation of second moment of intensity) causes an effect even for a system of perfect axial symmetry. This is obvious from the fact that the ratio of scattered intensity to incident intensity (Ig/lp) depends on |r|. Two sources contribute: (i) a geometric effect, which is known from tests with position modulation, and (ii) thermal effects caused by radial temperature gradients. Thermal effects continued to be a problem until we installed a new, more powerful blower system for the target gas. Thermal effects are illustrated in Fig. 6, which shows (Is/Ip) as a function of blower speed, i.e., the rate of circulation of target gas (top of figure). The bottom of the figure shows the effect of beam position modulation and emittance modulation for different blower speeds.Fig. 6. Top: Effect of blower speed on Ig/Ip* Bottom: Effect on R of a 0.1 mm vertical position modulation and of emittance modulation for different blower speeds.Fig. 5. Effect on R of a 1.4 mm horizontal and vertical motion of the beam as function of beam current. The straight line corresponds to a vertical temperature gradient of 0.3 K/mmxyA.13To study possible effect of emittance modulation on the parity measurements, the sensitivity to emittance modulation was measured by modulating the beam line quadrupole Q (Fig. 2) after every eight parity runs. Analysis of the quadrupole modulation data show that with a blower speed of 6000 RPM no detectable thermal effects remain. For the parity runs, the emittance modulation (second modulation moments) can now be measured with sufficient accuracy by the beam scanners, thanks to a new data acquisition system which permit rates up to 0.2 MHz, and is limited only by pileup within a single cyclotron micropulse.In order to obtain information about correlated moments between the two scanners, measurements must be made in a third plane, preferably near the center of the target. Measurements of modulation at the target center were made by periodically translating the scattering chamber along the beam axis so that scanner 2 is at the position of the target center. Several thin secondary-electro-emission foils with holes of different diameter and with slits were inserted in scanner 2 to measure emittance modulation near the target center.We have found no evidence for emittance modulation. For each series, the limit on the modulation is < 2xl0_3mm2 and < 0.2xl0_3mm2 for scanner 1 and scanner 2, respectively, which corresponds to a fractional modulation in the cross sectional area < 2xl0-4. The overall limit on the systematic error in Az from emittance modulation, including remaining thermal effect which are too small to be detected, varies between ±0.03 and ±0.09xl(T7 for the most recent six series.4.4 Energy ModulationA coherent beam energy modulation of only 1 eV changes the value of R by 3xl0~8. We have not been able to measure the energy modulation directly, but we make use of the fact that a contribution Rg to R+ and R from energy modulation can be determined separately from |pz |Az, since under solenoid reversal the PNC effect reverses sign but the energy modulation effect does not (see Eq. 5).The contribution Rg is determined separately for each series. A typical value (Series 5) is Rg = (-0.36+0.73)xlCT7. None of the results have ever given evidence for an energy modulation.In the determination of |pz |Az, the contribution Rg cancels, provided Rg is the same for either sign of solenoid current. If Rg for some reason were to change with time, the cancellation would be incomplete, so that a contribution pgRg would remain, where PE < 114measures how effectively solenoid reversal eliminates energy modulations.In our first measurements, it took many hours of effort to refocus the beam after solenoid reversal. Now, all beam transport elements can be addressed by our computer, so that the beam line parameters are stored and the sign is reversed after every other parity run.Our measurements depend on the assumption that (except for beam energy modulations) the beam scanners detect all coherent modulations of beam properties. We wanted to present to the system a complicated combination of intensity, position and emittance modulation to ascertain that after corrections are applied, no unexplained effect remains. For this purpose we modulated a lens at the ion source, which indeed produced large modulations. To our horror, however, the corrected valve of R was not zero, but could easily be 10 5. It was found, in fact, that the dc operating point of the lens can be adjusted such that the lens modulation causes no measurable beam modulation, yet gives significant values of R. We now know that these effects are caused by energy modulation, and we use them to make artificial energy modulation with every 20 min parity run. The results are used to determine the suppression factor pg separately for each series. Typically (e.g., Series 5) the average effect on R from the artificial modulation is (110±3)xlO-7, while the difference between positive and negative solenoid 1 is (-5±3)xl0-7. Thus the rejection factor is, in this example, pE = (0.044±0.026). Combining this with the value of Rg given above yields a limit for the contribution of energy modulation to |pz |Az of PgRg ^ (0.04)xl04.5 Residual Transverse PolarizationBesides the very small (-10 7) longitudinal analyzing power Az, there is in nuclear scattering a much larger transverse analyzing power A . For pp scattering at near 50 MeV, Ay = 1CT2, while for pa scattering Ay is of the order 0.5.It is relatively easy to precess the proton polarization vector such that, in the average over the beam, the polarization is nearly longitudinal.6 The real problem is that the polarization vector in the beam is not perfectly uniform in direction, so that the residual transverse polarization components px , py vary with position within the beam. Particularly dangerous is a first moment of px (or py ) with respect to y (or x), i.e., a linear variation of the residual polarization.20 The situation is easy to understand: if we look along15the beam direction, and assume that the left and right halves of the beam have residual polarization up and down, respectively, the analyzing power Ay will cause particles on the left to scatter predominantly to the left and particles on the right to the right.When the beam polarization is reversed, the preferred direction of scattering is reversed. As an estimate, the unwanted modulation of the count rate in the ionization chamber when the beam polarization is reversed, depends on the ratio of beam radius r to ion chamber radius R, and is proportional to PyAy . This yieldsRi  “ | ) PA  “ 10-6 (8)if each of the three factors is roughly 10-2. The existence of first polarization moments in the beam is not far fetched, as is illustrated in Fig. 7 which shows beam polarization profiles measured with the beam scanners. The cause of the nonuniform polarization is not understood in detail, but presumably arises from the magnetic field nonuniformities when particles enter or exit magnetic fields in the ion source, the cyclotron (spiral ridges) and the beam handling system.Fig. 7. Examples of transverse polarization distributions. At the end of the spectra (tail of the intensity distributions), the statistical errors become large.It is sometimes proposed that some of these problems can be overcome by dividing the detector e.g., into left-right and up-down quadrants. This is not the case, as is readily seen if one considers a perfectly symmetric scattering chamber containing a perfectly centered beam whose polarization py varies linearly with x. The resulting modulation of the current in the ion chamber is indistinguishable from a PNC effect.16Residual transverse polarization is the most important of the ion source effects, in that it causes a significant correction to R. The mathematical treatment of this effect is given in Ref. 20. This paper shows that corrections for these effects can be made if the polarization and intensity profiles are measured separately in two planes, without information about correlations. For our experiments, the corrections are based on the beam scanner measurements during the parity runs. Determination of the chamber sensitivities are made with transverse beam polarization (px and py) and some 10-20 different beam positions with respect to the chamber. Each of these measurements has a statistical accuracy of about +5xl0~7. With displaced beam, R is typically a few times 10-5. The large number of points is required in order to measure also the sensitivity to "harmless" polarization moments, e.g., moments of py with respect to y. For a perfectly symmetric chamber these sensitivities should vanish. In reality they are small but not consistent with zero. Thus corrections for these moments are applied as well. In addition, the analysis includes estimates of second and third polarization moments.Compared to the earlier work on pp scattering, the uncertainties of the transverse polarization corrections in the most recent results have been considerably reduced. This improvement was forced upon us by measurements on the pa system, for which some of the sensitivities to transverse polarization were larger by a factor 10 than for pp scattering. For pa scattering, significant improvement was obtained by optimizing the acceptance function of the chamber, taking advantage of the zero crossing of the analyzing power near 50°. Many other improvements were made, which eventually permitted the present, more accurate pp result. Among them was a new target room, which permitted some reduction in transverse polarization by more flexible beam transport; addition of solenoid 2 to permit frequent tests with horizontal transverse beam polarization; new data acquisition system for the beam scanners for higher count rate; improved monitoring of 12C(pp)12C pulse height spectra; implementation of computer control of the experiment including hardware parameters to permit more frequent test measurements.The correction to each run is typically AxlO-7. Depending on the focus properties of the beam, for some series there is considerable cancellations from solenoid reversal. Averaging uncorrected values of |pz |Az over a series usually gives very poor X‘> while after correction the consistency of all 350 runs is excellent. Over many measurements with different setup of the cyclotron beam, the corrections tend to average out, so that the overall correction for the 350 runs is17(-0.56+0.04+0.04)xl0~7, where the first error is the statistical error, the second the systematic error. Included are estimates of possible effects for higher polarization moments, based on the measured uncorrected second and third moments.The effects discussed above were not treated in detail for the pp experiment at 15 MeV, but at the lower proton energy the problem is reduced compared to 45 MeV by the lower analyzing power and the better beam quality of the tandem accelerator. However, for measurements of Az on nuclei with a significant transverse analyzing power Ay, very careful attention to the determination of the corrections for these effects is essential.5. HELICITY DEPENDENT EFFECTS5.1 Contribution from PNC in g-decayThe concern is that activation of the beam stop and the scattering chamber may produce polarized g-emitters whose z-component reverses with reversal of the incident longitudinal polarization. The PNC asymmetry of the resulting g-decays produces a helicity dependent background current, which contaminates the hadronic PNC effect.Initial studies, based on pessimistic estimates of activation cross sections, polarization transfer coefficients and spin relaxation t i m e s , s h o w e d  that the effects are quite small, particularly since the background currents caused by g's is small. Measurements were made of the decay of the background current immediately after turning off the proton beam.12 The dominant activities have x = 9 sec and contribute currents of (2.8±0.5)xl(T5 and (4.5±1.5)xlO-5 in Faraday cup and ionization chamber. Long lived activities are of little concern, because they are more easily measured and because the relatively rapid helicity reversal averages out the effect.A certain discomfort remained, because we could not exclude the possibility that the 50 MeV beam might produce some unknown g-activity, and that by unfortunate coincidence there might be a large production cross section for a process with unusually large polarization transfer and unfavorable spin relaxation.To attack this problem, a set of auxiliary measurements was made12 of the production rate of g-emitters in the chamber materials exposed to the proton beam (beam stop: graphite; scattering chamber: A1 alloy), and to measure the asymmetry of the g's in relation to the incident18proton polarization. Measurements were made of activities between 11 msec [12C(p,n)13N] and several hundred seconds, using a pulsed beam. These measurements were used to calculate by numerical integration the expected background currents in the parity experiment. Comparison to the measured currents of different decay times showed good agreement.A separate experiment was done to measure the 3-asymmetry. This has to be done on a very short time scale to exclude the possibility of a dangerous effect caused by a large initial asymmetry with rapid decay of the polarization. Thus measurements of the 3-asymmetry were made with scintillation telescopes during the -70 nsec time between adjacent cyclotron micropulses. Measurements on A1 were repeated for different proton energies, corresponding to the different energies in the scattering chamber, and for transverse as well as longitudinal incident proton polarization. The measured 3-asymmetries, which had a statistical uncertainties of typically ±2xl0~4, showed no significant nonzero result. As an end result of these rather laborious measurements and calculations,12 we can place a limit of 3xl0~9 on effects from 3-decay.5.2 Empty Target BackgroundIn the earlier experiments,6,7 2.7% of the current in the ionization chamber arose from neutrons and gamma rays produced in the beam stop and the windows of the target cell. If the background has no PNC effect, it simply dilutes the measured effect by a few percent, but parity violation in the production of the background also needs to be considered. It is probably quite safe to assume that in the production of background the PNC analyzing power is < 10xl0~7, particularly since for a beam energy as high as ours, many reaction channels contribute to background. This would lead to a limit on the error in Az from background of about 3xl0-8.For the new pa and pp measurements, the background was reduced to0.35% by moving the Faraday cup further away from the chamber. In order to maintain the small sensitivity to position modulation, a rather large diameter Faraday cup had to be built (compare Fig. 4 with Ref. 6). With the above assumption about background PNC, the possible effect on Az is < 3.5xl0~9. However, in keeping with the philosophy that errors for systematic effects should be based, if at all possible, on measurements rather than assumptions, a parity measurement was made with evacuated target. This included sensitivity measurements and corrections for transverse polarization moments and intensity modulation. Because of the small background current, good statistics is difficult to obtain. The measured value, Az = (5.0±15.5)x!0~7,19background. In cases like this, where a value and an uncertainty are combined to calculate an upper limit, the two are added in quadrature, and the final result is treated as a la probable error.We note that in spite of a significant effort (the empty target measurements are equivalent in statistics to our entire pp measurements of Ref. 6), the uncertainty from this source is the largest single systematic error of the new measurements. This could have been avoided by accepting a priori the assumption that PNC in the background is small.After scattering in the target, the protons have a transverse polarization component parallel to the first scattering plane which reverses sign with incident beam helicity p2. The corresponding polarization transfer coefficient, K*, can be calculated from the known scattering phase shifts. The model in Fig. 8 shows that double scattering from a nonuniformity T in the target wall, combined with a nonuniformity D in the ionization chamber, leads to a residual effect, because the left-right asymmetry reverses with beam helicity. It can be shown that there is no net effect as long as the arrangement has at least one plane of symmetry. In our scattering chamber, axial symmetry is broken by a joint in the foil which forms the inner electrode of the ionization chamber. Limits on target wall imhomogeneity are known for measurements with an ultrasonic probe.To place a realistic limit on these effects, measurements were made with artifically enhanced chamber asymmetries: a foil (T) increased the target wall imhomogeneity and a rod (D) shielded part of the ion chamber. Measurements were made with five different geometries, which were chosen to correspond to the worst conditions, as5.3 Double-Scat teringIONIZATIONCHAMBERFig. 8. Schematic diagram of test arrangement to study double scattering effects. T represents an A1 strip to locally increase the thickness of the target wall. Rod D shields part of the ionization chamber from scattered protons.20geometries, which were chosen to correspond to the worst conditions, as judged from data on pp (or pa) and p-Al scattering. For each of these measurements, the sensitivities to transverse polarization had to be determined. The measurements for each test configuration had an overall accuracy of about ±2xl0~7, and, after correction for systematic effects, the results showed no statistically significant deviation from the measurements of IPz IAz with the unperturbed chamber. Compared to the known limits on inherent chamber asymmetries,12 the tests enhanced the effects by a factor of at least 200. Double scattering affects our final result for Az by <0.03xl0~7 for pp scattering4 ’12 and by <0.15xl0~7 for pa scattering. 1® ’ 13In the first tests13 we used a skew holder to insert a foil over the target wall. We found that this holder all by itself produced quite a large double scattering effect [(-16±2)xlO-7], while a radial holder did not. While this was understandable, because the skew holder destroyed all symmetry, it also taught us to avoid all possible chamber asymmetries even if they appear harmless. Thus, for all parity measurements, the test equipment (foils, rods, supports, etc.) not only was retracted to the rear of the chamber, as we had done earlier, but was completely removed.6. HELICITY UNCORRELATED EFFECTSElectronic cross talk is tested by adding constant current sources to the inputs of the integrators, without removing any other leads and without turning off anything but the cyclotron beam. A typical test (e.g., 19 runs of 60 min each taken after Series 6) yields R = (-0.02+0.05)xl0-7. Since electronic cross talk cancels with solenoid reversal, the remaining effect is certainly < 10 9.The possibility of insufficient averaging of periodic modulations, in particular 50 Hz signals from the line frequency, and mechanical noise from the beam scanners through microphonics of the ionization chamber, was discussed in Ref. 6. Measurements of the beam characteristics, phase locked with 50 Hz, were done repeatedly. Such tests proved useful also for other reasons. One time it was found that the beam polarization disappeared for a short time once during every cycle of the line voltage. The problem was traced to a defective power supply on the RF transition magnet, which produced periodic current spikes.217. RESULTS AND CONCLUSIONSThe new result for pp scattering at 45 MeV is^Az = (-1.50+0.22)xl0-7, (9)where the error is the RMS of statistical error in the parity measurements (+0.19xl0-7) the error of the applied corrections (+0.05xl0~7) and of various systematic errors (+0.09xl0-7). This average includes, besides 350 new 20 min data (Series 2-8) also the results of 69 data published (Series 1, 2) earlier,^ which yielded Az = (~2.31±0.89)xl0-7. The earlier measurements have relatively little weight because the lower beam current available at the time, and the larger uncertainties assigned for systematic effects.All measurements show good consistency. The distribution of the 350 new data is shown in Fig. 9. The results of the eight Series correspond to (X2/ND)1/2 = 1.04, which is quite satisfactory (confidence level 33%). We note that no measurements had to be rejected, except that a priori results were not used when there were unusually large beam intensity fluctuations (more than 12% difference between the average charge collected in the Faraday cup for polarization states w and s, when averaged over sixteen 30 msec measurements). From the early measurements, an entire series was discarded (even though it is consistent with the rest of the data) because the data log showed isolated ambiguities in the bookkeeping concerning the sign of the solenoid current. For the new data, the sign is recorded automatically in the computer as well as manually in the data log.Fig. 9. Distribution of the new 350 20-min data for Az in pp scattering. The uncertainty AA7 of the 20-min run varies between 2.5xl0-7 and 4xl0-7, depending on beam current.22Sometimes the question is raised why the results we have reported over the years show a steady decline in the magnitude of the Az> The answer is that the very first series^ yielded Az = (-3.2+1.1)xlO 7, and this value has been included in subsequent averages,^ ^  since there is no statistically significant indication for an inconsistency. By now, of course, this series has little effect on the final result.The above results refer to an acceptance function (see Ref. 6) which extends from = 23° to 52°. To convert the result to Az°^requires a small correction:*7Azot(45 MeV) = (-1.57±0.23)xlO-7 (10)The known energy dependence of the strong pp amplitudes allows the prediction that Azot(45 MeV) = (1.75±0.l)Azot(15 MeV), which leads to Atot(15 MeV) = (-0.90±0.14)xlCT7. This is in agreement with the Los Alamos result3 Az(15 MeV) = (-1.6±0.8)xlCT7.Comparison of the pp longitudinal analyzing power Az with theory will be discussed in another paper in this issue. We note that the measurements of Az in pp scattering are the most accurate observations of PNC in the nucleon-nucleon interaction, and the only ones which have given a significant non-zero result.For pa scattering, the very large transverse analyzing power makes an accurate PNC measurement difficult. In the 15 MeV pp experiment,3 the large pa analyzing power was exploited by replacing the hydrogen in the target with helium for polarization adjustments prior to the measurements.33 A preliminary measurement for pa scattering at 46 MeV required transverse polarization corrections of the 20-min data of 20xl0-7 or more. The later measurements9,10 reduced these corrections by more than a factor 5, by redesigning the acceptance function of the chamber, and to a lesser extent, by reducing the transverse moments (new beam optics). We would very much like to know why there is at best, only marginal compatibility between the old [Az = (0.3±1.3)xl0 7] and the improved [Az = (-3.3±0.9)xlO-7] pa measurements. Of the many improvements which were implemented for the new measurements, the ability to make frequent measurements with transverse polarization without the need to move the scattering chamber10 may have been important in view of the large corrections. In addition, at the time of the first experiment, we did not take the precaution of removing all test equipment for double scattering from the rear of the chamber. Indeed, some of this equipment was partially illuminated by scattered protons.23It is interesting to ask whether measurements of Az by detection of scattered particles can be further improved. The largest single error in the new result is still the statistical error. Development work is under way to increase the polarized beam intensity by another factor of ten but this is useful only if it does not increase coherent beam modulations. In addition, one would require shielding of the beam scanner detectors from the increased background and would once more have to face thermal gradients in the target. Because of our preoccupation with instrumental effects (the number of individual test measurements exceeds the number of data runs by a factor of about twenty) a relatively small fraction of the measuring time is spent on parity measurements. The efficiency of data collection can not be improved very much without sacrificing the reliability of tests for systematic effects.Some of the systematic errors (e.g. empty target background) could simply be improved with longer measuring times. Some other problems would be alleviated if the symmetry of the scattering chamber could be improved significantly. In addition, one would welcome smaller corrections yet for transverse polarization. One can hope to reduce the transverse polarization distribution within the beam, or the sensitivity of the chamber, or both. The acceptance functions of the chamber was optimized separately for pp and pa scattering. The good agreement with the observed sensitivities10, 3 suggests that the calculations are realistic. On the other hand, one might explore completely different detector arrangements. Possibly, transversepolarized moments of the beam can be reduced if one were to study theorigin of these components, and if one were to build a beam transport system specifically for these experiments. We often tried to reduce transverse polarization at the expense of beam intensity by reducing the beam phase space with slits before the momentum analyzing magnet, but without success.There are no plans to further improve our pp experiment at 45 MeV. A precision measurement at significantly different energy would be worthwhile as a confirmation, as would be a pa experiment at a different energy to confirm the predicted variation of <AZ(0)> with energy. One measurement that would certainly be worth the effort is animproved measurement of Az in pa scattering at 46 MeV, using thepresent apparatus. This new measurement would benefit not only from the higher beam currents now available, but from the many improvements in equipment and analysis which were developed for the latest series of pp measurements.248. ACKNOWLEDGEMENTSI should like to thank my colleagues at ETH, Professor J. Lang,Dr. R. Muller and Dr. M. Simonius, who led the effort, for the opportunity of a long, fruitful collaboration on a challenging problem. Dr. Henneck made essential contributions in the early years of the experiment. We missed his competence and cheerful disposition after he left. Years of hard work and hosts of innovation and productive ideas were contributed by the Ph.D. students on the project: Ch. JacquemartF. Nessi-Tedaldi, and Th. Roser. The help of Dr. R. Balzer, Dr. W. Reichart and Dr. Ch. Weddigen in the difficult early phases of the experiments contributed much to the eventual success.An essential part of the experiment was Dr. S. Jaccard and the SIN cyclotron operating staff, as well as Dr. Th. Stammbach. Their dedicated effort provided the proton beam with the special characteristics required by the experiment.Finally, I should like to thank Dr. M. Simonius for helpful discussions during the preparation of this manuscript.References1. M. Simonius, Phys. Lett. 41B, 415 (1972).2. J.M. Potter, J.D. Bowman, C.F. Hwang, J.L. McKibben, R.E. Mischke,D.E. Nagle, P.G. Debrunner, H. Frauenfelder and L.B. Sorenson, Phys. Rev. Lett. 33, 1307 (1974).3. D.E. Nagle, J.D. Bowman, C. Hoffmann, J. McKibben, R. Mischka,J.M. Potter, H. Frauenfelder and L. Sorenson, AIP Conf. Proc. 51, 218 (1979).4. S. Kistryn, J. Lang, J. Liechti, Th. Maier, R. Muller,F. Nessi-Tedaldi, M. Simonius, J. Smyrski, S. Jaccard, W. Haeberliand J. Sromicki, Phys. Rev. Lett. 58, 1616 (1987).5. M. Simonius, this issue.6. R. Balzer, R. Henneck, Ch. Jacquemart, J. Lang, F. Nessi-Tedaldi,T. Roser, M. Simonius, W. Haeberli, S. Jaccard, W. Reichart andCh. Weddigen, Phys. Rev. C30, 1409 (1984).257. R. Balzer, R. Henneck, Ch. Weddigen, J. Lang, M. Simonius,W. Haeberli, W. Reichart and S. Jaccard, Phys. Rev. Lett. 44, 699 (1980).8. R. Henneck, Ch. Jacquemart, J. Lang, R. Muller, Th. Roser,M. Simonius, F. Tedaldi, W. Haeberli, and S. Jaccard,Phys. Rev. Lett. 48, 725 (1982).9. J. Lang, Th. Maier, R. Muller, F. Nessi-Tedaldi, Th. Roser,M. Simonius, J. Sromicki and W. Haeberli, Phys. Rev. Lett. 54, 170(1985), erratum Phys. Rev. Lett. 54, 2729(E) (1985).10. J. Lang, Th. Maier, R. Muller, F. Nessi-Tedaldi, Th. Roser,M. Simonius, J. Sromicki and W. Haeberli, Phys. Rev. C5, 1545(1986).11. Ch. Jacquemart, Dissertation, ETH-Zurich, Nr. 6833 (1981).12. F. Nessi-Tedaldi, Dissertation, ETH-Zurich, Nr. 7825 (1985).13. Th. Roser, Dissertation, ETH-Zurich, Nr. 7487 (1984).14. P. von Rossen, U. von Rossen and H.E. Conzett, AIP Conf. Proc. 69, 1442 (1981).15. D.M. Tanner, Y. Mihara, R.E. Tribble, C.A. Galiardi, R.E. Neese and J.P. Sullivan, in Proc. Internat. Conf. on Nucl. Phys., Florence (Tipografia Compositon, Bologna, 1983) p. FI.16. E. Adelberger, this issue.17. R.E. Mischke, this issue.18. W. Haeberli, R. Henneck, Ch. Jacquemart, J. Lang, R. Muller,M. Simonius, Ch. Weddigen and W. Reichart, Nucl. Instr. Meth. 163, 403 (1979).19. S. Jaccard, in High Energy Physics with Polarized Beams and Polarized Targets [C. Joseph and J. Soffer, Eds.], Birkhauser (Basel) 1981, p. 443.20. M. Simonius, R. Henneck, Ch. Jacquemart, J. Lang, W. Haeberli and Ch. Weddigen, Nucl. Instr. Meth. 177, 471 (1980).21. J.L. McKibben, AIP Conf. Proc. 69, 830 (1981).26DISCUSSION: van Oers:Do you use a deep Faraday cup to reduce false asymmetries due to backstrearaing of beta particles?HAEBERLI:Yes, we do have a deep Faraday cup with good shielding. A bigger problem probably has to do with beta-decay asymmetries in the ion chamber, where the walls of the chamber are activated by scattered protons and you don't gain from solid angle.Mischke:I'd like to confirm we did know that the angular distribution was an important consideration. Your remarks about the quality of that p-d measurement are valid.van Oers:So that was still done with scintillators?Mischke:Yes, with the same equipment.HAEBERLI:I looked through your [Mischke's] papers, and also McKibben's. I thought you put helium in your chamber for testing purposes.Mischke:I actually have a transparency on that, so will show it in my talk. Bowman:We did some measurements of the sensitivity to the distribution of polarization with hydrogen in by blocking off part of the beam at the source, assuming there's an imaging. That was our means of studying polarization moments at that time.van Oers:Willy [Haeberli], what did you want to say about Bonn?HAEBERLI:They have an experiment in progress at 15 MeV on the cyclotron. A year ago when I was there they were testing the chamber. They were using split-plate secondary emission monitors for beam centring and determination of beam breathing. The main detector was a scintillator.van Oers:What about the problem of afterglow in scintillators?Mischke:It's coupled with the reversal frequency. When we ran into problems with afterglow the afterglow time constant was about 1 ms and we27were reversing spin at 1 kHz, so it was the worst matching you could get - which means we found the problem sooner rather then later.HAEBERLI:What did you do about it?Mischke:We changed to a scintillator called perylene - very nice carcinogen - that had only a fast response.Simonius:With an atomic beam source you cannot anyhow switch as fast as 1 kHz.van Oers:So you say the reversal frequency at Bonn must be of the order of 30 Hz or so and then these effects are much smaller. But then, that depends on the scintillator.Bowman:I don't believe that. I believe that whenever you put a scintillator in the experiment you'll never understand it. You have to put in some detector like a Faraday cup or an ionization chamber whose physics you can understand.Simonius:I agree with you.Bowman:In our subsequent experiments we used ion chambers, van Oers:The Texas A&M people quit using scintillators because they also found these afterglow effects, but they had different time constants than you quoted in your private communication.Roy:You mentioned that your empty target test took as long as the parity measurement itself. How far does one push an empty target measurement?Haeberli:We stopped when we got an error of 5xl0-9 and this is the largest error.Mischke:But you had a suppression of 10-3 that you could apply to your raw statistics on that.HAEBERLI:Right, but the number of runs it took for that would have given us28an error in the parity measurement of 0.5x10 ^, so it's not easy. Simonius:Things have improved. In the last run a third of the time was spent on the parity measurement itself, when all was working well.Mischke:That is probably as good as you can get in two years, van Oers:In establishing the coefficients you do a set of control measurements. Do you get just a set of linear equations and solve a big matrix? There are no complications in that?HAEBERLI:You could make some simplifying assumptions. You could say that if you have a vertically polarized beam then that gives a left-right asymmetry, but if you displace the beam vertically it has no effect. We don't make that assumption. That is why we have 10 coefficients. At some point it becomes a question of where to stop. It's an expansion one is using. So, the success of the model is measured by the quantitative agreement you get for the whole set of over­determined data. You see how good a chi-square you get from that model. But there’s an assumption that these terms go as Ar/r to some power.Page:One thing that bothers me is how long it takes to make an accurate polarization scan compared to the parity measurement itself. How long does it take you to get a significant measurement of transversepolarization with the scanning polarimeter?HAEBERLI:The scanning polarimeter keeps running and is in the beam for the same time that the actual parity measurement itself takes. In the end, the error in the dirt effect is 4xl0-7, the error which comesfrom the polarization moment is a small fraction of that, so theresult is limited statistically basically by the parity measurement and not by the uncertainty of the moment.Simonius:The ratio between the uncertainty of correction and the statistical error of the parity measurement was 1:4.Adelberger:You mentioned that the solenoid reversal tended to cancel the effect of non-uniform polarization distribution. I assume the corrected result you gave was obtained by summing over both solenoid directions. How big were the corrections for one solenoid sign?29HAEBERLI:They could be as large as 6xl0-/.Simonius:We have a list in the old paper.Bowman:They're very big in p-d and p-alpha.HAEBERLI:Yes.PARITY NONCONSERVATION IN PROTON SCATTERING AT HIGHER ENERGIES*R.E. MischkeLos Alamos National Laboratory, Los Alamos, NM 87545ABSTRACTParity-nonconservation experiments in the scattering of longitudinally-polarized protons at incident proton momenta of 1.5 GeV/c and 6 GeV/c are examined. These experiments indicate a change with energy of the total cross section correlated with proton helicity that was unexpected. This energy dependence is due to the strong part of the interaction and may indicate the role of a diquark component in the nucleon. New experiments at higher energies are needed to confirm such a model. Future experiments can benefit from an analysis of sources of systematic error that have been encountered in the experiments discussed here.INTRODUCTIONThe first experiments to search for parity nonconservation in proton scattering at higher energies used double-scattering1 or triple-scattering3 geometries. This technique was limited to^  a precision of -10 . A new generation of experiments began in 1972 witha proposal to measure the helicity dependence of the transmission of1.5 GeV/c longitudinally-polarized protons through an unpolarized target.3 An interference between the strong amplitude and the parity- nonconserving weak amplitude is expected to produce a longitudinal asymmetry A, = (cr - cr_)/(a+ + a_) at the level of 10 ', where a+(a_) is the total cross section for positive (negative) helicity protons.Since 1972 experiments have been performed at four energies. In each case several years have been required to reach the required level of precision. The experiment using a 15-MeV polarized beam at the Los Alamos Tandem Van de Graaff was begun in 1972 and ended about 1980. When a 6-GeV/c polarized beam became available at the Argonne ZGS, an experiment was started in 1974 and ended when the ZGS was closed in 1979. The 1.5-GeV/c experiment at LAMPF was begun in 1978 and completed in 1984. These experiments, together with experiments at 45 MeV sample the energy dependence of A^. The group at SIN has continued to make improvements at 45 MeV and has just reported a measurement withunprecedented precision.4A common theme of all these experiments is the identification and suppression of sources of systematic error. This paper will discuss the ZGS and LAMPF experiments in detail. The lessons learned from these experiments can be applied to future experiments at comparable or higher energies.THEORETICAL AND EXPERIMENTAL BACKGROUNDWhen comparing experimental values of A^ with theoretical predictions, there is a contrast between the situation at low energies and at high energies. Measurements5 ’4 of AL at 15 and 45 MeV on30*Work supported by the U.S. Department of Energy.31hydrogen yield results of = (-1.7 ± 0.8) x 10“ andA. = (-1.50 ± 0.22) x 10 , respectively, in reasonable agreement withtheoretical predictions based on a meson-exchange model6-11 and a hybrid quark model.12 (See Fig. 1.)On the other hand, the experiment13 with 6-GeV/c protons on a 1^0 target has reported a value of A^ = (26.5 ± 6.0) x 10- , which is much larger than expected from calculations made prior to the experiment.14’15 Later meson-exchange calculations16’17 have confirmed the prediction of A^ ~1.0 x 10 . Other theoretical approaches includethe multi-peripheral model18 and heavy boson exchange,19 both of which also predict AL to be ~10 . The contribution20 from Coulomb effects isexpected to give only a 15% enhancement of the asymmetry. A recent calculation used Regge theory to calculate the contribution to the asymmetry from parity nonconservation in the nucleon wave function.21 The result is AL = +2.1 x 10 with an estimated error of 30%, but this calculation has been criticized22-25 because its extension to low energies yields predictions for several parity-nonconservation results that are much larger than the experimental values.Most recently a calculation has been reported that considers the effects of parity nonconservation at the quark level. This calculation included both the scattering contribution and the wave-function part.26’27 The wave functions were written in the SU(6) quark basis. The calculation was done as an operator product expansion and independently by writing amplitudes for one-loop graphs. Single-gluon exchange amplitudes were used for the strong interaction. The wave-function mixing effect is based on a sum of transitions toENERGY (MeV)Fig. 1. Measured values of A^ versus energy. The solid curve is a generic meson-exchange calculation and the dashed curve is the model of Ref. 26.32negative-parity excited-nucleon states. The interaction takes place in the nucleon between one quark and a vector diquark. The results aredominated by the wave function part with = +(0.7 to 2.7) x 10~ .Although this model is expected to be valid only at high energy and the uncertainty is large, the result is very encouraging.This and most other calculations have been for proton-protonscattering and have not considered nuclear effects and the role of theneutrons. A Glauber model calculation28 predicts that the effect for p-p scattering should be a factor of 1.7 larger than that measured onwater.The experiments at 1.5 GeV/c (800 MeV) are at an energy intermediate to that of the previous measurements. Results have been published for polarized protons on an HnO target29 of A, = (1.7±3.3) x 1(T7 and AL = (2.4 ± 1.1) x 10_/ for an LH2 target.30 These results can be compared with a surprisingly large range of values among published predictions for the asymmetry at 1.5 GeV/c. The variation is mainly due to the use of different parametrizations of the strong nucleon-nucleon interaction. The predicted values for At from meson-exchange models range from (-8 x 10“ )17 to (+3 x 10_ )16 with intermediate values of (-0.2 x 10~ )31 and (+2 x 10“ ).14 A hybrid-quark model12 predicts a value < 1 x 10- , and the wave-function renormalization model32,33 predicts a large positive value (+18 x 10" ). If the high-energy quark-quark model27 is extrapolated down to1.5 GeV/c, the result is +2 x 10“ .^ No theoretical approach describes the energy dependence of p-nucleon scattering at all energies. The meson-exchange approach can explain experimental results at energies up to 1.5 GeV/c, but underestimates the 6-GeV/c result. The QCD approach is consistent with the 1.5- and 6-GeV/c results, but is not applicable at low energies. These experiments were originally envisioned as a study of the weak interaction between nucleons, but the most difficult parts of the problem for theorists are the strong-interaction aspects. The indication that the diquark component of the nucleon is important is very intriguing. An experiment at higher energy can confirm the energy dependence of A^ predicted by this model.EXPERIMENTAL METHODThe usual technique to determine A^ at higher energies is to measure the beam intensity before and after the target in a transmission geometry. An alternative is to monitor the incident or transmitted beam and detect scattered protons. At high energy the fractional asymmetry could be large enough to compensate for the reduced statistics in this geometry. If the detectors cover a large fraction of the angular range, measuring the integral of the scattered beam is equivalent to measuring the total cross section.The ZGS experiment illustrates the transmission technique; the LAMPF experiments are similar. In the ZGS experiment, two independent detector systems measured the number of protons upstream and downstream of the target for each beam pulse. The detector currents were integrated, as the required beam intensities prohibited counting individual protons. The first detector system used scintillation counters. For this system, the transmission for one pulse of protons from the ZGS was measured as Z^ = T/I where T and I are the signals from the downstream and upstream counters, respectively. The second system used three identical ionization chambers. For each pulse, the signal33from the downstream chamber D was subtracted from the upstream chamber, U, and normalized to the monitor chamber, M (located upstream). Thus 1 - Z, = (U - D)/M .Because each successive beam pulse had opposite helicity, the fractional change in transmission for each pair of pulses is ( = AZ/2Z = (Z+ - Z_)/(Z+ + Z_) where Z+(Z_) is the transmission (from eitherdetector system) for the positive (negative) helicity pulse.Fluctuations in AZ resulted from statistical uncertainties in the measurements of Z and from changes in Z due, for example, to random fluctuations in beam properties. The dependence of Z on beam motion and intensity fluctuations was removed by defining a corrected transmission, Z', for each pulse given byZ' = Z - a1(x-xQ)2 - a2(y-y0)2 - a3(<i2>/I). (2)Here ( x - X q ) and (y-yQ) are horizontal and vertical deviations of the beam from the symmetry axis of the experiment (given by Xq, Yq). A measure of the time structure of the beam within a beam pulse is given by the square of the instantaneous beam intensity, <i >, normalized to the beam intensity for the whole pulse, I. The dependence of Z on position is quadratic in lowest order because the beam was centered on a collimator and a displacement in any direction caused Z to decrease. The coefficients ai were determined from a linear regression analysis to minimize fluctuations in Z'.An average <C'> was calculated for each run. The uncertainty in <C'> was determined from rms fluctuations in C' and is designated S<C'>. Corrections were applied to the <C'> from each run for known background processes such as residual transverse polarization that could give a change of transmission correlated with helicity, yielding<C'>' = <£'> - I y - d • <AHO (3)iwhere Y(cm_1) is the sensitivity constant for the term; d(cm) is the displacement of the beam from the symmetry axis; and <AH> is the average change of a polarization-correlated quantity. The values of the H and d quantities were monitored each beam pulse and the y values were measured in calibration runs.An unanticipated source of asymmetry in the ZGS experiment was due to beam scattered by the small amount of material in those parts of the beam channel where the polarization was fully vertical. The scattered beam produced a signal in the I counter and U chamber that wascorrelated with beam helicity (to the extent that the beam was displacedfrom the effective center of the upstream detectors). In the runsmeasuring this so-called beam-matter interaction, the interaction probability was increased by adding a known amount of material in the channel and measuring the asymmetry.After all runs were combined, a correction for the correlationbetween transverse polarization and position within the beam was applied to the weighted average. This last correction is given by ye where y is the sensitivity to transverse polarization and e is the spatial first moment of the beam polarization distribution:e = JJ dx dy (xRy(x,y) - yRx(x,y)) B(x,y) . (4)x and y are particle coordinates at the collimator, R (x,y) and R (x,y) are the transverse polarization components for a given beam helicity,34and B(x,y) represents the intensity distribution of the beam. It can be seen that a transverse component of polarization that averages to zero can produce a spurious parity signal.5 ’34After all corrections have been applied, the value of <C’>' is converted to the corresponding value of AL.ZGS EXPERIMENTPolarized proton beam and target aThe-6 GeV/c beam from the ZGS had an average intensity of 3.2 x 10 protons/pulse, a spill width of roughly 700 ms, and a repetition rate of 0.3 Hz. The polarization direction was reversed at the source each ZGS pulse. The polarization was vertical during acceleration in the synchrotron and remained so in the external proton beam.A plan view of the beam line and apparatus is shown in Fig. 2. Most of the beamline was evacuated but the beam encountered the vacuum windows and air in some regions. A septum magnet separated the beam from the external proton beam. The magnet B2 deflected the beam upward through 7.75 to rotate the transverse polarization into thelongitudinal direction.Solenoids in the beam line were used to control the transverse polarization of the beam at the target. A quadrupole triplet focused the beam on the aperture of a brass collimator, C, located after the target. The spectrometer consisted of two bending magnets and four quadrupole magnets. Each bending m§gnet bent the beam downward and rotated the spin direction by 90 in the vertical plane. Quadrupoles focused the beam onto the transmission detectors.The target was distilled water, enclosed in an aluminum cylindrical container. The container windows were made of flat quartz glass and were aligned parallel to each other and perpendicular to the incident beam direction. This design ensured that each beam particle encountered the same amount of material in the target. The transmission coefficient of the target was Z = 0.18 ± 0.01.Detector systemsMost of the detectors were mounted on two rigid rails. Three scintillation counters were used for the transmission measurement. Each of these counters had a block of scintillator viewed by four photomultiplier tubes (PMT). The symmetrical arrangement of the PMTsFig. 2. Schematic diagram of the beam line and apparatus. Detectors and beamline components are described in the text.35about the beam direction helped to minimize the dependence of the summedsignals on beam position. A fiber-optic cable, attached to each lightguide, was used to inject a light pulse between each beam pulse for gainmonitoring purposes. Counter I was located upstream of the target, T'was just downstream of the target, and T was placed after thespectrometer.Each of the three ionization chambers had 20 collector plates and 21 high-voltage plates. Each circular collector plate had guard rings on either side. A gas mixture of 90% argon and 10% methane was used in all three chambers. The operating pressure for the M and D chambers was~40 psia and ~8 psia for U.The beam position and polarization were measured every beam pulse by several sets of scintillation counters. The horizontal and vertical beam positions were measured by three sets of detectors with wedge-shaped scintillators, P p  P2 , and P p  Two complementary wedges, forming a block of scintillator, were optically isolated and connected to light guides and PMTs. Two such assemblies were mounted at rightangles to each other and to the incident beam direction to form one position detector.The and R2 polarimeters measured the scattering asymmetries at the entrance to the experimental area due to material in the beam line. The Ro detector monitored residual transverse polarization by measuring the left-right and up-down scattering asymmetry of the beam scattered from the water target. The R^ detector monitored scattering in the magnetic spectrometer. Each detector consisted of four plastic scintillator counters with active regions located to the left, right, above, and below the beam line.The beam centroid at position detector P-^ was stabilized pulse-to-pulse with the aid of a feedback loop. The voltage signals from the two horizontal PMTs of detector P-^ were used to control the current in magnet SB1. The time constant of the feedback loop, including the magnet response time, was ~100 ms.The PMTs were selected to provide linear, noiseless gain with capability for a large dynamic range. Only five accelerating dynodes were used for the P p  I, and T' detectors, which were exposed to the full intensity of the beam. Detectors R p  R 2 , and R^ used ten-stage PMTs. The current in each PMT was converted to a voltage by an operational amplifier and digitized by voltage-to-frequency converters (VTFs). The gain of each member of a group of detectors was matched to within 5%. The VTF output pulse train was scaled and recorded on magnetic tape each beam pulse.Experimental procedureThe beam-line magnet currents were adjusted to maximize the transmission of the beam through the apparatus. Information from the calibration runs allowed the beam to be positioned on the null orsymmetry axis of the experiment where contributions from beam-matter effects were minimized. The beam was focused at the collimator, thesmallest aperture in the beamline, to minimize the noise due to beammotion. A transmission quality factor Q was defined as the ratio of measured fluctuations in the difference of transmission through the target for each helicity state to those fluctuations expected from statistical variations in the absorption process. By systematically adjusting the beam magnet currents, online Q-factors between two and seven could be attained, for both detector systems. The offline36analysis could not remove these beam-induced transmission variations satisfactorily if the online Q-factors were larger than about ten.There were 184 data runs with -1600 pulses/run for a total of -9 x 10 protons on target. The rms variation of beam intensity was 4%. The rms resolution of the wedge detectors was about 15 ym. The pulse-to-pulse fluctuations of the beam position were 1 to 2 mm horizontally. In the vertical direction, beam motion was a factor of two smaller. Fast motion within a beam spill had amplitudes of up to -5 mm and these were unaffected by the slow feedback. The difference in the beam position between positive and negative helicity protons as measured by P p  averaged over all the data, was consistent with zero.The polarimeters monitored scattering asymmetries throughout the experiment and the results are incorporated into the correction terms in Eq. (3). Averaged over all data runs, the residual vertical polarization was (3.62 ± 0.15) x 10~4 and the horizontal polarization was (-2.69 ± 0.40) x 10 , compared to the longitudinal polarization of0.71 ± 0.03 at the target. The average left-rightscattering asymmetry measured by the R2 detector is (3.500 ± 0.002) x 10 due to material in the beam line.Three types of calibration runs were taken to measure the sensitivity coefficients y, for the correction terms in Eq. (3). A horizontal or vertical polarization of -5% increased <AH> = <AR3x> or <ARo > to -2.5x10 . Data were recorded with <AR3 > non-zero atincrements of a few mm in ^P3x^ an<^  similarly at intervals of <P2y> for <ARo > non-zero.Added-absorber runs to measure the beam-matter interaction effects were taken with 5 cm of Lucite placed about 2 m upstream from th^ center of B2, which increased <AR2 > by a factor of ten to -3.5 x 10 . Thebeam was moved a few mm left and right of beam center while the absorber intercepted the entire incident beam. Additional data were taken with Lucite absorbers of 1-cm and 2-cm thickness. By extrapolating the asymmetry from the data with Lucite to zero added absorber, the amount of scattering taking place during nominal data runs was determined to be equivalent to 5 mm of Lucite absorber.Beam-partially-blocked runs to measure the polarization distribution in the beam were taken with either the top, bottom, left, or right half of the beam removed with a collimator.Analysis and resultsThe signal from each phototube for each pulse was obtained by subtracting electronic offsets and dark current as measured in the appropriate gating intervals. The data selection procedure eliminated data from beam pulses with poor beam quality. The procedure used "quads" where a quad for variable X is defined as Xq = Xi - Xi+1 - Xi+2 + X. o, where pulse i has positive helicity. A quad has zero net polarization, an average value, <Xq>, of zero, and is not affected by a linear gain drift during the four pulses. Acceptable quads had all beam pulses with more than 5 x 10 incident protons and no variables with negative offsets. The distribution of values of Xq is generally Gaussian and centered at zero but has enhanced tails. Because of thejj>e tails, the width of the distribution is defined as 8<Xq> = 0.69(X ) where Xm is the median of the absolute values of the quads. If Xq was greater than 2.5 S<Xq> for any of the six variables listed above, the data from the four beam pulses were rejected. These criteria removed about 10% of the data from each run.37A regression analysis was employed to reduce the effects of beam properties on the measured transmission. The evaluation of thecoefficients in Eq. (2) was based on an analysis using the quad values of the variables. This made the results insensitive to any correlation between beam helicity and position or intensity. There is no evidencefor a helicity correlation with these variables since each of thecontributions is consistent with zero. The data from the R p  R2, and polarimeters were treated in a similar manner to remove position sensitivity in the polarization values.The next stage of the analysis corrected for known helicity correlated quantities based on Eq. (3). The terms for residual transverse polarization mostly affected the T and D signals. The terms for beam-matter interaction mostly affected I and U. For each run,including calibration runs, the values of <C'>, <AR?>, and <P^AR<> were found. The coefficients were determined with a X minimizationprocedure applied to these values. The 10% of the runs that contribute a X > 5 to the fit were rejected. The X /df =1.17 for both systems.The result is <C'>' = (-2.92 ± 0.80) x 10 for the scintillatorsand <£'>' = (-4.96 ± 0.99) x 10” for the ion chambers. The correlationcoefficient between values of <£'>' for the two detector systems is 0.20 and was determined from the values for each run after all thecorrections were made. The small value of this coefficient indicates that the measurements are essentially independent. A weighted average gives<£'>' = (-3.73 ± 0.62) x 10”6 . (5)For the final correction, the average helicity correlatedcomponents of polarization, <AR > and <AR >, were measured with the beam partially blocked. Then e = a \ <ARX> - <AR > ) where the coefficient a depends on the beam shape and the distribution of polarization across the beam. The polarization distribution arises from the process ofextraction from the ZGS and from the effect of fringe fields in the magnetic transport of the beam. The known air and solid matter in the beam line broaden the beam size due to multiple scattering. Thus the beam profile is Gaussian and any higher-order components of the polarization distribution are washed out. As a result, a linear variation of polarization with position is expected with a Gaussian beam intensity shape, yielding a = -S2n/8. The value of y is that determined for transverse polarization, leading to a correction of(-0.50 ± 0.37) x 10”6 to <£'>'.The parity-nonconservation asymmetry A^ is related to the net <C'>', 4n the limit of small AZ, by the expression Al = 1/(|P|8nZ) <C'>' . The result isAl = (2.65 ± 0.60 ± 0.36) x 1CT6 . (6)The first error is statistical; it is dominated by the uncertainties in the individual measurements of the transmission that have been propagated through the analysis but also includes contributions from the statistical uncertainties in the corrections.The second error is an estimate of systematic uncertainties. Because the largest correction to <C'> comes from beam-matter interaction, several possible sources of error in the assumptions have been studied carefully. Data taken with the beam displaced 4 mm off the central axis yielded an unwanted 15% increase in the asymmetric halo38measured at the I counter, due presumably to scattering from upstream apertures. However, the sensitivity of <C'> versus position agrees with a linear dependence within statistics. In addition, placement of the Lucite scatterer along the beam line was studied and the position chosen was representative of the real distribution of matter. Finally, theintroduction of additional scatterer upstream of the I counter did not change, within statistics, the asymmetric halo measured just downstream of the target. From these considerations a plausible systematicuncertainty is 20% of the correction, or 0.3 x 10“ .Another possible systematic error comes from uncertainties in the correction for the effect of polarization correlated with position within the beam. One contribution comes from a lack of direct knowledge of the shape of the polarization distribution across the beam profile. Another possible contribution is from the fact that the blocked-beam measurement was not taken at the location of the collimator. The total estimated uncertainty in the correction is 30%, leading to an estimated systematic uncertainty in the result of 0.2 x 10~ . The measurement of this contribution was made near the end of the experiment; as a result there is no direct information on its stability with time. However, there is no evidence for drifts in the observed longitudinal asymmetry. If the observed asymmetry is the result of position-correlated polarization, this quantity must be large and constant during the long period of the data runs and then change abruptly to a small value at the point when it was measured. Such a change is very unlikely.Other sources of systematic error, such as the treatment ofresidual transverse polarization and the effect of hyperon decay products, are negligible. From the energy dependence of the cross section and an upper limit on the correlation between beam momentum and helicity, this effect is estimated to be < 2 x 10“ . The effect of purely electronic sources of a false parity effect are tested by analyzing data taken with the beam off; the result is AL < 10“ . The result of analyzing the data grouped in a helicity suppressing pattern is Al = (0.5 ± 0.6) x 10 . A test of drifts in the signals is ananalysis of alternate runs starting with the opposite polarization; the results with this analysis are unchanged.LAMPF EXPERIMENTSThe experiments performed at LAMPF utilized 1.5-GeV/c longitudinally polarized protons. A transverse magnetic field in the Lamb-shift-type ion source35 reversed the proton helicity with a 30-Hz periodicity. The reversal frequency was chosen to be near a minimum in the spectral density of beam noise. The beam was accelerated to 800 MeV as H~ ions and reached the apparatus in macropulses of 500-psec duration with a 120-Hz repetition rate. The beam intensity varied between 1 and 5 nA and the average polarization was |P| = 0.70 ± 0.03.The layout of the version of the experiment with a 1-m LH2 targetis presented in Fig. 3. The stripper foil was located 50 m upstream of the rest of the apparatus. An aperture in the foil defined the beam by stripping electrons from the H~ in the outer parts of the beam. The resulting H+ were removed by a magnet. The transmission of protonsthrough various targets was measured by two integrating ion chambers(II and 12), located upstream and downstream of the target. The statistical sensitivity of the measurement was limited by the available beam intensity as well as by detector noise due to nuclear spallation39_JmuBEAMPI□Iw-EBSga----- S□II 1 2W STP2□nFig. 3. Schematic of experimental setup. Detectors are described in the text.reactions in ion-chamber surfaces. To reduce the second effect, spallation-minimizing ion chambers36 were developed and used.For the two helicity states of the beam, the fractional change in transmission was determined from the analog difference of the II and 12 signals. This difference signal was amplified before digitization to reduce round-off error. For each group of four pulses an analog to Eq. (1) was calculated. The helicity reversal pattern for the group of four pulses was + - - + to reduce the effects of drifts and to remove 60-Hz effects. At the end of a run, which consisted typically of 4 x 10 pulses, an average of £ was calculated and a statistical uncertainty was computed from the fluctuations of C. For the 1-m LH2  target the transmission was 0.85.The beam position, intensity, size, and net transverse polarization (T 1 ) were monitored for every pulse. In addition, the transverse-polarization distribution across the beam profile was sampled to determine e.Integrating multi-wire ion chambers,37 W, monitored beam position and size for each pulse. Split-collector ion chambers, S, also monitored beam position and were part of a dual-loop feedback system that stabilized the average beam position and incident angle. A four-arm polarimeter, PI, used the LH2 target as an analyzer to measure T 1 the beam. A second polarimeter utilized a narrow target, ST, that continuously scanned the beam profile to measure e. The upstream ion chamber of the transmission measurement recorded intensity variations of the incident beam.To cancel contributions to A^ from beam changes uncorrelated to the beam helicity, the experiment was run for equal time periods in two different operating configurations (N and R) of the spin filter35 in the polarized source. In both configurations protons exiting the source were longitudinally polarized, but positive helicity for the N and R configurations occurred during opposite phases of the spin-flip field of the source. The combination (C^-Cg)/2 measures the longitudinal asymmetry while canceling some systematic effects and is referred to as the PNC signal. The combination (C^+C^)/2, called HIS, is expected to be zero and serves as a test for unidentified systematic errors.To correct C for systematic contributions, its sensitivities to different systematics were determined. During the transmission measurement, each beam systematic was monitored. Final corrections to £ were applied in the off-line analysis. Z values were corrected pulse-by-pulse for changes in beam intensity, position, and size. Corrections for Tp0i were made for each group of four pulses, while40corrections for e and for unwanted electrical couplings were applied on a run-by-run basis. As a further test for unidentified systematic errors, the data were analyzed using a shift in the four-pulse grouping that eliminates any helicity dependence from the calculated A^. The resultant value, AL (shift), was consistent with zero.The sensitivity of C to intensity modulations was determined using an apparatus38 consisting of a set of stripper grids that were moved in and out of the H~ beam path to produce a 10% intensity modulation at 30 Hz. Stripper-grid data were taken as the DC intensity and size of the beam were varied. An analysis of these runs indicates a dependence: dC/dl = Aq + A-^ I + A2IZ + 2^,/ax + A4CTv/ax’ where I is the beam intensity, ax(a ) is the horizontal(vertical) width of the incoming beam, and the A^ are coefficients determined from the data. The terms containing I result from nonlinearities in the detectors and electronics. The size-dependent terms are consistent with recombination effects within the chambers.During the experiment, contributions from polarization systematics were minimized by locating the beam along the symmetry-axis of thetransmission detectors. To determine this axis, the transversepolarization was deliberately increased, and changes in C were measuredas the beam was scanned across II and 12. The position servo-loopsystem held the beam on the symmetry axis. As a result, transversepolarization gives the smallest of all systematic corrections: acorrection to A^ of < 1 x 10" .At each transmission detector, position scans were performed to measure the sensitivity of Z to position. The largest measuredsensitivity was dZ/dy = 1.3 x 10 /mm for vertical motion at the downstream detector. Small corrections for size variations werecalculated from the quadratic components in the position dependence of Z. For approximately one third of all the runs the beam spot fell mostly on only two wires of the beam size monitor, and hence the beam size could not be accurately determined. Size corrections were not applied for these runs. In the runs where size corrections wereapplied, their contribution to AL was negligible.Any 30-Hz electrical pickup was kept out of the difference signalin two ways. First, a 15-Hz digital signal was used to transmit thehelicity-reversal information from the polarized source to the experiment. Second, optical or analog isolators were inserted in allimportant signal paths. Residual pickup contributions were measured in runs taken with the beam off.Applying the corrections improves the consistency of the data inseveral ways. First, within each run, corrected data have decreased pulse-to-pulse fluctuations in AL because the correlations between C and various beam systematics have been removed by the application of the corrections. Second, when the data from all runs are tested for the hypothesis that HIS = 0 and that PNC has a definite value, the X value for the corrected data is nearly a factor of 2 smaller than X for the uncorrected data. The corrected HIS result is consistent with zero. The measured parity-nonconserving longitudinal asymmetry is Al = (+2.4 ± 1.1 ± 0.1) x 10_/.DISCUSSIONIn both the LAMPF and ZGS experiments, each version of the experiment benefited from the earlier ones. The experience gained from these experiments may also be applied to future experiments. Most41immediately this applies to the experiment underway at 230 MeV at TRIUMF. Other possibilities for future experiments include Saclay at 3 GeV, BNL at energies up to 22 GeV, and Fermilab at 200 GeV or higher.The first measurement39 at the ZGS found AL = (5.0 ± 9.0) x 10 using a Be target. It was found that the dominant contribution to the fluctuations in the measurements of Z was due to nonuniformities in the target coupled with random motion of the beam. This lead to the use of a water target with flat and parallel end windows in subsequent runs to ensure a uniform length and density for the target.In the second version of this experiment,40 was found to be(-15.0 ± 2.4) x 10“ . This value of A^ was attributed to the productionof polarized hyperons in the target. Specifically, in the decayA0 -> pn~, the protons emerge preferentially along the direction of the lambda polarization and the pions preferentially against. The proton has an angular distribution peaked more forward in the laboratory frame than the pion. As the beam helicity is reversed, the angulardistribution of the decay products is modified, which gives rise to a helicity-correlated signal. A collimator was inserted downstream of the center of the target that transmitted only 10% of the protons and 1% of the pions from lambda decay and less than 0.5% of the lambdas. Inaddition, a focusing magnetic spectrometer was installed to transmit only particles with the beam momentum minus the momentum loss in the target. This eliminated the decay products of the polarized hyperons produced in the target and therefore removed the spurious parity signal that could be caused by hyperon decay products striking the transmission detectors. A study40 of the decay distribution of polarized lambdas with a Monte Carlo computer program, in which the longitudinal polarization transfer to the lambdas was assumed to be (0.26 ± 0.18), produced a cross section asymmetry A, = (31 ± 23) x 10 . The result ofthe final experiment41 using the T9" detector, which reproduces the geometry of the detectors without the spectrometer, does not confirm the large negative asymmetry for the value of AT but finds Ajj(T') = (3.9 ± 0.72) x 10“ after all corrections.The third experiment42 included the spectrometer to eliminate hyperon decay products. A large transverse scattering asymmetry due to the beam-matter interaction was discovered (six times greater than the present experiment). The result was (-26.3 ± 7.5) x 10 . Since theexistence of the beam-matter interaction was not realized until the end of the third experiment, the data from the second experiment were not corrected for beam-matter interaction, nor was there an attempt to position the beam on the symmetry axis. Thus it is probable that beam-matter interaction was responsible for the large negative result in the second and third versions.In the final version the contribution from beam-matter interaction was reduced by evacuating the beam line where possible, adding helium elsewhere, and enlarging the aperture at the entrance to the experimental area just upstream of B2. Even so, the largest systematic correction to AL in this experiment comes from the beam-matter interaction. The correction to Aj. with the beam carefully positioned on the symmetry axis, is -1.2 x 10 . Transporting a longitudinallypolarized beam to the experimental area would eliminate this contribution to A^. Otherwise beam halo can be a very subtle and time-dependent source of systematic error.An attractive feature of the ZGS experiment was the ability to make two simultaneous independent measurements of AL. Two detector systems with different properties increase the confidence in the final result by42aiding in the understanding of systematic and random backgrounds. This experiment measured A. with an accuracy of better than 6 x 10~ in abouta six-week period of data taking. The error is roughly three timesgreater than expected from the statistical fluctuations of the beam absorption in the target (Q-factor ~3).With beam intensities above 5 x 10 protons/pulse, the Q-factor increased rapidly, precluding a more precise measurement of in areasonable amount of time with these detectors. The extra fluctuationsin the transmission measurement in each detector system are uncorrelated and therefore did not originate from a common source. The dominant source of noise for the ion chambers was due to spallation in the plates.36 Beam motion during the spill, 60 Hz and greater, contributed to the noise for the scintillation counters. To improve the Q-factor, a regression analysis removing beam motion from the transmission and adata-selection procedure, during the spill, could be accomplished by electronically dividing the beam spill into small time segments. The gain drifts of both detector systems were random and negligible.Ion chambers perform well in intense beams but scintillation counters do not because of radiation damage to the plastic scintillator. The use of liquid scintillator instead of plastic scintillator is a possible solution to this problem. Alternatively, an experiment that measures only the scattered beam from the target with scintillation counters and the transmitted beam with ion chambers could utilize high beam intensities.Early versions of the LAMPF experiment were plagued by high noise in the system. This was eventually traced to spallation in the ion chambers and lead to the design of new chambers. Next data were taken with an H2 O target. Control data were taken with a Pb target and no target as a check on the validity of the corrections and a test that other beam properties do not contribute to the PNC signal. The ideal control target would have all the properties of H2 O except the PNC contribution. The thickness of the Pb target was chosen to give the same amount of multiple Coulomb scattering as the H2 O target but with a factor-of-ten fewer nuclear interactions. Both the Pb and target-out measurements are less than ideal as controls because of reduced sensitivity to beam polarization effects due to low analyzing power,sensitivity to intensity that is different from H2 O, and statistical uncertainties about twice that of The net corrected value of A^for HoO is (1.7 ± 3.3 ± 1.4) x 10~ .The credibility of such experiments depends on the identificationand study of all sources of systematic error greater than approximately half of the desired statistical accuracy. This is no easy task as there is no global test to determine the presence of a systematic contribution to A^. Therefore, careful consideration should be given to detector systems that monitor beam properties and the models used to make corrections should be experimentally tested. Also, classes of systematics may be studied with unpolarized beam. The ZGS experiment had only a simple reversal of spin between pulses. A reversal pattern of +— + can remove linear drifts. In addition there should be a method of reversing the proton spin external to the source. This helps to separate spin related systematics from those due to other beam properties. The LAMPF experiment included data with both helicities relative to the reversal signal.The method used in these experiments to measure residual transverse polarization contributions to A^ could be repeated in a more sensitive measurement of AL- A position feedback loop controlling the current in43an upstream bending magnet is necessary to minimize beam motion and maintain the beam position on the symmetry axis to minimize effects of residual transverse polarization. The correlation of polarization with phase space should be measured at apertures that intercept scattered beam and can be determined by passing a thin scatterer through the beam and measuring the resulting transverse scattering asymmetry.43Calibration runs should be repeated frequently during the experiment to compensate for changing conditions. In spite of the similarities of the sources of systematic error in the experiments described here, each accelerator is different and has its own potential for surprise.CONCLUSIONSThe existing measurements of AL at 1.5 GeV/c and 6 GeV/c indicate a strong energy dependence of the amplitude for the interference between the strong and non-leptonic weak interactions. New measurements at higher energies are needed to confirm this energy dependence and validate the quark-model predictions. These experiments are very difficult, but with adequate beam intensity and quality, the lessons of previous experiments should guide new efforts to a successful conclusion.ACKNOWLEDGMENTSMy colleagues on the LAMPF and ZGS experiments are listed as the authors of Refs. 13 and 30. I am indebted to them and to the others listed in these references for the success of these experiments.REFERENCES1. E. J. Gucker and E. H. Thorndike, Phys. Rev. D 4, 2642 (1971).2. P. Limon, L. Pondrom, S. Olsen, P. Kloeppel, R. 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Ohlsen et al., AIP Conference Proceedings No. 69,(American Institute of Physics, New York, 1980), p. 830.J. D. Bowman, R. Carlini, R. Damjanovich, R. E. Mischke,D. E. Nagle, R. L. Talaga, R. W. Harper, and V. Yuan, Nucl. Instr. and Meth. 216, 399 (1983).D. W. MacArthur, R. E. Mischke, and J. P. Sandoval,Nucl. Instr. and Meth. A245, 262 (1986).D. W. MacArthur, Nucl. Instr. and Meth. A243, 281 (1986).J. D. Bowman, C. M. Hoffman, C. F. Hwang, R. E. Mischke,D. E. Nagle, J. M. Potter, D. M. Aide, P. G. Debrunner,H. Frauenfelder, L. B. Sorensen, H. L. Anderson, and R. Talaga,Phys. Rev. Lett. 34, 1184 (1975).R. L. Talaga, Ph.D. dissertation, University of Chicago, 1976 (unpublished).N. S. Lockyer, Ph.D. dissertation, Ohio State University, 1980 (unpublished).D. M. Aide, Ph.D. dissertation, University of Illinois, 1978 (unpublished).W. Haeberli, R. Hennick, Ch. Jacquemart, J. Lang, R. Muller, M. Simonius, W. Reichart, and Ch. Weddigen, Nucl. Instr. and Meth. 163, 403 (1979).45DISCUSSION: van Oers:When the beam was servoed onto the axis did the envelope of the beam change?MISCHKE:Yes, but we used the average position for correction on a run by run basis.van Oers:Were there two sense regions in the perpendicular field ion chambers?MISCHKE:Yes, but we tied them together. The plates should be made symmetric by making the plates at ± high voltage.Haeberli:Would you like to say something about the quietness of liquid H, targets?MISCHKE:There seems to be enough averaging over one meter to be no contributions, but we used only 2 nA of beam.Haeberli:Would you like to comment on your 800 MeV experiment? You made some suggestions that we should look into. I don't remember the limitations on the accuracy of your numbers. Why did you stop where you stopped?MISCHKE:We had exhausted the patience of the experimenters and the management at LAMPF.McDonald:Not necessarily in that order...?MISCHKE:We were statistics limited. We could certainly consider going back to redo the experiment with the bright new optically pumped polarized source now being installed there.Bowman:It's much harder to do parity experiments with pulsed accelerators because the beam comes down and you can't control its properties and there are transient effects.Simonius:Your control of the beam, is it done on a pulse by pulse basis?46MISCHKE:It was done with a time constant on the feedback that had a drop off time that was fast compared to the 60 Hz pulse rate. The dominant position modulation was at 13 Hz due to mechanical instabilities in the drift tube section of the linac.Simonius:It's not on a pulse by pulse basis, but on the average.Roy:Did you use just one scanning polarimeter at LAMPF in the 800 MeV experiment?MISCHKE:Yes.Roy:Unlike the SIN experiment, in a sense, they did the trajectories with two scanning polarimeters. Wouldn't you suggest getting the trajectory is best?MISCHKE:Well, what's relevant for the correction is the polarization distribution at the defining aperture, which was the downstream ion chamber and the transmission detector. And the scanning polarimeter was located close to that; and the only difference in the transported beam is due to multiple scattering which doesn't affect the correction. In their (SIN) case they had a completely different size scale —  maybe you could comment on if you could have done with one polarimeter.Haeberli:No!MISCHKE:They have a different situation. They're putting the raw beam into their ion chamber. Whereas our aperture is downstream of a one meter long H2 target and the beam has essentially forgotten what it looked like coming into the apparatus.Simonius:Have you checked that experimentally?MISCHKE:Well, look, in a transmission experiment you're inherently much less sensitive, because you're sensitive to analysing power at the defining aperture and that's small because the defining aperture is near 0°.Bowman:... whereas in a 3-dimensional experiment you're much more sensitive to the 3-dimensional geometry.47Simonius:Are you outside the Coulomb region?Bowman:You can analyse all that stuff, but I just want to say that you can get away with a simpler polarization monitoring apparatus in a transmission experiment than in a scattering experiment.Haeberli:It is true in our case that we could not have gone anywhere without two polarimeters.Bowman:It turns out that the only thing that you need is the phase space correlation at the defining detector downstream of the target and not at the target as you might think.Roy:You're not concerned about nature being unkind and faking you out? Simonius:Have you checked this experimentally? You can also go in with a different direction with the beam.Bowman:I think I can convince you with some simple analytical arguments that it was true.Adelberger:In the Glauber model where the ratio of the Az in H2 to H20 is calculated, what physics do they assume? After all, in one case you have p-n interactions as well as p-p interactions, so what did they assume about the p-n interactions?MISCHKE:They do basically a p-nucleon model, so they break it into two pieces with the p-p and p-n cross sections and then it depends on the nucleus. So the 1.7 factor is calculated specifically for oxygen.Adelberger:What about the weak interaction?MISCHKE:They don't do anything about the weak interaction part of the absorption.Simonius:This is a strong absorption.48Bowman:The 1.7 comes from the denominator because the denominator H20 is strongly influenced by shadowing or by screening.Simonius:The correction is not on the weak part, but it's on the strong part.Adelberger:The weak part is different in the two cases.MISCHKE:Maybe, but they don't take that into account.Adelberger:The calculation only has meaning to the extent that the weak interaction between two protons is the same as between a proton and a neutron.van Oers:Your last transparency was entirely based on the model of Goldman and Preston. Is that correct?MISCHKE:That's correct.49Editors n ote: Dr. Adelberger’s manuscript was not available. What follows is es­sentially a verbatim transcript o f his talk. The diagrams are based on transparencies presented during the talk and the text has been edited slightly for  clarity.50PARITY VIOLATION IN NUCLEAR SYSTEMS, SCATTERING AND DECAY EXPERIMENTSE. AdelbergerNuclear Physics Laboratory, University of Washington Seattle, Washington 98195In principle, the low energy PNC NN interaction could be com­pletely determined for 6 independent experiments in the two-body sys­tem, involving p+p, p+n and n-t-n interactions. However, in spite of intensive efforts, a definite parity-violating effect has been seen in only one observable in the nucleon—nucleon system, although for this one observable the experiments have been done very beautifully indeed, and that is the longitudinal analyzing power in proton-proton scattering. So, it turns out that the additional experiments that you might imagine in the nucleon-nucleon system are even harder than the ones in the proton-proton system. I will return to the question about the neutron-proton system towards the end of the talk but, for the moment, I will take the point of view that, to learn something additional, we have to turn to complex nuclei to look for more phys­ics. Clearly we need systems that amplify the effects. That was one of the reasons for going away from the beautiful simplicity of the nucleon-nucleon system. That means we need low-lying levels in sim­ple nuclei, and then in complex nuclei we can take advantage of the isospin structure of the states in order to learn something about the isospin structure of the underlying nucleon-nucleon interaction.That is very interesting because, for instance, the part of the nu­cleon-nucleon interaction that behaves like Al = 1 is the part that comes from weak pion exchange and if there is any part of the nucle­on-nucleon interaction that we think we ought to understand then it should be the longest range part. It's interesting that it is also that part which is most sensitive to the contribution of the Z° to the weak interaction.To go in general to some random nucleus and try to measure some observable such as, for example, the circular polarization of a par­ticular l~->-2+ gamma ray, the circular polarization comes from admix­ture in either the initial or the final state of the neighbouring, or not so neighbouring, states of the same spin but opposite parity.And so you can measure something, let us assume with great accuracy, but then to try to learn something about the nucleon-nucleon parity- violating interaction you would need to have a very good theory of nuclear structure. You would need to know where all the 1"*" (admix­ing) states were in principle, their excitation energies, their Ml matrix elements, their weak matrix elements, where all the 2” states are, and it's obvious that the problem is a great mess and that nu­clei are too complicated to calculate some ab initio kind of thing like we might be able to do in atomic physics .Fortunately, there exist simple systems where the nuclear struc­ture part, this big sum over all initial and all final states, col­lapses down with reasonably good accuracy to a single term, and hence you can measure one quantity, some parity-violating observable, and deduce one thing, which is the weak matrix element connecting a par­ticular pair of levels. You are still left then with the problem of understanding this weak matrix element between two nuclear states in51terms of the more fundamental weak interaction between nucleon-nucle- on states and I will address that issue because, in fact, that is really the Achilles' heel of this business.The simple systems are nuclei in which the two levels have the same spin and opposite parity and one of the levels has a much larger decay amplitude than the other. If you study the parity non-conserv­ing property of the longer-lived member of the doublet then you can get an amplification, and I'll remind you of how that goes. The amplification factors in light nuclei range from in the order of 10 for the 0“ and 0+, 1=1 levels near 8700 keV excitation in iltN and the■?r and i- , 1= ■i- 110 keV level and ground state of iyF, up to 300 forl+ l- ithe j  and j  , 1= j  levels at 2790 keV in 2iNe. In all of thesecases, when one works out the details, this approximation of two-level mixing is really quite solid. Just to remind you of how this goes, I'd like to show you the famous case of 1BF. It's famous be­cause the mixing of the 0~, isospin 0 level with the 0+, isospin 1 level probes the part of the weak nucleon-nucleon interaction that carries one unit of isospin and that's the part that is the pion ex­change, the longest range part, the part most sensitive to the neu­tral current. If we just do elementary quantum mechanics, the circu­lar polarization of the gamma ray comes from admixing into the domi­nant El transition a little Ml piece. It turns out that this El is very retarded, it's forbidden by isospin conservation, on the other hand the Ml transition is one of the strongest known to man. As a result, the circular polarization is very, very sensitive to the lit­tle mixture of this very rapidly decaying level. If you carry through the perturbation theory you see that the circular polariza­tion is twice the weak matrix element between the two levels divided by the energy splitting, which is only 39 keV, and then some factor which gives you the El over the Ml matrix elements which just can be inferred from the lifetimes of the levels.l<Hpnc>l / t (1081)I P y (1081) | -  2  m   V - (1042)Now, you measure one thing, the circular polarization, the energy splitting and lifetimes of the levels can be measured, and therefore you can deduce one thing, the weak matrix element between these two levels. For this part of the talk, I'm going to neglect the experi­mental problems, namely, how do you really know what the circular polarization is, and I'm going to focus on the question "what do we do with this quantity that we extract?"Computing these parity non-conserving nuclear matrix elements turns out to be far from trivial and the reason is that the matrix elements are considerably suppressed with respect to what you would expect using what Gerry Garvey calls the idiot minded wavefunctions. These are suppressed by physical phenomena, the deformation of nuclei, the pairing force and the shape of the nuclear surface. How­ever, it's fortunate, as Wick [Haxton] I believe will explain to you later that although the parity non-conserving operator is really a two-body operator, its effects are really quite well described in terms of two effective single body operators, a part that's an iso­spin scalar, cr.p, a part that's an isospin vector, (a.p)Tg. For52example, in fluorine, because the isospin is changing by one unit, it is really primarily sensitive to this second single particle operator.Therefore, in special cases, unfortunately, all too rare, one can calibrate the nuclear matrix element by independent experiment. You don't have to know how to calculate the matrix element (a.p)T3 between these two states in fluorine; there's a way to measure it.In these special cases one can measure the first-forbidden beta-decay between the analogues of the states that have the parity mixing. The parity mixing and the beta decay share the same single particle nu­clear matrix element. Hence measuring the beta-decay calibrates the same matrix element as involved in the parity mixing and the situa­tion is very nice.This shows how it goes<0+lH™ J 0_> M ,p = o --------1 Pn C   — Ly AE E i ><0+ | Hpnc I 0_> = <0+ | © ( A J *  = O-AI = 1) | 0“>firThe weak matrix element for fluorine is some nuclear matrix element times something that stands for the strength of the nucleon-nucleon matrix element involving weak pion exchange and this nuclear matrix element is the same one that's involved in the beta-decay of 18Ne to the 0“ level. And so for these particularly simple cases where you can do the beta-decay correctly, there is really a very clean situa­tion. As a dramatic illustration of how this works let's just con­sider this case of 18F and compare it to another case in the nucleon- nucleon system where there's an experiment sensitive to exactly the same physics, ie to the weak exchange of pions. That is the experi­ment where you have polarized, thermal neutrons on protons and you look for the asymmetry of the gamma ray. Let's assume the standard Desplanques, Donoghue and Holstein best values that we know now, in this particular case, are at least 3 times stronger than what's ob­served, but let's use them for the moment for comparison here. Youwould predict an asymmetry of the gamma ray in n-p capture which is 5xl0-8. The circular polarization in i8F is 1.5xl0-3, so the nuclear amplifier, so to speak, has bought you a gain of 3xl0+lt, so it's really a very significant gain that you get by running in a complex nucleus. In this case the gain does not induce much noise, because of the two-level nature of the problem, measuring a circular polari­zation gave us a nuclear matrix element, and because one was able to do the analogue, first-forbidden beta-decay, you could calibrate the nuclear matrix element and therefore you could extract this coef­ficient f,j from the expression. This is, what you might say, a shining success story.In the case of iyF parity mixing, one can play the same trick with a little less certainty, because two nuclear matrix elements, the isospin vector and the isospin scalar, are involved. One can measure the decay of lyNe, which is the analogue of the ground stateof iyF, to the y  state. Unfortunately, for the case of 21Ne, one cannot play this trick because neither of the parity mixed states is the analogue of a ground or beta decaying state. For other examples53that I talked about as well, one cannot play that trick. However, these other doublets, where you can't play the trick, are still in­teresting. For example, 18F probed the part of the nucleon-nucleon interaction that was Al = 1, laF is a mixture of 0 and 1, but they come in one sign, and the mixture comes in with the opposite sign in 21Ne and the example of 11+N that I'll be talking about in more detail today, because there are some new results, is primarily sensitive to the Al = 0 part of the interaction. So, in principle, we want to do all of these kinds of cases, so that we can learn the complete iso­spin structure of the interaction.One can analyze the results in iaF, in 18F and 21Ne and p + a in terms of two parameters. The crudest way to do this is just to trun­cate the problem and have the weak pion exchange for the isovector interaction and, for the isospin scalar part, weak p exchange. The strength of the zero-energy amplitudes for these exchanges we can represent by fu and F0. Each experimental result and its interpre­tation implies a constraint. In the case of 21Ne the interpretation relies on a shell-model calculation, in p+a on what you might call an ab initio nuclear calculation, in 18F on the measured beta decay cal­ibration, and in iaF on a beta decay calibration plus a scaling of the uncalibrated isoscalar matrix element to the calibrated isovector matrix element. You see that *aF and p + a provide very similar con­straints, that says the experiments were done with roughly the same kind of precision and that the physics sampled by these two cases is roughly the same. You see that iaF is an odd-proton system, as is p + a, so it's plausible. One immediately notices that these one-o constraints in all four experiments don't overlap. The 21Ne con­straint (which is completely uncalibrated) appears to be anomalous. Clearly, we need more experimental results. I would like to talk about two things that haven't appeared in print. One will be a novel experiment in the nucleon-nucleon system, that will set a different constraint. The other is the mixing of 0”, 1=1 and 0+, 1=1 levels in 1‘*N, that's primarily sensitive to the Al = 0 part of the inter­action.Before I start talking about experimental details, I'd like to just make an observation. There are two ways you can do PNC decay experiments. You can look for some pseudoscalar, as I have talked about a little bit already, for instance, the circular polarization of some decay particle, it might be a proton or a gamma—ray, or you could deal in an experiment where you just measure a rate, for exam­ple, alpha-decay. The famous example here is the 2~ level of i80 to i2C plus alpha. Clearly, the rate probes the intensity of the parity admixture, while the polarization measures the amplitude of the pari­ty admixture. One might think naively that therefore polarization experiments would be greatly favoured on statistical grounds because you're looking for a very tiny number instead of the square of a very tiny number. But it turns out that, in fact, there is no difference statistically between these two types of experiment. The disad­vantage of the rate experiment is not statistics, but that we can't measure the sign of the weak matrix element.I'd like now to give you a progress report on an experiment which probes the isospin scalar parity mixing in 1I+N. It's the PhD thesis of Val Zeps. Val, Erik Swanson, Cindy Gossett and I along with54our collaborators, Willy Haeberli, Jurek Stromecki and Paul Quin, are doing the experiment at the university of Wisconsin. The situation is as follows. The 0+ and 0“ levels in 14N are unbound to proton decay, so it is not practical to look for parity violation in the gamma-decays. For a long time I believed that if you had levels that were unbound to proton decay, the physics would be so complicated that it wasn't really worth doing. In general, I think that's still true but, in special cases, and this is an example of it, the physics is still simple enough so that you can really learn the weak matrix element connecting the two levels with very minimal uncertainty from reaction theory of the decay process. With gamma-rays there is no uncertainty. In this particular case of proton decay it's very, very small. You can think of the experiment as measuring the circular polarization of the longer-lived of the two states, so it's an analo­gy to iaF, but one is measuring the circular polarization of protons instead of gamma-rays. In fact, of course, we do it the other way around, we do an experiment of the kind that Willy and Dick talked about, where you have a circularly polarized beam from an ion source, you form this level as a resonance and you look at the circular polarization dependence of the resonance cross-section.The attractive feature of this system is that the mixing is es­sentially pure isospin zero. The Wigner-Eckart theorem allows both Al = 0 and Al = 2, but, in all cases in complex nuclei, it turns out that the Al = 2 matrix element is roughly 1/A of the Al = 0 matrix element and the 1/A factor just comes from the fact that I cannot make a single particle potential for nucleons that carry two units of isospin, so there's no cooperative behaviour of all the nucleons, and that's the 1/A factor. In fact, a detailed shell model calculation that Wick Haxton has done bears this out. The reaction theory in this case is simple. It's simple for two reasons. Elastic scatter­ing is the only open channel, that means we can describe the scatter­ing by real phase-shifts. The 0+ and 0~ resonances have unique L and S, in other words, each resonance corresponds to a particular partial wave, S1/2, P1/2' It's a favourable case because the ratio of the widths of the two levels is roughly 1 0 0 :1 , so, if you look at the narrow level, the long-lived one, you can see a big effect. As a result of the big difference in widths, the two-level approximation is valid for the analyzing power. The reason is that the 0“ level exhausts a big fraction of the 2 S i / 2  sum rule and the weak matrix element is expected to be large. The expectation was ~ 1.2 eV, but we now know it's not that large. We can achieve high statistical accuracy in the experiment because elastic scattering cross-sections are very large and you expect a big effect, approximately 3x10  ^ us­ing the DDH parameters. So, to give just a little general observa­tion, if you're scattering spin-1 / 2  particles from unpolarized nuclei and you include parity conservation, the cross-section can be written as the familiar unpolarized cross-section, an analyzing power terma.n where n is the normal to the reaction plane, then there are 2  parity non-conserving terms. A helicity-dependent term, which is the king studied in p-p scattering, but there's also another term. You can think of it as a transverse analyzing power that is rotated 90 degrees in phi.55da da„ da.dft (E »Q) + dfi (E, 0 ) a.n(E,0) a.u, & = uThis is also parity-violating. It’s a difficult thing to measure in many cases and, for this case, it would not be a good thing to measure, because the only way you can tell it's parity-violating is that the asymmetry from one side of the beam to the other is off by 90 degrees and so you've got to ask how well you can establish the angle. (There are other reasons why it's not good for our case.)There's also another term that you have with a particle such as a neutron that is not electrically charged and that corresponds to the spin precessing about the momentum of the particle. For the case of charged particles the Coulomb scattering makes that not a practi­cal interesting thing, but for neutron scattering it's very interest­ing as I will come back to.The first thing to do in any of these experiments is of course to understand the character of the levels that we're studying. The 0 + resonance shows up as a sort of glitch in the elastic scattering cross-section. The 0“ resonance doesn't even show up, it's so broad, against an intense Rutherford scattering background, so the first thing we did was to measure the properties of the 0 + and 0 “ levels. The (p,y) yield from iaC, measured at 90 degrees, taken at the Seattle tandem from all the way down to 500 keV, up to a little over 2 MeV shows a big, broad bump, which is the 0“ level, and a little spike which is the 0^ level. We made a prediction for the yield, treating the resonances as poles in the S-matrix with widths and energies chosen to match the data. It works very well. These data were taken with a target whose thickness accounts for the width of the 0+ resonance. The actual width of this resonance is much smaller. The data taken with a thin target and careful attention to the resolution of the beam, reveal that the width of the narrow resonance is slightly under 4 keV. This was very good news to us because it is almost two times narrower than the value given in the compilation. Two times narrower is an advantage for us, because it means that we have a bigger amplification factor.There's a little cost. We have to run with a thinner target, in fact that's a problem of scattering experiments compared to decay experiments. In decay experiments you separate the decay process from the production process, so you can produce a state like 1BF by some strong interaction and the fact that it produces a 0 “ level whose decay is isospin-forbidden has nothing to do with the size of the cross-section for forming it. So you can form a level, whose decay is very inhibited, at no cost. In a scattering experiment, that's not true. If the way it decays is highly inhibited, so is the way it's formed and so the more inhibited the dominant amplitude you've got, the smaller cross-section you've got. That's a disadvan­tage of scattering experiments. It's one reason why there will never be enormous amplifying factors in scattering experiments.The shell model calculation that Haxton and Dubach did of the levels in 11+N look very good at the experimental level. All the56known levels up to 10 MeV are shown. There'sal to 1 correspond­ence. Every level seen has a predicted analogue and every predicted level has an experimental analogue. In studying the elastic scatter­ing of protons we found some parity assignments that weren't correct and Wick [Haxton] said, yes, I knew that already because I couldn't get the 2“ level, I predicted a 3“ level there. So this gave us a lot of optimism that the wavefunctions were under control.The optimism turns out to have been a little unwarranted. The 0level is very, very narrow. Why is it so narrow? It's so narrow,not because it decays by p-waves, but because the reduced matrix ele­ment for decay is very small. It's very small because the level looks almost purely like 2 particles in the s-d shell, 4 holes in the d-shell and this doesn't like to decay to i3 C, which is 3 holes in the p-shell primarily. And in fact the spectroscopic factor is 0.04. You can see that this is true also when you look at the gamma-ray decays of the 0"*" level (fig. 1)* There's a whopping big B(M1) to the— r— 1— r ~  ° 2+ ;i— ----------------  B(M1) * -  i 3+ ; o—   B(M1) 1 12 ;0— -----------------------------  B(M1)— *■ 1T;0Fig. 1. B(M1) values for decays from 0+2;l state of ll+N.third 1+ state, 14 ± 5 yN2. It was so big that in Ajenberg-Selove'slatest compilation she left this branch out because she thought it must be wrong. It's not wrong, it's that strong, it's so strong because it is similar to the very strong Ml in 1BF.The experimental setup looks like this. Beam from the Wisconsin tandem is focussed through quadrupole triplets to give us as symmet­rical a focus as we can get on a 25 yg/cm2 13C target viewed by 4 scintillation detectors, azimuthally symmetric around the back angles, averaging 155 degrees and 4 scintillation detectors, azimu— thally located around an angle of 35 degrees. The rest of the appa­ratus is all designed to make sure that the beam is, in so far as we're able, steady on the target in position and angle and the polar­ization vector is as longitudinal as we can make it. Because the effect is predicted to be so big, we're able to count the individual protons. The photomultipliers go to discriminators and we just count fast discriminator pulses.=  14±5 ,uN=  1.3±0.4 /i-jq — 0.11±0.04 /xn57The feedback systems work very well. We have two magnetic steerers based on fast ferrite core dipoles that are activated by a pair of slits that then feed back to a dipole to keep the beam halo centred on these slits and then there's a split Faraday cup and that drives another dipole and keeps the beam on the centre of the Faraday cup. That's suffucient to control the position and angle on the target in both x- and y-planes.In addition, the target can be moved around under the control of the computer. We take data continuously rastering the target around so we average over any nonuniformity of the target and the computer controls the spin precessor back at the ion source in a digital feedback loop to keep the spin always longitudinal on the target.Some details. The beam energy is 1.16 MeV. We're able to get about 200 nA of polarized protons on the target, the polarization is about 85%, the detectors are thin plastic scintillators. The signal we use is the difference in longitudinal analyzing power between the front and back counters. This is a useful signal because, as I will show you, the longitudinal analyzing power changes sign in the forward and backward hemispheres, and so we get a slightly bigger signal and we can reject certain false effects from this.13C(p,p)E*pb ( k e V )Fig. 2. Observables in 1 3 C(p,p)1 3 C. Left: counting rate in forwardand backward detectors; centre: predictions for the parity-violating analyzing power, the band represents the error corridor resulting from a measurement over 5 days at 200 nA; right: the parity-allowed analyzing power, Aj.Fig. 2 shows you a combination of predictions and data. The panels to the left are the counting rates in the front detectors and the back detectors. The curves are predictions and the dots are what we actually measure. The panels over to the right show the trans­verse analyzing power in units of 10“3. The centre panels are pre-58dictions based on Wick Haxton's nuclear matrix elements, the DDH best values of the weak couplings. The predictions are actually the centre of this band and the band is + 1  statistical error bar you have after running for 1 day with 1 microamp of beam, in other words, running 5 days with 200 nA.Before I show you our results, I'd like to discuss the question of systematic errors because, as you doubtless know, that's the heart of all these problems. I consider here three sources of systematic error. The first two have been referred to by previous speakers.If the polarization of the beam is not uniform over its profile, or, more generally, if the polarization is correlated either with the position of the beam on the target, or with the angle of the beam on the target. We have plotted as a function of the beam energy in the vicinity of this narrow 0 + resonance the measured sensitivity to correlation of polarization with position or angle. We measure this by using the superposition principle. We displace the transverse polarized beam and measure what the apparent parity-violating effect is. The calculation of systematic errors and it agrees very well with the measurement. This is quite impressive because it's an inte­gration over finite solid angles and energy loss in the target. The second error considered is the correlation of polarization with angle. We do this by blocking off parts of the beam with slits at certain angles. The bottom plot is the apparent Az you'd get if the energy of the beam is different in the two spin states. No parity violation, just the energy of the beam being different. That data is taken by keeping the beam in one definite helicity state and then modulating the voltage on the target by ± 50 volts and looking at the apparent Az you get from that source. So we understand these, starting from the reaction model and doing all the finite solid angle effects and so on.The fortunate thing to notice is there's a magic energy, where the sensitivities to all of these effects are very small. So, clear­ly the thing to do in an experiment like this is to take advantage of the fact that there's a magic energy at which the parity-violating effect is still approximately 80% of its maximum value, to take data here, where we're insensitive to any of the known, identifiable sources of systematic error.I've described to you how we measure the sensitivities to trans­verse polarization gradients and to energy modulation. Next I must tell you how we measure the energy and transverse polarization modu­lations of the beam so we can determine upper limits on systematic errors. We measure the energy modulation by going to a very narrow resonance. We go to the same resonance in the actual experiment, a very narrow resonance at 1.7 MeV in *3 C(p,y). The cross-section goes from one quarter of maximum to three quarters of maximum in 300 and some eV and we sit at the place where the fractional cross-section is changing most rapidly with energy, then look to see how does the (p,y) yield vary when you flip the spin from right to left handed. Because the slope is so steep here it's very sensitive. We've actually done this with Al(p,p), which is even better.We also measure the transverse polarization components in the beam, as was described earlier but, because this is a very low ener­gy, the technique has to be different, because the beam couldn't pos—59sibly go through a self-supporting little rod of carbon. So what we have is a 2 yg/cm2 carbon foil with a horizontal 150 yg/cm2 strip of carbon evaporated on it and this is mounted in a target holder. The target holder is driven by computer control and we sweep it through the beam. We have another such strip that's oriented vertically and we sweep it through the beam left-right. Typically, the polarization components aren't uniform across the beam.All Al measurementsEp lab (keV)Fig. 3. Results of measurements of AL in 1 3 C(p,p)1 3C.The results, based on three successful runs, are shown in fig 3. I've plotted the PNC signal, the difference of longitudinal analyzing powers between the front and back counters, as a function of the pro­ton energy. The magic energy is shown and that's where the data points are clustered. We also took data away from the magic energy to test experimentally that the bounds we have on these things are right. The curve at the bottom is the prediction we started with.You can see it's quite inconsistent with the data.The best fit signal has about one third of the magnitude and the opposite sign to the prediction. I have to say it's conceivable there's a problem somewhere with the sign just because the shell model uses a different phase convention than does the reaction theory on the magnitude question, though there is no doubt that there's disagreement. The weak matrix element is -0.37 ± 0.33 eV, with a systematic uncertainty of ± 0.12 eV. The theoretical prediction was 1 eV.There's really no reason to suspect the experiment. What one has to do is suspect the theory. Let me address the question briefly60of what could be wrong with the theory. We must first direct our attention to the nuclear wave-functions.I 0 +> = 6 1 0hw> + I 2tra)> + 6 ' 14frto + • • •t s ' t / ' t| 0”> = | lhw> + e | 3trto> + e 1 | 5frm> + • • •The 0+ level looks mostly like a state of 2 tio that has little pieces of 0 trw and 4 trm and so on. The 0“ level is mostly 1 irco with pieces of 3 ta and 5 -trio. The biggest single particle parity-violating matrix elements connect states differing by 1 hw (heavy arrows) . The dominant transition is clearly 0"(1 -tito) + 0+ (2 ■hm) that is of order 1. If we want to take in all the first order terms, all the terms proportional to the small components, then we have to include two things: the 0"*"(0 drm) and the 0 (3 "iToj). Thishas not yet been included in the calculation, so it's one short­coming. How well do the wavefunctions do on the gamma-ray decay^transitions? I show the transitions from the 0 + state and the 0state, expressed in Weisskopf units for El and Ml transitions com­pared to theory.Gamma Decay S (Weisskopf limits)Transition Experiment Theory°+ 1 + 0.089 0 . 0 1 1o+l * i+ 0.59 0.320 + 2 + i+ 2 7.1 6.70 + 2  * 1~. 0.069 0.042o-i - i+ ;0.17 ± 0.05 0.071Proton Decay0 + -► 1 3 C+p 0.043 ± 0.003 0.13o-i ■* 1 3 C+p "1.3" 0.78Theory looks somewhat like the data, the strong transition is strong, agrees very well, the next agrees somewhat less well and the weakest agrees only within a factor of 8 . Similarly, the proton de cay width of the narrow level is off by a factor of 3. The level is predicted to be wider than it is. In fact, if you look in detail at the shell model predictions, one sees the states have too much configuration mixing. That's what's wrong. Lastly, one has to note that a large decay width of the 0 “ level has not been included in the calculation. So this is an incomplete story, because, primarily, we have to do more work in understanding the nuclear matrix elements.How can we learn more about f^? We could try to do bet­ter, but that's almost impossible. You could try to do the asymmetry when polarized neutrons are captured by hydrogen better, but that's a difficult experiment. I'd like to now show you that there is a very interesting experiment that Heckel and Lamoreaux and Rogers and I are planning to do in the n + p system that has a lot of possibilities61for making progress. The experiments that have already been done in the n-p system all involve the np+dy reaction - the circular polariz­ation of the gamma ray and the asymmetry of the gamma. Two other experiments that you could imagine would be the parity-violating spin rotation, (the neutrons come in to a hydrogen target spinning up and when they come out they’re slightly twisted to the left or to the right) or a transmission asymmetry from longitudinally polarized neu­trons going through a hydrogen target. Now, you want to have para- hydrogen in this case (parahydrogen is the hydrogen molecule where the nuclear spins couple to zero). Cold neutrons do not have enough energy to excite the parahydrogen to orthohydrogen transition. Therefore the neutrons cannot flip their spin in the scattering, which is good and you also get a long mean free path.It turns out that the quantity A. in ftp ->■ np is hopelessly small, but the spin rotation is, I believe, accessible to experiment. We can understand why the transmission asymmetry is so small. The transmission asymmetry comes from the interference of S and P-waves in the elastic scattering. There has to be a factor, which in the decay language looks like the neutron width for P-wave divided by the neutron width for S-wave and that is a factor of /(kr)3 /kr, which is kr, and for 5 A neutrons it's a factor of 3xl0-5, which is very tiny. On the other hand, in the Lobashov type of experiment you're looking for circular polarization of the gamma ray, A^, not the neutron.How it's the square root of the El width divided by the Ml width thatmatters. The kinematic factors here are k^ 3 and k 3 in both cases, giving a factor of 1. So, you expect that JL is going to be 1•10b of A^. That, in fact, is what Avishai and Grange, who sat down and did all the hard work, predict. Avishai and Grange show that the spin rotation is sensitive to fu . That's what we want, and it's got roughly 1 0  times less sensitivity to the other parameters.So the observable is interesting, it's sensitive to f^, it's not been studied before, and the effect predicted using DDH best values, which we know are too large, are that, in a mean free path, you will get roughly 2xl0“y radians rotation. Now, the question is,how do you measure such a small angle? Before I go on, let me justshow you the status of the other experiments in the np system.Expt. DDH Predictionnp -> dy PY= (18 ± 18) X  10- 8 + 6 xl0 - 8np + dy Ay = (-4.7 ± 4.7) x 10" 8 - 5xl0- 8yd + npal = 2+, ~ ? < 1 0 - 8  a +  +  a~You see that none has enough sensitivity to test the best value predictions of DDH, which we know, in the case of f_ are 3 times too big.62Let me remind you of what we want. We want to have a neutron beam from a reactor, a polarizer to select the neutrons spinning up, let them go through a parahydrogen target and now have an analyzer to detect that the spin has rotated by a small angle, 1 0 “ ' radians.Fig. 4 shows the proposed experiment of Heckel, Lamoreaux, Rogers and myself. We have dual liquid parahydrogen targets that are connectedp o la rizer  m a g n etic  sh ie ldInto target0PNCJOut of target \/After 7i / 2  coil 0PNCFig. 4. Schematic of proposed experiment to measure the parity- violating spin rotation angle <f>pNC-by a pipe that allows us to pump the liquid back and forth from one target to the other.The neutrons come in and pass through a supermirror polarizer that polarizes them up. The beam then is split into two. One beam half travels through a vacuum all the way along, and through some coils, whose function I'll explain in a minute, the other half passes through parahydrogen which is either in the upstream or downstream chamber. The flip in the experiment is moving the hydrogen back and forth. In between the two targets the neutron beams pass through a coil that rotates the neutron spins by 180 degrees. After the second target the beams pass through a coil that rotates the spin by 90 de­grees and then go through an analyzer which selects spin up and into63two separate counters. One beam is our monitor beam, the other one is the probe beam. A comparison of what happens to the monitor beam is the signal.Some numbers which are interesting. The mean free path is about 25 cm, the ILL beam is about 5xl08 polarized neutrons/cm2 /sec, the area of the beam is about 15cm2 and the polarization is about 95%. It's sobering to realize that a cold neutron precesses at 0.2 radians/G/cm. Remember that we're talking about a PNC signal of 10_/ radians. We must ask how do magnetic fields screw things up? The most damaging thing that could happen would be that the hydrogen had a significant magnetic susceptibility, because then, if there were some ambient magnetic field around the rotation would be different, depending on whether the hydrogen were in the upstream or the down­stream chamber, and that would look like a real PNC signal. Fortu­nately, liquid parahydrogen has a susceptibility which is 1 0 -/, and you can control the B-field inside with well constructed magnetic shielding to be about 1 mG. Therefore the systematic errors from this effect are small compared to what we're looking for. After 30 days of counting we'll get a 4 sigma effect if DDH are right. OK, now they're not right but this allows you to estimate counting time. This experiment clearly has a log of promise. Running on a reactor for 30 days is certainly more pleasant than running on an accelerator for 30 days. The PNC neutron spin rotation in hydrogen, in con­junction with the very precise results for PNC in the NN system reported by Haeberli, should yield a reliable value for f^.Now for conclusions. At low energies the parity non-conserving meson exchange model seems to be quite successful. There used to be many anomalies and they've all disappeared. They were either wrong experiments or incorrect interpretation of correct experiments.There is nothing that forces you to believe in new physics of the weak interaction, such as multiple Zs, which have become a fashion­able extension of the standard model and which could show up by some very different form for the Al = 1 matrix elements. Nevertheless is quite a bit smaller than the best value. This could be a very interesting thing. It could be something similar, for example, to the Al = 1/2 rule for flavor changing decays. As for the future, we should definitely study the n-p interaction in more detail, particu­larly what we can learn about f^. It would be very nice if we could have a better theoretical understanding of what these weak amplitudes are supposed to be. Clearly, it would be interesting to study p-p scattering at higher energies, for example at the AGS. Lastly, if we want to learn anything from nuclei, parity violation being just one example, we really must have better understanding of nuclear structure physics. Because these experiments are so hard and we invest so much in them, we who do them feel this quite particular­ly strongly.64DISCUSSION:McDonald:What h° and etc. does the 11+N experiment predict?ADELBERGER:I don't want to do that because I have so much reason to worry about the nuclear wave functions that I think it would be misleading.Page:Does it land on 2 1 Ne?McDonald:I accept your answer. In p-p scattering there is a combination of AI=0 and 2; if the results of 18F and 19F were taken together, then you get a number for the AI=0 part that's about a factor of two larger than the best values, and yet the 45 MeV p-pscattering result is already smaller than that result. Is that comparable with the AI=2 part taken all out or is there any consistency there?ADELBERGER:It looks as if the AI=0 part is enhanced by a factor of two, and I expected when we did this experiment we would see a bigger signal than predicted, being naive and optimistic. But, with the p-p scattering result the consistent picture would be to say that the AI=2 part isn't what we thought it was. You have to change that a little bit, then you can still achieve the new result from SIN and the others - not 11+N.McDonald:Does the AI=2 part get so large that it's outside the range? Is it still within a reasonable range of Desplanques, Donoghue and H o l s t e i n ?ADELBERGER:They had a very small reasonable range on that one.Holstein:There's not much range. It all comes from factorization.McDonald;So there is only a number in their paper on that?Holstein:There's basically only a number. The only uncertainty has to do with finding ...McDonald:If you put those things together is there anything significant in the fact that the AI=2 part must be different from what you get from factorization?65Holstein:If you want to assume that the problem is the AI=2 part, there is a conflict, but I think there are other possible explanations of this.Mischke:On your results for 14N versus energy you have one at the magic energy and like three at the other energies. The results that you've plotted are presumably corrected for the fact you're not at the magic energy. How big are those corrections?Adelberger:No corrections are applied we took the most conservative upper limit.Bowman:Is a means of getting f^ to go to some energy like a TRIUMF or LAMPF energy, where you make pions about half the time, and try to calculate the inelastic channels? With 20 mb of pion production getting weak-strong interference, you could possibly beat the error bars down a factor of ten or something.McKellar:The difficulty with that is not only does f^ enter, but the equivalent for the ttNA vertex - which is expected to be about the same size.Bowman:I would also think you could add all the other exchanges between the two nucleons and the pion still emitted by the strong force.McKellar:You probably can separate some of that by isospin.Miller:You have to have a reliable control on the calculation of the strong amplitude. You can't calculate the strong force within a factor of three; so there's a factor of three error from the start.Henley:It doesn't isolate f^ because you can get a strong pion emission and a weak nuclear force.Simonius:In the 11+N calculation, was it an R-matrix calculation?ADELBERGER:It's a calculation with levels treated as simple poles in the S—matrix with widths having the energy dependence of ...66Simonius:Are there other measurements of the same nature?ADELBERGER:The trouble is that the 1" levels are not well understood. It's not just a question of nuclear matrix elements.Haxton:It is not possible to say which level of the shell model corresponds to the physical state.67PARITY NONCONSERVATION IN THE N N  SYSTEM: NUCLEAR STRUCTURE ISSUESW. C. Haxton University of Washington, Seattle, Washington 98195ABSTRACTI discuss some of the nuclear structure issues that arise in nuclear tests of parity nonconservation.To date we have only one definitive measurement of parity nonconservation (PNC) in the two nucleon system, the analyzing power in the p+p  system for longitu­dinally polarized protons. 1 Since matrix elements of the weak N N  interaction depend on five elementary S-P amplitudes, experimentalists have turned to few-nucleon and nuclear systems in order to obtain additional constraints. This has the somewhat regrettable consequence that the complications of the nuclear many-body problem must be surmounted in order to relate experimental observables to the underlying two-nucleon amplitudes. I would like to discuss some of the techniques that have been developed for this purpose, and point out some of their apparent limitations. A more complete discussion of these matters can be found in the review by Adelberger and Haxton. 2Modern PNC experiments concentrate on special transitions involving parity doublets, closely spaced pairs of states having identical spins but opposite parities. Some examples are shown in Fig. 1 . If the separation A E  between the doublet states is sufficiently small, the parity admixing with more distant states can be ignored. In Fig. 1 both A E  and A E ' , the next smallest energy denominator governing parity mixing, are given. The two-level mixing approximation greatly simplifies the task of the nuclear theoretician: he is only asked to provide accurate wave functions for the doublet states, and perhaps for a third state to which they may decay. Typically the doublet states are low-lying levels in well studied nuclei, so that electromagnetic decay rates, spectroscopic factors, and other information may provide checks on the nuclear wave functions.An important example is the 0+ l <-»■ 0~0 doublet at E ex ~  1 MeV in 18F, as illustrated in Fig. 1. The energy denominator, A 2? =  39 keV, is favorable. Assuming that a two state approximation is valid, the weak interaction mixes these states|0“ 0; Eex =  1.08 MeV) —> |0~0) +  e|0+ l)|0+1; E ex =  1.04 MeV) (0+1) -  e|0~0)_  (0 + l|^PAfc[0 ~ 0 ) , ,A E ' 'where V p^c  is the weak NN potential. The 0 ~ and 0+ levels decay by dipole tran­sitions to the 1 + ground state of 18F. Because the A l  =  0  E l decay of the 0“  level is isospin forbidden, this state has a comparatively long lifetime, r_ =  27.5 ±  1 . 9  ps. In contrast, the A I =  1 M l decay of the 0+ level is isospin favored and fast,r+ =  2.5 ±  0.3 fs, rendering it one of the strongest known M l transitions (10.3 ± 1 .5W.u.). Because of this large difference in lifetimes, a small admixture of the 0 "1" state68Al=0 (-2) A 1= I AI=0+l, odd n36623  1 2 2l+ 12795 2 i 22789 2 ,221 NeA E  152-206i keVA E ' 3703 keVv V /ro+sl0539keV 3134 keV|m i/ e i |= 112110 keV 5337 keVM l/E l = II5.7 keV 3662 keVI Ml/El I * 296Fig. 1. Parity-mixed doublets in light nuclei. The transitions displaying the amplified PNC effect are indicated. The quantities A E  and A E '  are the smallest and next smallest energy denominators governing the parity mixing. The quantities shown in the bottom row are “amplification factors.”into the 0 ~ level produces a sizeable circular polarization:e(gs | r r a 9 ;7 |+)P7(1081) «  2 Re39 keVH  2 T l!7 l->( + l V p i v c ] - ) ( g s i r r a9 ;7 l + )Re (2)As -  a  \{f\T?\i)\2E* the magnitude of the dipole matrix elements can be determined from the known lifetimes of the 1.04 and 1.08 MeV states, yielding1/2  /  1081 keV \ 3 / 2  1042 keV I(gS|r1m a9;7 1+)(gs|rxel;7|- )T— 111 ± 8 . (3)The factor (gs|T1ma9 ;'T|+)/{gs|riel;7|-) can be regarded as an “amplifier” of the PNC effect. The relative sign of the amplification factor and (Vpjvc) is not fixed by the 7 - decay lifetimes, but must be taken from a nuclear model calculation. (In the example69chosen here,the isospin-forbidden El transition is so suppressed that it is probably impossible to calculate its sign reliably.)To summarize, the 18F case illustrates three advantages of two-level PNC sys­tems:1. A measurement of a single PNC observable (such as P1) in conjunction with known lifetimes, energy splittings, etc., determines a well-defined matrix element of V p n c  that can be compared to the predictions of theory.2. Because only two levels are involved in the mixing, the transition “filters out” specific components of the PNC N-N interaction. In 18F, where the doublet levels have 7 =  0  and I  =  1 , P7 determines the A I  =  1 component of the PNC N-N interaction and therefore probes the weak n± exchange amplitude.3. If the members of the parity doublet have very different decay (or formation) amplitudes, the PNC observable can be enhanced significantly, making it practical to measure very small PNC matrix elements. In our example, the large amplification factor and the small energy splitting combine to produce an expected3 P7 of ~1.5 x lO-3 . This can be compared to the natural scale for PNC in the two-nucleon system, 4n G f  m * /g lNN ~  1 0 ~7, where GF is the Fermi constant and gVNN the strong pion-nucleon coupling. Thus the net enhancement in 18F is about four orders of magnitude.There are also certain disadvantages inherent in nuclear tests of PNC. One is the loss of sensitivity to certain aspects of the PNC interaction. It is probably easiest to illustrate this by reducing the two-body potential to an effective one-body potential. This one-body potential UpNC, which approximates the interaction of a valence nucleon with the nuclear core, is defined by(a \UpNC\ft) =  Y ,  (<xt\VpNc\PS -  60) (4)6 < Fwhere the sum extends over occupied core states. For a Fermi gas model and a spin-symmetric core, one obtains2u PNcil) =  2 P('G)X{ ( 1 .6  Fq + 0.4 G0) +  r3 (t)(3.7 Fr +  0.5 Px -  0.4 Gj +  0 .2  H ^ }  (5 )where p(rt) is the nuclear density, mp is the rho mass, and the various weak meson- nucleon coupling constants are defined in Table 1 in terms of the couplings of Des- planques, Donoghue, and Holstein (DDH ) . 4One sees that, in general, only two constraints can be obtained from nuclei:“ F o ” =  1 .6  T o  P O A  Go“F„” =  3.7 Fff +  0.5 Fi -  0.4 G x + 0.2 H x~ 5 %  for  DDH best  values(6)70Table I. Weak coupling constants from the “best value” and “reasonable range” re­sults of Desplanques, Donoghue & Holstein4 for the Glashow- Weinberg-Salammodel.“Coefficient Equivalent “Best value” (xlO 6) “Reasonable range” (xlO 6)F,r g ^ N  n  f  ir /  \/32 1.08 0:2.71F0 - 9 Ph°p/ 2 1.59 -1.59:4.29Fi -9phxp/ 2 0.027 0:0.053Fi ~9phl / 2 1.33 -1.06:1.54Go -gujh^/2 0.80 -2.39:4.29Gi - g ^ h l /2 0.48 0.32:0.80Hx -Qph'p / 4 0 .0“ We have taken gKNN — 13.45, gp =  2.79, and gw =  8.37.The isovector coupling aF„” is almost equal to the standard pion coupling provided the strengths of the isovector heavy meson couplings are not too different from the DDH best values. The isotensor interaction is washed out in summing over the core nucleons. One can also anticipate the sensitivity of the various PNC experiments illustrated in Fig. 1:14N A l  = 0 ,2 “Eo”1 8 p A l  == 1 “FV”19F A l  = 0 ,1 “E0 +  Fnodd proton21Ne A l  = 0,1 “F0 -  Fnodd neutronA second disadvantage has been mentioned previously, the nuclear physics uncer­tainties in evaluating (’Vp n c )■ I begin a discussion of these uncertainties by focusing on the one-body potential of Eq. (5) and introducing the density matrix Thematrix element of Up^Q between the doublet states of Fig. 1 can be writtenp{ oM MOp TM P (7)where |J+ ) and \J~) denote the states of the doublet, It =o(0 =  1 an  ^ It = i(0  = t-3 (0 , and the sum over the one-body density matrix elements extends over a complete set of single-particle quantum numbers a and (3. The notation | indicates a matrix element reduced in angular momentum and isospin.Equation (7) is exact. The nuclear theorist must find a reasonable approximation to the unknown nuclear structure coefficients xjja3- For instance, in the shell model71description of the nuclei in F ig. 1 one m ight adopt a model space consisting o f the ls- lp - 2 s ld - 2 p lf  orbits, thereby truncating  the infinite sum s in Eq. (7), and a harmonic oscillator single-particle basis. Regard less of the com plexity of the m any-body  wave functions in th is space, the m atrix  elements of the operators in Eq. (7) are given by~  M b  ^ 2(2 T  +  ^  P i/a 1* 1/2 ”  V ^ 0 2 «1/aiP l / 2  +  v/^ i d 8/2 ip8/2+  V /5V'2P l /22Sl/2 -  2^ 2p 8/2l d8/2 +  v /21^ 1/5/2l d s / 2 ) ’ (8 )where b is the oscillator param eter and tpaj3 =  rpap — ippa . Fo r the case of 1 9F, we now  consider the predictions of successively more sophisticated nuclear models for the density m atrix  elements in Eq. (8 ):• Perhaps the sim plest reasonable description of the parity  doublet in 19F  is p rovided  by the N ilsson  model. T he  l/ 2 +  and 1/2-  states are viewed as proton holes in a deformed 20Ne core, w ith  the holes hav ing  the asym ptotic quantum  numbers [N n 3 A K ] =  (220 1 /2 ] and [ 1 0 1  1/2], respectively. Then  the nonzero density m atrixelements for Eq. (8 ) are ^ s l/2i Pl/2 =  V>2 . I/2 iP l / 2  and </>r<i8 / 2 l p s / 2  =  ^ id 8/2 iPs/2.However, in the spherical lim it the [220 1/2] orbital is in the l d 5/ /2 shell. Thu s the m atrix  elements in Eq. (7) vanish. Likewise, in the lim it of large deformation they van ish  because the orbits differ by A n 3 =  2. The  m atrix  elements obtain their m ax im um  values (about 0.3 of the “single-particle value” obtained w ith  4>2sl/2iPi/2=  1 ) at a deformation 6 ~  0 .2 , while 19F  is best described5 w ith  6 ~  0.3. T hu s th is sim ple exercise dem onstrates that reasonable calculations of the parity  m ixing m atrix  element in 19F  m ust do well in describ ing nuclear deformation.• Now  consider more “realistic” wave functions, those generated by the popular Zuker-pds effective shell model H am ilton ian  for the space l p i / 2 — 25^2  — 1 ^ 5/2 - W h ile  such wave functions m ay appear to be more sophisticated than  those of the single-particle N ilsson  model, they lack essential physics. O m itt in g  the 0 id 8/2 ips/2  density m atrix  element destroys the A n 3 =  2 forbiddenness, w h ich  occurs because of an exact cancellation between V 'ids/2 ips / 2  and 4>2sl/2\Pl/2 in the asym ptotic limit.Therefore, for a well-deformed nucleus such as 1 9F, the Zuker-pds wave functions m ay overestimate m atrix  elements of the axial charge.• One m ight attempt to “cure” the deficiencies of the Zuker-pds calculation by d iagonaliz ing a shell m odel H am ilton ian  in a full 07kj +  1 huj space. A  num ber of calculations of P N C  m atrix  elements have adopted th is basis . 6 , 7  Unfortunately, while the effects of nuclear deform ation m ight be well described in this approach, two other deficiencies arise.T he  first has to do w ith the effects of the pa iring force. A x ia l charge m u l­tipole operators and the more fam iliar transverse electric m ultipoles are both  odd under particle-hole conjugation. The  pa iring  force suppresses m atrix  elements of odd operators . 8 In  the density m atrix  form alism , the pa iring suppression  is embodied in the coefficients xjjpa that, hav ing  the same sign  as rjjap, generate cancellations inV’op =  ^a/3 -  0/3a- Unfortunately, in the shell m odel the tjjpa enter only in verylarge-basis calculations. For instance, ipiPl/22s1/2 is produced on ly  if 2hu “pairingexcitations” are included in the l / 2 + wave function (corresponding to the prom otion o f two p-shell nucleons to the 2 s Id  shell). Note that, despite other shortcom ings, som e correlations of th is sort w ould  be included in the Zuker-pds calculation. Be­cause of the sim ilar behavior of the axial charge and electric m ultipole  operatorsunder particle-hole conjugation, predictions of low -lying E l  transition  strengths are an im portant test of the pa iring properties of wave functions used in P N C  calcula­tions.The  second deficiency is due to sensitiv ity  of the axial-charge operator, v ia  the derivative p( i ) ,  to the shape of the nuclear surface. The  harm onic oscillator radial wave functions fall off too sharp ly  at the surface. In  the shell m odel the nuclear surface can be “softened” by includ ing the 2 hw l p l h  excitations that generate thecoefficients ^ i p 1/2i8l/2' ^ 2Pi/2 2 si/2 ’ e^C- ne  ^ e ^ e c ^9 1S f a th e r  suppress them atrix  elements of the axial charge operator. In  effect, such excitations redefine the single particle basis. A lternatively, one can adopt a more realistic single-particle basis at the outset. M illener and W a rb u rto n 10 have show n that calculations using W ood s-Saxon  wave functions often yield m atrix  elements of the axial charge operator that are sign ificantly sm aller than the corresponding results for harm onic oscillator wave functions.It is apparent that the m in im um  realistic calculation of parity  m ix ing  in nu ­clei around  160  requires a full 2ku basis. T h is  is generally very difficult, thoughcalculations in 14N  and 18F  have been p e r f o r m e d .9 -11Because of the uncertainties associated w ith nuclear m atrix  elements, a rela­tionsh ip  between parity -m ix ing  of “two-level” system s and first forb idden (3 decay has become quite im portant. The  relationship is illustrated in F ig. 2 for the two practical cases, the /? decay transitions 1 8N (0 + 1) —> 1 8 F (0 ^ 0 )  and 1 9 N e ( l/ 2 ~  1/2) ->■ 1 9 F ( l/ 2 ~ l/ 2 ) .  Note that these transitions connect the isosp in  analog of one state in the parity  doublet w ith  the other state.72Fig. 2. F irst forb idden /?-decay analogues of the parity m ix ing  in 18F  and 1 9F. The ratios u th/ u exp refer to the 0 +  lfiw — K u o  calculation of Ref. 2.73In  general, five independent operators contribute to first-forbidden /? decay. However, for an odd-parity  J  =  0 —> J  =  0 transition  [such as 1 8 N e (0+ ) —+ 1 8 F (0 - )], sp in  and  parity  selection rules elim inate all but two of these12,where is the axial vector charge-changing hadron ic current and q is the three- m om entum  transfer from  the nucleus. The  second operator in Eq. (9) vanishes in the long wavelength limit. Since the m om entum  transfer in the 18Ne decay is small (qrl «  0 .0 2 ), the decay rate is determ ined p rim arily  by  |(0 ~ |Mo0 |0 + )|2.T he  dom inant M q 0 operator contains, in principle, one- through  A -b o d y  axial currents. The  m ost im portant of these are the one- and tw o-body currents. The one-body contribution, M q 0 ( 1 ) ,  is obtained from  the nonrelativistic reduction of the axial current for a free nucleon. To 0 (1/M ) one find s12Jow = F *  E  r±(')i—\o{i)  • Si] +  8 \ x -  S i ] ~ l  ■ o(i)2  M 2 M— ii=i(10)where H  is the nuclear H am ilton ian  and F a  — -1.25. We use partia l conservation of the axial vector current ( P C A C )  to evaluate the pseudoscalar coupling constant, F p =  2 M F A / m \ .  The  first term  in Equation  (1 0 ), a lthough forbidden in the sense that |p ( i ) / M  & 1/5, is finite as q —> 0, while the second term does not contribute to M q 0 in the long wavelength limit.N  7r~ 7rlJ ------- i> 6 ?)--------P N C0N  N' N  N'(a) (b)Fig. 3. Schem atic representation of the relation between semileptonic axial charge and isovector pa rity -m ix ing  operators.74T he  tw o-body contribution  to the axial charge operator, M$0(2), is dom inated by p ion  exchange (see F igu re  3) and can be evaluated from  a low-energy theorem based on P C A C  and current a lgebra . 1 3 ’ 14 The  result, to leading order in (1/M ), is g iven entirely by the seagull termJ 5± \ y ^ [ f ( i )  x T(j)]±[o(i) • fij6\x—  Xj]°(2 ) 87tM 2F a  2 Z ^ 1 V 1 J*76ja ( j )  ■ f i jS\x -  Xi ] ]^[mnrij ] ,  (H )where f tJ =  r l — f j , [ ]±  =  ([ ]i ±  t[ ]2 )/2, (j>{x) =  (e I / x ) ( l  +  1 /x), and F x — 1. T he  one-pion exchange current p lays an im portant role in axial charge transitions because it is of the same order (\p\/M «  v/c) as the one-body contribution.A s  in the earlier example of V p jv c ? °ne can reduce M q0 (2) to an effective one- body  operator by averaging over core nucleons. A  sim ple resu lt2 ’9 emerges: the ex­change currents effectively renorm alize M<f0 ( 1) by  a m ultip licative  factor ~  1.7. Th is  suggests that, in more sophisticated tw o-body calculations, the ratio of ( M q0(2)) to (M o o ( l) )  ought to be approx im ately  constant. In  fact, a lthough  various calculations predict values of (M $ 0( 1)) that differ by  as m uch as a factor of four, this ratio is con­stant to better than 10% . The  significance of this is due to an observation9 ’ 15 made independently by Bennett, Low ry, and K rien  and by me: the pion-exchange contri­bution  to Vpjvc, w hich  is expected to dom inate A l  =  1 parity  m ixing, is identical to M q 0 (2), apart from  overall coup ling constants and an isosp in  rotation. Since the m easured 18Ne /?-decay rate2 ’3 ’ 16 determines, effectively, |(M |0 ( l)  +  M £ 0 (2 ))|2, one can derive \{VP N C }\ from  calculated values of < A ^ 0 (2) )/ ( ^ 0 0 f1)). a quantity  that is given reliably by theory. T he  net result is the prediction(0~0|Fp iVC|0+ 1) =  (0.324 ±  0 . 0 5 3 ) M e V|P1 (’8 F)| = (2.0±0.5).10-3 ( i^ 5r) (1 2 )where F ® DH is the best value of Donoghue, Desplanques, and Holstein. The  m atrix element used in (1 2 ) is s im ilar to that found in a full 2 hu  shell model ca lculation . 9 F rom  the experim ental resu lt17P1 =  (0.17 ±0.58) • 10- 3one then obtains F n <  0 . 5  F ^ ® ^ , a som ewhat su rp ris ing  result. Adelberger and I considered the im plications of this result together w ith  the constraints on P N C  from A lLot{p +  p ) 1, A L { p + 4 H e )18, A L {p +  d )19, ( 1 9F ) 20, and P 7 (2 1 N e )21. F rom  theseobservables, the only evidence for a nonzero isovector com ponent in V PNC  depends on the shell m odel pred iction6 ’ 7 that the isoscalar m atrix  element in 21Ne is not small. A s  the existing calculations do poorly  in reproducing relevant E l  m atrix  elements in 2 1 Ne, it is difficult to have confidence in this prediction. In  contrast, the isoscalar com ponent of V P n c  is somew’hat stronger than, but in reasonable agreement with, the D D H  best value prediction. Perhaps the isosp in dependence of V PNC  w ill prove75as su rp ris ing  as the A I  = 1 / 2  rule in A S  =  1 interactions. One should  recall that the principa l m otivation for study ing  P N C  in the N N  and nuclear system s was to m easure the neutral current enhancement of the A I  =  1 com ponent of V p ^ c -C learly  the 18F  results need to be cross checked. W hen  the 14N  experim ent22 is completed, the results could be com bined w ith those from  A 7 ( 1 9 F ) to yield a second value for the A I  =  1 com ponent of Vpjvc- Alternatively, a considerably more precise m easurem ent of in n +  p —> d +  7  could provide a sim ilar constraint. There is also theoretical w ork underw ay23 to determ ine whether the nuclear anapole moment (the axial coupling of the photon to the nucleus) can be used as a new probe of nuclear P N C :  in cases where a ground  state opposite parity  doublet exists, nuclear wave function adm ix ing  due to V p jvc  should  determine the strength of the anapole moment. T he  anapole m oment could be measured in electron scattering or atomic P N C  experim ents w ith  polarized nuclei. T h u s  we can look forward to uncovering more pieces o f the P N C  puzzle in the next few years.R E F E R E N C E S1. M .  S im on ius, in Intersections between Particle and Nuclear Physics, A IP  Conf. Proc. No. 150 (New York, 1986), p. 185; R . Balzer et al., Phys. Rev. C  30, 1409 (1984); J .M . Potter et al., Phys. Rev. Lett. 33, 1307 (1974); P. von  Rossen  et al., 5th Int. Sym p. on Po larization  Phenom ena in Nuclear Physics, A IP  Conf. Proc. 69 (Santa  Fe, 1981), p. 1442; D .M . Tanner et al., Proc. Int. Conf. on Nucl. Phys. (Florence, T ipografia  Com positori, 1983), p. 697.2 . E .G . Adelberger and W .C . Haxton, Ann. Rev. Nucl. Part. Sci. 35, 501 (1985).3. E .G . Adelberger et al., Phys. Rev. C  27, 2833 (1983).4. B. Desplanques, J.F. Donoghue, and B .R . Holstein, Ann. Phys. 124, 449 (1980).5. W .C . Haxton, Ph .D . thesis, Stanford  Un iv. (1975).6 . W .C . Haxton, B .F . G ibson, and E .M .  Henley, Phys. Rev. Lett. 45, 1677 (1980).7. B .A . B row n, W .A . R ichter, and N.S. Godw in, Phys. Rev. Lett. 45, 1681 (1980).8 . A. B o h r  and B .R . M ottelson, in Nuclear Structure I I  (New York, Benjam in, 1975), p .  650.9. W .C . Haxton, Phys. Rev. Lett. 46, 698 (1981).1 0 . D .J. M illener and E .K .  W arburton, in Nuclear Shell Models, ed. M .  Vallieresand B .H . W ildenthal (W orld  Scientific, Singapore, 1985), p. 365.1 1 . W .C . Haxton, unpublished.1 2 . J.D . Walecka, in Muon Physics, ed. V .W . H ughes and C.S. W u, Vol. 2 (New York, Academ ic, 1975), p. 113.13. K . Kubodera , J. Delorme, and M .  Rho, Phys. Rev. Lett. 40, 755 (1978).14. P .A .M . Guichon, M . Griffon, and C. Sam our, Phys. Lett. B74, 15 (1978).15. C. Bennett, M .M .  Lowry, and K . K rien , Bull. Am . Phys. Soc. 25, 486 (1980).16. W .W . Daehnick, Phys. Rev. C  25, 2957 (1982).17. H .C . E va n s  et al., Phys. Rev. Lett. 55, 791 (1985); M .  B in i, T .F . Fazzini, G. Poggi, and N. Taccetti, Phys. Rev. Lett. 55, 795 (1985); C .A . Barnes et al.,Phys. Rev. Lett. 40, 840 (1978); P.G. B izetti et al., Lett. N uovo  C im ento 29,167 (1980); G. Ah rens et al., Nucl. Phys. A390, 486 (1982).18. J. La n g  et al., Phys. Rev. Lett. 54, 170 (1985).19. D .E . Nagle  et al., Proc. 3rd Int. Sym p. on H igh  Ene rgy  Physic s w ith  PolarizedBeam s and  Polarized Targets, A I P  Conf. Proc. No. 51 (New York, 1979), p. 24.2 0 . K . E lsener et al., Phys. Rev. Lett. 52, 1476 (1984); E .G . Adelberger et al.,Phys. Rev. C  27, 2833 (1983).7621. K .A .  Snover et al., Phys. Rev. Lett. 41, 145 (1978); E .D . Earle  et al., Nucl. Phys. A396, 221c (1983).22. E .G . Adelberger and V . Zeps, private com m unications.23. W .C . H axton  and  E .M . Henley, unpublished.77DISCUSSION:McDonald:There is another nucleus about which one might be able to get information, 1 9 Ne, which has an analogous ground state and first excited state to 1 9 F. It seems as though if one had parity violation information both in 19F and 19Ne then it could be combined to extract the purely isovector part of the parity violating matrix element with little uncertainty. Basically the isospin impurity is the only uncertainty there. That then is the part which is calibrated by beta decay of 1 9 Ne. Is that correct?HAXTON:Yes.McDonald:So that is another experiment which when combined with 19F could give basically the same information as 1 8 F. In the same light, if one looks at the 21Ne and 19F data as plotted on that graph that [Eric] Adelberger showed earlier today, basically that which leads to the contradiction is the fact that they form a cross, which you referred to earlier in your talk as the fact that one is odd proton and the other is odd neutron. My question is whether you could determine the relative slopes of those two, such that they could be combined to just get an isovector part for example?HAXTON:Yes. What you're saying is that the unknown quantity is the isoscalar element of 1 9 F. There simply is no check on that matrixelement. I don't know where it appears in nuclear physics apartfrom, say, neutrino neutral current scattering on the nucleus. You just can't test that matrix element, it has to be a calculated quantity. That's the thing that changes the slope of that line, and you could rotate it toward the vertical by turning the isoscalarpart off, but you want to do it to fit the rest of the data.McDonald:Is it possible by applying your shell model and asking a more restricted question to be more accurate about the relative slopes of the *9F and 2*Ne results?HAXTON:The only information we have is 1 9 F. We can do all these calculations, one of which we think might be relatively realistic, all the others of which we think are definitely wrong. We can ask in that set of calculations, how does this ratio of matrix elements behave? The only statement we know how to make is that they tend to scale with one another. You turn off the isovector in order to test beta decay, you also turn off the isoscalar in about the same proportion. Actually, that was an assumption we made that appears in that graph of Adelberger's , that we do divide by a special scale factor which is one-third of that needed in a naive calculation to reproduce the beta decay results. And it is globally put in 1 8 F,7819F and 2 1 Ne. But it's not at all clear to me that the physics is exactly the same in those nuclei.Adelberger:It is also apparently true in ll+N.HAXTON:It is a global factor, so you can multiply the theory by a third. Bowman:But can you take the sum or difference in mirror nuclei to cancel things out?HAXTON:All you get is the product of a coupling times the matrix element. Bowman:But you get to see if the f^ is consistent.HAXTON:Yes, you can always get at the f^.Bowman:Well, that would be something.HAXTON:The 18F has already done that. You can certainly use the mirror nuclei to get exactly the same test; you get out the f^. But the fQ, the other linear combination you can extract, is always fQ times some unknown matrix element that we don't know how to get from e x p e r i m e n t .Adelberger:The trouble is subtracting two experimental results to get one quantity is a tough way to make a living.Bowman:You take the calculation and normalize to f^ and there is an inconsistency. So if you could take some experimental numbers and subtract them and say "Ah, at least I know that f^ is okay", that would tell you what the problem was, now we are speculating about what the problem is.HAXTON:It could be even worse, it could be that not even 18F is right.Gai:I have one comment and one question. First, expanding on your comment about the similarity of calculating the El matrix element, the PNC matrix element, I would like to comment that in 180 some 4579out of 50 possible, in fact 90% of the possible El matrix elements are all measured and they all range from some 10- 7  to 10- 2  Weisskopf units and that makes 1 8 0  a good case to do such a measurement; in fact there is a 0“ state in 180 very similar to 1 8 F, not the same isospin, but similar in shell model structure to 1 8 F. My question is: The Russian school in or around Dubna claim to get theories,which are based on QCD, which get f^ a factor of three smaller than the DDH value. They also make the statement that perhaps the hard pion will destroy the analogy with the first forbidden beta decay. Should we take this seriously?HAXTON:The diagram that was actually evaluated is one pion exchange. It happens that as f^ approaches zero that the one pion exchange diagram gives the low energy theorem. So essentially there is a low energy theorem here, that the pion is essentially a hard pion, the pion has a mass with the off—shell effects treated in some way.There is a lot of literature looking at the coupling say, between the proton coming in and the pion coming out of the nucleon and expanding that vertex and try and figure what happens when you put in heavy meson exchanges and so forth. Because of this exchange current connection, axial charge operators are interesting operators in beta decay and muon capture. There is a lot of literature actually on 1 6 0 , about partial muon capture rates, beta decay rates, looking at the relative sizes of the exchange currents. I haven't studied the article in detail, but I know the kinds of effect people talk about in previous work is 15-20%. In addition, in that paper there is at one point use of better wave functions, in the sense of Wood-Saxon types of wave-functions, at the end they multiply by the canonical factor of one-third. But that's double counting, because of that factor of one-third; at least half of that represents the effect of going to better wave functions and things of that sort. I don't want to get into the details, I haven't had a chance to study it.Falk:What's the nuclear structure situation like in 1 0 B?HAXTON:Well, the fewer nucleons you have the better off you are. Below 14N things are relatively simple. I am certain that one could do a calculation of the energy levels of 1 0 B, get the first fifteen levels right and get most of the gamma decay right. The question is what does it do for the axial charge? Maybe llfN is special because one of the states that mixes has a special 2 hco character, this may be a bizarre case, so, if you can do a measurement in *8B I encourage you. If you have to do an absolute theoretical calculation of the mixing matrix element there is nothing better than a nucleus with ten nucleons.80WEAK NNM COUPLINGS AND NUCLEAR PARITY VIOLATION*Barry R. Holstein Department of Physics and Astronomy University of Massachusetts, Amherst, MA 01003ABSTRACTAfter many years of careful theoretical and experimental study of nuclear parity violation, rough empirical values for weak parity violat­ing nucleon-nucleon-meson vertices have been deduced. We address some of the physics which has been learned from this effort and show that it has implications for work going on outside this field.I. INTRODUCTIONThe study of nuclear parity violation has been pursued now for nearly three decades.! Despite vigorous effort on both experimental and theoretical fronts considerable challenges remain to be overcome be­fore a real understanding of the nonleptonic AS=0 parity violating in­teraction can be said to exist. Nevertheless, progress has been made to the stage where one can say that there is interesting and significant physics which has been gained by careful analysis of such effects and we shall attempt to document this claim below. This is encouraging news in that over a decade ago, the characterization of the weak neutral cur­rent, which originally propelled work in the area of nuclear parity vio­lation, was accomplished via alternative techniques, and with a preci­sion to which the nuclear work can never realistically aspire.The canonical approach to the theoretical analysis of parity vio­lating nuclear effects is based upon the single meson exchange picture. Thus, just as the usual, parity conserving, nucleon-nucleon potential can be represented reasonably accurately in terms of a sum of exchange diagrams associated with all light m e s o n s . 2n, p» 4*» o . . . ,one assumes that the parity violating nucleon-nucleon interaction can be represented similarly, but with two important differences. Firstly, one nucleon-nucleon-meson vertex is strong and parity conserving, with the other being weak and parity violating. Secondly, because of the stric­tures of hermiticity and CP invariance, the exchange of neutral spin- less mesonsis f o r b i d d e n .  ^ (Two pion exchange effects must also be considered, of course, but it has been shown that such contributions are small at least in the dominant 1 = 0 , 2  channels, where they lead only to slight modifica­tions in the shape of the p exchange potentials.^)The strong NNM vertices can be represented asResearch supported in part by the National Science Foundation81p ' y 2M y v(1)+ g/ ^ y + ^ yvv C Nwith the couplings g , etc. taken from experiment and the vectordominance hypotheses. Thus we can use2 2 2 S-rrNN g n 1 g (ii- S T -  1 4 '5  • ° ' 6 2  < 2 >Xv»Xs being identified with the iso-vector, -scalar nucleon anom­alous magnetic moments.The weak NNM vertices may similarly be categorized as- 1/2 _H = (2 )~ f ^N(t x N3 „ (3t3 c()P - t*(£ P)+  N [hp°T V  +  h p \ P + % 2  ^ - 1 7 2 ^ 1  A N2 (6)+ N[h °(j) ^ + h T^^ d) ^Iy^y^N (3 )a) ry u) Ty J ' ra1 _ -> -v p 3 ^UVk- h 'p', ( T X V ) - a r Vwhere here the subscript, superscript identifies the meson, isospin car­ried by the particular coupling form - e.g., hp° (1 = 0 ), h^ 1 (1 = 1 ), etc. - except for f^ which is purely 1=1 and needs no such designation. These weak parity violating couplings can be, with considerable uncertainty, deduced from experimental results and compared with various theoretical attempts to predict their size, as will be described below. The nonrel- ativistic parity violating NN potential which results from combining eqns. 1 and 3 then has the formT7p.v. _ f7r8TTNN . ,T1 * T2,3/+ , -* N rpl “ p2 „ , ,,12 2 l/2 l( 2 } (ai + a 2),[ 2M ’ 7Te (h °t -t + h 1 (-T1 + T 2 - , 3 , u 2 ( 3 t 1  T 2 ~ Ti‘T2 ) ,Sp(hp T1 X2 +  hp ( 2 }  +  h p 2 ( 6 ) 1 / 2X ((a'l- 5 2 )‘f^ ^ i ' fp (r)}+l(1 + X,)S1 x J 2- [ \ ^  ,£p(r)]-+-*■ 3 ,uo , , 1  , T 1 + t 2 , ,“ su (hW + hu (--- 2 >82x ((CT1 o2).{ 2M , fa)(r) > + x (1 + Xs^ai x a 2’ ^ 2M ’V r)]-*■ 31 i — T„ p ,  Pp(g<A) ' Sphp )( (CTl + a 2)*{-7 M ~  ’fp(r)}T XT, J p -p„' gphp i( 2 } (ai + a 2 )*[— 2 M —  ’fp(r)] 5 (4a)where-m rfTr(r) = h t r  ’-m re Pf (r) = f (r) = 6(4b)pv ' tov ' 4TTrwhich can be utilized, together with nuclear wavefunction information, in order to predict the size of parity violating effects in particular reactions.My purpose here, however, is not to review this connection between theory and experiment. Indeed it has been eloquently summarized pre­viously . 5 The results of such analyses reveal thati) From measurements of asymmetries in longitudinally polar­ized pp scattering^ and in the decay of 110 KeV, 1 /2 level of 7 the existence of parity violation in the 1=0 channelseem definitely established with an inferred valuedh ° n  ° ^ - 15 g (5)u> p P ofor the corresponding weak couplings. (Here gp = 3.8 x 10 represents a canonical weak interaction scale.) ii) From the recent measurement of the absence of circular polariza­tion in the radiative decay of the 1.081 MeV, 0 state of F,one finds an upper boundf < 3g (6 )7T pfor the weak pion-nucleon coupling, which is expected to domin­ate the 1 = 1  channel.These values do not, of course, give an ideal fit to all current experimental results in this field. In particular, with a value of f^ this small it is difficult to understand the very tiny experimental upper bound for circular polarization in the radiative decay of the 2.789 MeV, l+/2 level of 2lNe.9 However, shell model calculations of this nu­cleus are particularly complex. Also, unlike lgF and l^F there does not exist for this case an analog beta decay with which to normalize the shell model calculation.10 If we thus neglect results in the 2lNe system83as being theoretically unreliable, the remaining low energy experiments in light nuclei can be reasonably represented in terms of the above values for h^0 , hp°, f^ as shown by Adelberger and Haxton.5 Thus we shall ask below in section II what these empirically determined couplings teach us about the underlying physics, and will summarize our conclusions in a final section III.II. ANALYSIS OF WEAK NNM COUPLINGSAs outlined above, a fundamental challenge which theorists face is in the calculation of the weak nonleptonic parity violating NNM couplings hp°, f^ p, etc. defined in eqn. 3. However, before reviewing the current state of the art, it is useful in this regard to put this work into a proper perspective.A seminal contribution to this field was the paper of F. Curtis Michel-*--*- who utilized the so-called "factorization" approximation, wherein one attempts to saturate matrix elements of currents by including only their vacuum to single particular values. Thus, for example, one approx­imates- | l-i2 .l+i2 „l+i2 .l-i2. i <7T Pl(Vy y y Ay > ln>* <1T_ I Ay_ i 2  I °><P I Vp+ i 2  I n>- | , l-i2 .l+i2 l+i2 .l-i2 . i <p p (V A + V A ) l n>y y y y(7)0 ><p|Aj+l2|n>The justification for such a brutal truncation is basically that at the time (1964)i) only rough estimates were neededii) alternative techniques for evaluation of such complicated matrix elements were unavailable.The 1960's, of course, was the heyday of group theory, with the dominant idea that what was fundamental was the group structure. Quarks were only imaginary tools which made the group calculations easier. By the 1970's this situation had become completely reversed - quarks were the fundamental entities and the group structure followed from their exis­tence. The existence of this quark substructure had important ramifica­tions for the factorization techniques. The point is that according to the original factorization prescription a matrix element for emission of a neutral meson via a charged current interaction as in eqn. 7 would have to vanish since, oI,l-i2 i<TT IA^ 1 0 > = o (8 )However, if the currents are in fact constructed from quarks, they are subject to the so-called Fierz identity84uY]J(l+Y5)d dYy (l+Y5)u = dYy (l+Y5>d u ^ C l + Y 5>u (9)which results from completeness of an expansion in terms of combinations of Dirac matrices. This is simply a mathematical statement but it has the important physical ramification that emission of neutral mesons from a product of charged currents must now be permitted. (Of course, inclu­sion of color degrees of freedom implies that such "Fierzed" emission is color suppressed, but this is easily accounted for.) This inclusion of the vacuum insertion in all possible ways came to be called the "modified factorization" technique-^ and was the first real impact of the quark model in this area.That more than just factorization or even modified factorization was required, however, was indicated by a persistent sign discrepancy - the calculated sign of parity violating 1 = 0  effects seemed to be consis­tently opposite to that found experimentally even though the order of magnitude was usually correctly predicted. The key step in the resolu­tion of this problem was taken by McKellar and Pick.13 Using group theoretical techniques - specifically the group SU(6 )y - they were able, in the case of the charged current weak Hamiltonian, to relate pieces of the desired AS=0 weak NNp, NNoo couplings to experimental values meas­ured in AS=1 nonleptonic hyperon decays, e.g. A -*■ Ntt, H -*■ Att, etc. Such terms were larger than the previously calculated factorization terms and seemed to be able to resolve the above mentioned sign discrepancy. How­ever, the group theoretical approach was limited in thati) there existed contributions to the NNM vertices which could not be related to analogous AS=1 amplitudes,ii) the treatment of the neutral current - Z° exchange - contri­butions was difficult to include completely.Again the quark model offered new insights.1  ^ In fact, when one con­siders the weak nonleptonic matrix elements from the point of view of the quark model there exist only three nonvanishing topologies, as shown in Figure 1. The first two - Fig. lc, lb - are in fact completely equiv­alent to terms identified by McKellar and Pick which can be related toFigure 1: Quark diagram topologies which contribute to weak NNM vertices(o) (b)(c)85analogous AS=1 B-*-Btt amplitudes. The graph shown in Figure la, which cannot in the group theoretical analysis be so-related, can in fact be identified with the factorization diagrams previously discussed. These two contributions - lc, lb and la - have opposite signs, but the factor­ization terms are somewhat weaker. Then the sign anomaly is explained. Finally these quark techniques are equally applicable to W- and Z-exchange so that a complete and consistent calculation is in principle possible.The quark picture thus allows for significant progress to be made in such calculations but is in no way a panacea. In fact even in 1987 evaluation of hadronic matrix elements of 4-quark operators is beset with enormous difficulties and uncertainties.15 Nevertheless, it has been possible to learn significant physics in this regard.Consider first, as a warmup, the related AS=1 hyperon decay system.In the parity violating (S-wave) sector there exists no significant fac­torization (Figure la) contribution, since<TT | | 0><B' | Vy | B> = F7TqV‘<B' | vy I B> ^ F^CB' | 3yV | B> (10)which vanishes identically in the SU(3) - mu =m<i =ms - limit, and is in general small in the real - broken SU(3) - world. The dominant diagram - Figure lb - is equivalent to the familiar commutator term which arises in the usual current algebra - PCAC analysis of such processes. The re­maining term -_Figure lc - is less familiar and corresponds to a non­valence - qqqqq - component of the baryon wavefunction. In fact, we learn that inclusion of such a quark sea contribution is required in order to produce a good fit in this sector, as shown in Table I.Table I. S-Wave Hyperon Decay AmplitudesMode Expt. X 1 0 ? Valence Fit x 10? Quark Sea Fit x 10?A_° -3.27 + 0 . 0 2 -3.27 (Input) -3.27 (Input)> oo 2.40 + 0.04 2.36 2 .36►7 -4.52 + 0.04 -6 . 6 8 -4. 54Oo[I] 3.40 + 0.06 4.72 3. 2 2L_" 4.29 + 0 . 0 2 8.17 4. 51£++ 0.13 + 0.04 0 03.29 + 0 . 1 0 5.78 3.29 (Input)Moving to the AS=0 vector meson emission amplitudes, we find that the factorization diagrams — Figure la — now are an important component of the weak NNp, NNu) vertices. However, as mentioned above in the non- relativistic quark model limit, the remaining two pieces - Figure lc, lb - can be evaluated exactly (in the SU(3) symmetry limit) in terms of experimental S-wave non-leptonic hyperon decay amplitudes and are found to be even larger than the factorization contributions. In the "real world", of course, there are two important modifications of this simple picture which must be accounted for86i) the effective AS=0 weak Hamiltonian must include both W- and Z-exchange contributions and must be corrected for hard (short distance) gluon corrections via renormalization group techniques, andii) the relativistic nature of the quark motion must be included, which tends to reduce the contribution of the non-factorizable terms.There exist considerable uncertainties associated with both these aspects. However, Table II lists typical results of such a calculation. We see then that strong interaction effects and relativistic corrections to theTable II. Calculated Values for 1=0 Vector Meson CouplingsK=1FactorizationK=4NonrelativisticK=4RelativisticV 30gp-80 gp -30 gph 0 15g -27 gn - 5 gn(A) P P Pquark wavefunctions are both important contributions and are required for a realistic calculation.Moving finally to the AS=0 NNir vertex, one finds three separate con­tributions to this coupling which must be includedl6i) a piece which can be related via PCAC and SU(3) symmetry to a linear combination of AS=1 hyperon decay amplitudes, yielding(1 -2 sin20 ) 0.48 -0.24' V 1  +  T T - ^  <V 4 8 ^ - 0 . 2 4 >’ (11>r s m  0 K + Kcwhere 8 ,0 is the Weinberg, Cabibbo angle and w c.2. M 2 a (y)v — 1 4 - g ^  ) v.0n W. - = _J?____  (1 2 )K - 1 + 2 b*n 2 a (M ) K )16tt y s wis the usual strong interaction renormalization group enhancementfactor.17ii) a piece involving the product of left- and right-handed currents which can be Fierz-transformed into a product of scalar and pseudoscalar densities and thereby evaluated via factorization, yieldingF  m 2 Zf P = ■§■ sin 0 F(K) x G — ^ —  (13)IT 9 w m +m,u dwhere87F(K) = 1.04K0,85 - 0.19K0'43 - 0.09K °'13+0.24K 0 , 3 5  , (14)Z = <p|ud|n> (15)is a renormalization constant which is unity in a nonrelativistic quark model and 't 0.5 in the MIT bag model with massless quark, and mu ,m^ are the "current" quark masses which appear in the Lagrangian.iii) a piece involving the product of left-handed currents which can­not be related to the hyperon decay amplitudes by a sum rule, but which can be evaluated in a quark modelQ 2 4  S(A_)f = sin 0  r]E(K)  —  (16)w 3^r sin9c COS0Cwhere S(A°) is the S-wave amplitude for A->pTT ,E(K) =-0.33K° ‘ 8 5  + 0.03K°"43 +1.61K~°'13 - 0.31K_°'35 , (17)and ri is a ratio of two reduced matrix elements which is unity in a nonrelativistic quark model and ^ 0 . 7  in the MIT bag model with massless quarks.In Table III, we list estimates of these three contributions for reasonable values of the strong interaction parameter K. ComparisonTable III. Calculated Values of the AS=0 NNir CouplingK=3 K=44tS 4 - 8  Sp 5 - 7 8 p£ /  6 - 3  Z§p  S - 1  ZSpfpQ l - 9 %  ° - 6  rigpwith the experimental result f^ < 3gp reveals a significant challenge to our theoretical evaluation of f^, in that all three components - f^S, f^P, f^Q - are expected to have positive signs (i.e., no significant cancella­tion is anticipated) and both f-^ E ancj are individually larger thanthis upper bound (here the latter is evaluated in the nonrelativistic quark model, Z=l, and with the "Weinberg" mass values m ^ m d  = 4.3, 7.5 MeV respectively.18)For f-n-S it has been noted that it is likely that the sum rule may bea considerable overestimate in that the linear combination of hyperondecay amplitudes which is involved in said sum rule tends to cancel and thus be significantly affected by SU(3) and/or soft pion uncertainties.12 However, this expectation is difficult to quantify.In the case of f^P, the violation of the experimental upper bound88reveals that a relativistic quark model such as the MIT bag is required. Use of such a model reduces the nonrelativistic quark model prediction by a factor 't 1/4. This is because in Weinberg's original derivation of the quark mass values,18 what is determined are actually the valuesm* = Zm =4.2 MeV u um* = Zm = 7.5 MeV (18)d dm* = Zm = 150 MeV s si.e. the masses times the scalar density matrix element between baryons. The value of f^P becomesf P ~ 8 Z2 as 2 (19)7Twhich is then consistent with the upper bound given in eqn. 6 .Thus the analysis of nuclear parity violation has yielded interest­ing physics. The particular aspect which has the broadest implications outside this area, is that of the current algebra quark mass values.These values are crucial to factorization calculations which are utilized in other applications, e.g. CP violation, the Al = 1/2 rule etc. and tend to reduce some estimates for these processes which exist in the liter­ature.Thus, for example, in the case of the KM model of CP violation, the dominant CP violating interaction arises from the so-called "penguin"diagram,19Le f  £ ‘  1 ^  5 V 1 + Y5)Xa<i S i V Sq <20)agA m 2G1 = Gs1 s2 s3 s5 Y 2 tt £n 2 ^21)mc1.3 < A < 2.5 (22)whereandis a strong interaction, renormalization group factor. One can evaluate matrix elements of this interaction by a Fierz transform and use of the factorization approach, yielding, e.g.Q<7T+TT S <,n'+ |uY3d 10><7T |su|k°>■Zi- | dd 1 0 ><0 |sy^d|K°>- . G1 32 , mK2 - " /  , , ,+m — ) ( 1 + — 2------ 1 + -">✓2 u d s A2 2 2 2 ,2 „ 32 W  “K mK “mTTiZ G, -re -7— x--- rr— x x1 9 (m*+m*)m* .2u d s A89where here A ^ 1 GeV is the chiral scale parameter.20 Note the factor of z2 , indicating that estimates of a lower bound on e' based upon use of the factorization scheme and using the Weinberg masses should be re­duced by this factor.A related application of the factorization approximation has been to the so-called "penguin" contribution to the Al = 1/2 rule, which yields a contribution very similar to the above, the only difference being that the coefficient of the operator (usually called C5 ) is real and CP even.19 Recent work in this regard has been encouraging, sug­gesting that Re C5 may be as much a factor of three larger than pre­viously expected.21 However, inclusion of the Z2 factor dampens this possibility.Actually, these larger values of quark masses required by the nu­clear parity violation calculation have previously been suggested by a QCD sum rule analysis of the axial current divergence22 which yielded mu = 7.6 + 2.2 MeV, m^ = 13.3 +3 . 9  MeV (using Aj^-=100 MeV), so that the present results may be considered a useful confirmation.III. CONCLUSIONSThirty or so years ago the study of nuclear parity violation was undertaken as a probe of the structure of the weak interaction. At the present instant in time, it is not yet clear whether we have a completely satisfactory picture of such processes - indeed vexing problems remain such as the understanding of the very tiny ^ N e  circular polarization or the surprisingly large asymmetry observed in the cross section for scattering of 6 GeV longitudinally polarized protons on water.27 Never­theless we have learned a number of valuable lessons via these experi­mental and theoretical studies.i) There is no evidence suggesting that anything beyond the usual Weinberg-Salam picture is necessary in order to understand the AS=0 parity violating interaction,ii) It appears that the basic features of such processes are under­standable in terms of the simple meson exchange picture and a relativistic quark model for the weak NNM vertices,iii) The upper bound on the NNtt coupling implied by recently com­pleted experiments requires a larger value of the currentquark mass than that given by the "Weinberg" value, a result which has implications in other sectors of weak interaction physics.We conclude that future experimental and theoretical work should firm up our picture of the weak AS=0 parity violating interaction and should eventually lead to the long-sought "holy grail" - the extraction of basic weak interaction parameters themselves, e.g. 0 W , from such processes.REFERENCES1. A very nice summary of much of the early work in this field is givenby E. M. Henley, Am. Rev. Nucl. Sci. 19^ , 367 (1969).2. M. M. Nagels, T. A. Rijken and J. J. de Swart, Phys. Rev. D15, 2547(1977) and D12, 744 (1975).903. G. Barton, Nuovo Cimento _19, 512 (1961).4. M. Chemtob and B. Desplanques, Nucl. Phys. B78, 139 (1974); H. J.Pirner and D. 0. Riska, Phys. Letters B44, 151 (1973).5. See, e.g. E. G. Adelberger and W. C. Haxton, Ann. Rev. Nucl. Part.Sci. 35, 501 (1985) and references therein.6 . J. M. Potter et al., Phys. Rev. Letters 33^ , 1307 (1974); D. E. Nagleet al., In Proc. 3rd Int. Symp. on High Energy Physics with PolarizedBeams and Polarized Targets, AIP Conf. Proc. No. 51, AIP, New York (1979); R. Balzer et al., Phys. Rev. C30, 1409 (1984).7. K. Elsener et al., Phys. Rev. Letters 52, 1476 (1984); E. G. Adel­berger et al., Phys. Rev. C27, 2833 (1983).8 . C. A. Barnes et al., Phys. Rev. Letters 41), 840 (1978); P. G. Zizettiet al., Lett. Nuovo Cimento 2j), 167 (1980); G. Ahrens et al., Nucl.Phys. A390, 486 (1982); H. C. Evans et al., Phys. Rev. Letters 55,791 (1985); M. Bini et al., Phys. Rev. Letters 55^ , 795 (1985).9. K.A. Snover et al., Phys. Rev. Letters 41, 145 (1978); E. D. Earleet al., Nucl. Phys. A396, 221 (1983).10. W. C. Haxton, Phys. Rev. Letters 4h, 698 (1981); E. G. Adelberger et al., Phys. Rev. C27, 2833 (1983).11. F. C. Michel, Phys. Rev. B133, 329 (1964).12. J. F. Donoghue, Phys. Rev. D13, 2064 (1976) and D15, 184 (1977).13. B. H. J. McKellar and P. Pick, Phys. Rev. D6 ,^ 2184 (1971) and D7,260 (1973).14. B. Desplanques, J. F. Donoghue and B. R. Holstein, Ann. Phys. (NY)124, 449 (1980).15. See, e.g., J. F. Donoghue, E. Golowich and B. R. Holstein, Phys.Rept. 131, 319 (1986).16. J. F. Donoghue and B. R. Holstein, Phys. Rev. Letters 46^ , 1603 (1981); V. M. Dubovik and S. V. Zenkin, Ann. Phys. (NY) 172, 100 (1986).17. G. Altarelli, R. K. Ellis, L. Maiani and R. Petronzio, Nucl. Phys.B 8 8 , 215 (1975).18. S. Weinberg in "Festschrift for I. I. Rabi," N.Y. Acad. Sci., New York (1977).19. F. Gilman and M. Wise, Phys. Letters 93B, 129 (1980) and Phys. Rev. D27, 1128 (1983).20. J. F. Donoghue, E. Golowich and B. R. Holstein, Phys. Rev. D30,577 (1984); A. Manohar and H. Georgi, Nucl. Phys. B234, 189 (1984).21. W. A. Bardeen, A. J. Buras and J.-M. Gerard, Phys. Letters 180B,133 (1986).22. J. Gasser and H. Leutwyler, Phys. Rept. 87^ , 77 (1982).91DISCUSSION:Adelberger:The diagram where the boson went between the bottom two quark lines, does that not appear because it is a wave function renormalization or something?HOLSTEIN:If you actually calculate the wave functions you will find that it vanishes by parity, those are all S states.Adelberger:They would correspond to parity impurities in the baryons?HOLSTEIN:Yes. In the lowest order they don't matter.Adelberger:Is there some reason for that to be a sacrosanct thing?HOLSTEIN: No. In higher orders that would come in. This is thelowest order.Adelberger:How do I know that the nucleon hasn't a bit of the wrong parity?HOLSTEIN:It does, a little bit.Adelberger:Why is it of higher order? Why isn't that diagram of the same order in the sense that it has one boson in it?HOLSTEIN:There has to be a resonance in the intermediate state and that puts you in a different mass scale. The scale one is talking about is the scale of y  resonances which is around 1600 MeV. That means things should be relatively small.Adelberger:But if I wanted to consider the nucleon emitting a rho? Am I being too naive?HOLSTEIN:The rho is off-shell, it is basically a very soft rho. If the energy scale is up there then you've got to worry about that situation. If the rho is inside the nucleus, then the scale is set by the Fermi momentum.Adelberger:What's the uncertainty of the relativistic correction, the correct mass to use?f92HOLSTEIN:The calculation of gn is very simple for two reasons: Firstplace, its a matrix element between a baryon and a baryon by a two quark object, that we can do. What we're trying to calculate is the baryon to baryon meson matrix element of a four quark object. Its much more complicated and you have four quark operators.Iqbal:How would a 20% qq admixture change all your results (because of all the three quark diagrams)?HOLSTEINYou want to put a quark sea in?Iqbal:That's right.HOLSTEIN:I don't know off-hand how much of an effect that would have.Iqbal:That would come back to Adelberger's question because there will be other quarks that will contribute to that.HOLSTEIN:My calculation is purely within the valence quark picture, that is true. The one anomolous thing that one does have at this point in time is the indication that the strange quark content of the nucleon may or may not be significant. Its a mystery right now, why the strange quark content seems to be as big as it is from measurement of the sigma term.Jennings:It seems that all your problems are coming up not because of the weak interaction but because of the strong part of the force; If that's the case, isn't measuring a weak process a perverse way of learning about the strong interaction?Adelberger:You want to study it with a process that's got inherently shorter range and the weak process is one of the shortest range things we know.McKellar:You take p(k) as only logarithmically dependent on y2, but unfortunately, if you want to push y2 down . . .HOLSTEIN:Then you get into the nonperturbative region.93McKellar:Then you move out of the region where you can believe the k. HOLSTEIN:Once I get down to 300 MeV that's of course a problem because I'm no longer in the perturbative QCD sector. If p is around one GeV I'm right on the border.Miller:Suppose p is one GeV, what would you get for k? It's only a 30% change?HOLSTEIN:I agree, that is a problem. 300 MeV is really the scale to be using this.Miller:First order of QCD or second order?HOLSTEIN:This is leading log, so it's all orders in a. You're just doing the leading log approximation. It's nonperturbative, you're summing all the leading log corrections. If you just did a perturbation theory calculation of some of these effects you get numbers much bigger than these, but if you do the leading log it cuts them down.McDonald:With respect to the question of whether you're doing weak interaction or strong interaction physics, as you pointed out at the beginning this is the only place where you get evidence about ZQ exchange between quarks as opposed to between a lepton and a quark. You have to understand the strong interaction in order to understand that, but the objective is also to see if there is something strange.HOLSTEIN:Yes, I think that's right. On the other hand, I would change what Jennings said. Jennings said isn’t this a perverse way to study the strong interaction. You could also say the same thing about the weak interaction. If you believe in the Weinberg-Salam model, then yes, this is a tough way to measure the Weinberg angle. On the other hand, as McDonald says correctly, as an experimentalist you have to ask is there something else there?Simonius:You need similar calculations in K decay. If you ever want to understand p-p parity violation you have to have other grounds.HOLSTEIN:Yes, any four point operator is tough ground.94Jennings:What I don't know is how to get from the basic weak interaction vertices to what you're measuring. We know sin2 0w from somewhere else, and all his discussion was about how he put in the strong interaction corrections. So you're not going to learn about the neutral part of the current because it's obscured by the strong interaction. And so, back to my question, is one doing a very complicated measurement to learn about what is basically a strong process?McKellar:The point is not that. What you want to learn about is: "Is thereanything else around and what can we say about it?". There have been people who have tried to put in some supersymmetric couplings on composite models and things like this from . . .Jennings:Do you believe you know the strong interaction well enough, that you believe in that?McKellar:You can believe some ball park numbers.HOLSTEIN:Super-symmetry does indeed predict parity violation coming in at some level and this has been used to set limits on quark masses. These are ball park estimates. Ball park estimates are direct.When you try to get into fine detail you get into some trouble.Ng:I didn't get the numbers that you put down. Is there some possibility that the massless up quark is completely ruled out?HOLSTEIN:I think most people believe that. I don't know that this speaks directly to the massless up quark being ruled out. I think that from other points of view, most people believe that the massless up quark is ruled out. The d to u quark mass ratio is known pretty well.Page:As an experimentalist I'd like to know whether I should work harder to measure f^ ten times better or would you rather have hp or another quantity?Adelberger:They haven't measured f^ yet!Page:It's zero.95HOLSTEIN:The question is well taken. I guess I would like to see the isoscalar pinned down. In some sense I think we've pinned down the isovector with 18F pretty well and I would like to see the isoscalar pinned down with the same sort of precision. That's tough; that brings in the nuclear uncertainties. I'm not quite sure how to do it, but that's what I would like to see done.McDonald:The isoscalar, in a sense, is pinned down, if you know the isotensor in pp. Do you see any way to have a combination of experiments that would get the two of those? Do you know the isotensor any better than the isoscalar theoretically?HOLSTEIN:In principal, yes. Because it comes from the factorization it cannot have any contribution from the other pieces.McDonald:If you measure at 230 MeV do you get a different combination of isotensor and isoscalar in pp?Simonius:You don't get the omega exchange, you have only the rho exchange. Adelberger:What you have to do is neutron-proton scattering at 45 MeV or something like that.Adelberger:You could compare the helicity dependence of np scattering at 45 MeV with pp scattering.McDonald:Or, if you had a good circular polarization number.Adelberger:That's very hard.HOLSTEIN:That would be a nice test, if you could somehow isolate the isotensor in testing this whole calculation.96MEASUREMENT OF THE PARITY VIOLATION IN THE QUASI-ELASTIC SCATTERING OF POLARIZED ELECTRONS FROM 9 Be+Reiner Neuhausen Institut fur Kernphysik, Universitat Mainz, 6500 Mainz, GermanyCollaboration with: W. Achenbach, D. Conrath, K.J. Dietz, W. Gasteyer,H.J. Gessinger, W. Hartmann, H.J. Kluge, H. Kessler, L. Koch, F. Neu- gebauer, E.W. Otten, E. Reichert (Institut fur Physik, Universitat Mainz, 6500 Mainz, Germany), H.G. Andresen, A. Bornheimer, W. Heil,T. Kettner, B. Wagner (Institut fur Kernphysik, Universitat Mainz,6500 Mainz, Germany), J. Ahrens (Max-Planck-Institut fur Chemie, 6500 Mainz, Germany), J. Jethwa and F.P. Schafer (Max-Planck-Institut fur biophysikalische Chemie, 3400 Gottingen, Germany)ABSTRACTThe standard theory of electroweak interactions predicts a parity non-conserving asymmetry in the order of lxlO- 5  for the quasi-elastic scattering of 300 MeV longitudinally polarized electrons from nuclei at backward angles. The experimental set-up for the detection of such small effects is described and the procedure of data taking and analy­sis is discussed with regard to systematic uncertainties. The experi­mental asymmetry is found to be Aexp = (-3.51 ± 0.68 ± 0.20)xl0~6, where the first quoted error specifies the statistical, the second one the systematical error. Aexp is Interpreted in terms of model inde­pendent coupling constants between the weak neutral current and the nucleon.INTRODUCTIONIn the energy range of about 300 MeV for the incident electrons, available from the Mainz electron linear accelerator, the quasi-elas­tic scattering process dominates the total cross section at backward angles. Parity violating effects may arise from the interference of the electromagnetic and weak amplitudes, because in contrast to the parity conserving electromagnetic amplitude the weak amplitude depends on the helicity of the electron. The asymmetry of the order of lxlO-5, predicted for the quasi-elastic scattering process in the kinematical range of the present experiment, 1 is reduced in the experiment to approximately 5 xl0 - 6  due to background contributions and an electron beam polarization of 40%, and is therefore one order of magnitude smaller than the asymmetry effects measured in an earlier experiment performed at higher momentum transfers at SLAC . 2 ’ 3The detection of the expected experimental asymmetry at a level+ Supported by Deutsche Forschungsgemeinschaft (SFB 201)97of 10 6 demands at least 101 2  events. Assuming an upper limit for the measurement time of 100 hours, 3xl04 events must be detected during a 4 us long electron pulse given from the Mainz linear accelerator with a repetition rate of 100 Hz. Detectors accepting very large solid angles are required to acchieve such high counting rates which, in addition, have to be handled by analogue detection technique rather than by counting individual particles.EXPERIMENTAL SET-UPFigure 1 shows the overall arrangement of the Mainz parity viola­tion experiment. The usual thermionic electron source of the linear accelerator is replaced by a polarized electron source which consists of a GaAsP photocathode irradiated with circularly polarized light from a flashlamp pumped dye laser . 4 The construction of the source is similar to that used in the SLAC experiment. 2 However, the require­ments regarding pulse stability and lifetime are much more demanding, since the asymmetry effect to be measured is a factor of 1 0  smaller in our experiment. Satisfactory linac operation requires electron pulses with a fast rise time, a flat top and a stability of better than 1 %. Since the laser light pulses are nearly Gaussian shaped and exhibit amplitude variations on a 5 to 10% level, the light pulses are shaped and stabilized electronically by a combination of forward and feedback regulation. At present the source reaches the necessary current (150 mA) and pulse width (3.5 /is). The pulse top stability could be improved to 3xl0-3. The lifetime of the source during operation was increased up to 2 0 0  hours with interruptions of 2 to 3 hours every 2 2  hours, necessary for changing the flashlamps and the dye of the laser system.The circular polarization of the laser light is achieved with the help of a Gian prism followed by a Pockels cell. To avoid influences of systematic drifts in the experiment the circular polarization was statistically reversed between two pulses by reversing the voltage applied to the Pockels cell, and in turn the helicity of the electrons was reversed. The sign of the circular polarization can also be re­versed by rotating the halve-wave plate following the Gian prism from its "X/2+" to its "X./2-" position. Therefore, keeping all parameters unchanged, any observed asymmetry effect must change its sign by switching the position of the half-wave plate.After being accelerated to 300 MeV the electrons are transported to the target by a beam line consisting of two achromatic deflecting systems with deflecting angles of 90° and -90°, respectively. Since the electron beam runs parallel to the direction of the linac axis after the twofold deflection, the longitudinal polarization of the electron beam is unchanged.98G.P .= Gian prism PC = Pockels cell WF = Wien f i l t e r  MD = Mott detectorMP = M i l le r  polarimeterMPo oy. - /« *♦33P . E HCt n V  Mvvzc=-i,m z M m w m Wall= achromatic deflection  systemM2 , M3 = d ipole magnetsQ3 , Q2 = quadrupole magnets= energy cav ityC2 = y -po s it ion  cavityC3 = x -pos it ion  cav ityF = fe r r i te  current monitor qT = "Be target EM = e l l ip t ic a l  m irrors PM = photomultiplier VZ = forward Cerenkov counters CP = Compton polarimeterFig. 1: Overall arrangement of the Mainz parity violation experiment.99The best alternative for meeting the boundary condition of high counting rates in large solid angles was a gas Cerenkov detector system (Fig. 2) consisting of a ring of twelve elliptical mirrors. Cerenkov light emitted by electrons scattered into angles between 115° and 145° is focussed onto the cathodes of photomultipliers. Due to the high counting rates per pulse the flux rate was integrated over the beam pulse width of 4 (is. The mirrors cover a total solid angle of 20% of 4ff. Operating with air at normal pressure the threshold energy for generating Cerenkov light is 21 MeV for electrons. This means that the low energy part of the electron spectrum is cut off. The mirrors can be tilted by remote control to scan the focus across the photocathode and to optimize the signal. By moving the mirror far enough out of focus the background contribution to the Cerenkov counter signal is determined. Special background counters positioned out of focus pro­vide an on-line control of the background signals. Considering the analogue detection technique, this procedure permits the determination of the signal-to-background ratio for each individual counter during the experiment.Fig. 2: Section through the gas Cerenkov detector system. T = 9Betarget (2.4 g/cm2), ES = elliptical mirrors, PM = photomultipliers, UZ = background counters, Cx , Cy = microwave cavities, VZ = forward angle Cerenkov counters, EA = magnetized iron absorber, IK1, IK2 = ionization chambers.100The parity violating asymmetry will be obtained by comparing the scattered electron yield from a right-handed to that from a left- handed electron beam. This procedure requires that the right and left- handed electron beams are identical in all other respects. Small changes in the beam position, in the angle of the beam at the target, in the beam energy and in other parameters should not be correlated with the sign of the electron helicity. Otherwise, they may simulate parity violating effects. A beam monitoring system was developed to give a quantitative measure of such systematic effects. It consists of several microwave cavities, a torodial ferrite current monitor, and four lucite Cerenkov counters positioned at forward angles in the horizontal and vertical plane symmetrically to the beam line. The requirement, that all systematic asymmetries should be less than 1 0 -6, sets upper limits on the systematic drifts of the beam position, the beam energy, the beam intensity and the background contributions. During the experiment all systematic effects due to the switching of the polarization could be kept less than the measured asymmetry.The polarization of the electron beam was measured by three dif­ferent methods: (1) By reversing the field of the dipole magnet bet­ween the source and the injection line of the linac the electron beam with the injection energy of 44 keV can be deflected towards a Wien filter followed by a Mott detector for electron spin analysis. (2) In a separate experiment the polarization of the 300 MeV electron beam has been measured with a Miller polarimeter sitting in the beam line upstream behind the first 90° achromatic deflection system. (3) This measurement has also served for calibrating a Compton polarimeter which is positioned approximately 6  m behind the target and consists of a magnetized iron absorber between two ionization chambers (Fig. 2). A very important aspect for our experiment is that the Comp­ton polarimeter is the only polarimeter which provides an on-line determination of the longitudinal polarization of the electron beam at the target position during the experiment (Fig. 3).DATA TAKING AND ANALYSISDuring data taking the polarization state of every second beam pulse was statistically chosen with a random number generator, that of the following beam pulse defined to have the opposite sign. To avoid influences of drifts of the apparatus we used pairs of consecutive beam pulses to form the single asymmetrieswhere N+ and N~ are the integrated outputs of any given counter for the positive and negative polarization state, respectively. For an101example, N can be the summed yields of the twelve Cerenkov counters divided by the measured charge.P (%)5050i i. . . .  M ( . R U N . . •: ____ff'i.L.'M■I.•r i ": : m { l ■:j; j ;1 . 8 )*;. . . . . . . . . F  = .J  : : • \A  i : > . . N D.V.E M .B .E R . R . U R . . . . ------ .........;----j___i•jH- • ■ •1W H / L .,%------J-------; | P  = ( 4 3 , 8 2 1 8 )f  : :L~T"" 1; : ; : i100time (hours)Fig. 3: Polarization of the electron beam at the target as a functionof time.With unpolarized electrons, we have not observed any significant asymmetry of the apparatus. Under stable conditions of the linac the measured asymmetries have proven to have a normal distribution. The width of the distribution is compatible with the number of scattered electrons and indicates that the influence of fluctuations of the beam parameters is sufficiently suppressed. For a run of 30 min and an averaged current of 2 0  /M the mean value of the asymmetry is less than 10 6 and consistent with zero. Equivalent checks for the production runs have established this "zero asymmetry" for uncorrelated fluctua­tions to a 1 0 - 6  level.Two production runs have been performed, one with a data taking time of 100 hours in May 1986, the other one with 120 hours in Novem­ber 1986. The data taking time was divided into single runs of 15 min.102asymmetryFig. 4: Frequency distribution of the single asymmetries measuredwith polarized electrons for a 15 min run. The solid curve is the Gaussian function fitted to the measured distribution.After each single run the halve-wave plate was rotated from its X./2+ to its "X/2-" position or vice versa. This procedure allows us to distinguish between physically real asymmetries and systematic asym­metries generated by the fast switching of the Pockels cell. The single asymmetries formed from pairs of consecutive beam pulses were also normally distributed (Fig. 4). The mean value A and its standard deviation AA = cr/t/fT, where cr2 is the variance of the sample of the n single asymmetries of a 15 min run (n = 22500), were determined by two different methods:i) The well known estimators are used:1 _  V ( A  - A ) 2* - : 1 a a’ - r -(2 )ii) The probability function of the normal distributionP(A) = P 0 exp[(A - A)2 /2ff2] ( 3 )103is fitted to the measured distribution by the method of least squares. The results from both methods agree within the statistical errors as demonstrated in figure 4 for a typical 15 min run.The asymmetries of the 15 min runs do not show any obvious devia­tions from a statistical behavior (Fig. 5, left side). In the next step, the mean value and its standard deviation was determined for this sample. Since the variances of the 15 min asymmetries were slightly different from run to run, the equations (2 ) were used with weights proportional to the inverse variances. The fitted normal distribution curves (Fjg. 5, right side) exhibit a statistically sig­nificant offset from zero with the expected change in sign. The re­sulting asymmetries Agc are compiled in table 1 together with other measured quantities of the two production runs.The integrated outputs of the background counters were treated in the same way. Again, the 15 min asymmetries proved to be in agreement with a normal distribution. The background asymmetries A^g , obtained as the mean values of the 15 min asymmetries, are consistent with zero within the statistical errors (see table 1 ).Table 1: Parameters of production runs and experimental asymmetriesMay 1986 November 1986polarization in % 44.9 ± 1.8 6.9 5.0243.8 ± current in /jA signal-to-background position of X / 2  plate + +number 15 min runs 191 182 225 225asymmetries in 1 0  6Ajjg (method 1 ) 2.96±5.52 -4.34±5 . 6 8  2.32±5.31 -2.91±5.295.22±5 . 6 8  -1.30±5.85 6.53±5.44 -2.81*5.432.59±1.52 -4.08±1.55 2.71*1.22 -4.59±1.222.78±1.56 -4.96*1.59 2.81*1.28 -3.88*1.27-1.51±0.56 -0.18±0.46 0.16±0.34 -0.68*0.354.10±1.52 -3.90*1.55 2.55*1.22 -3.91*1.22(method 2 )Agc (method 1)(method 2 )±0.56 ±0.36 ±0.34 ±0.35-4.00 ± 1.09 ± 0.36 -3.23 ± 0.86 ± 0.24-3.51 ± 0.68 ± 0.20asymmetry _ asymmetry104Fig. 5: Left side: Sequence of the asymmetries of the 15 min runs forthe "X./2 +" (top) and the "X/2 -" (bottom) position of the halve wave plate. The 450 runs correspond to a total running time of 120 hours (run of November 1986). Right side: Fre­quency distribution of the 15 min asymmetries with the fitted normal distribution.105CORRECTIONS FOR SYSTEMATIC ASYMMETRIESUntil now the measured asymmetries Agc are the results of the comparison of two measured quantities N+ and N_, which are assumed to be different due to the parity violating structure of the weak neutral current. The question as to what extent systematic asymmetries, which are not of parity violating nature, contribute to the measured asym­metries, can be answered from the recorded data from the various con­trol systems. The response functions of these systems were investi­gated in extensive test runs with unpolarized and polarized electrons before the production runs were performed.Systematic asymmetries could be traced back to three effects:(1) Jamming electronics effects correlated with the switching of the Pockels cell were suppressed to such a level that an induced system­atic asymmetry is less than 8x10 e. (2) Parity conserving processes which depend on the polarization of the electron beam are Miller and Mott scattering. The contribution of M011er scattering to the measured asymmetry was estimated to be less than 8x10 The Mott scattering contribution is suppressed by the axial-symmetrical construction of our detector system and was determined to be less than 4 xl0 ~8.(3) Small deviations of the beam position, angle and energy correlated with the helicity of the electron beam were quantitatively measured. They led to the systematic asymmetries Agys, listed in table 1.To obtain the corrected asymmetry Acor for each data set, the systematic asymmetry, Asys, was subtracted from the measured one, Agc, where the statistical and systematic errors were treated separately. Combining the asymmetries for the "X/2+" and"\/2-" runs, we obtain the asymmetry Acom for each of the two production runs and, subsequently by combining those, our final experimental asymmetry:Aexp =(-3«51 ± 0.68 ± 0.20)x \0 (4)where the first quoted error specifies the statistical, the second one the systematic error.INTERPRETATION OF THE EXPERIMENTAL ASYMMETRYThe interest in the parity violating effect in electron scatter­ing lies in the fact that both parity violating amplitudes of the weak neutral current contribute, the vector part of the nucleon as well as its axial vector part. The effective Langrangian which describes the parity non-conserving part of the electron-hadron interaction is given by5106Here, the a, 0, y and 6  denote the coupling constants which have to be determined experimentally. Whereas atomic parity violation experi­ments6 for heavy atoms are only sensitive to the vector part of the nucleon, the deep inelastic electron scattering experiment at high energies at SLAC2 * 3 as well as our quasi-elastic electron scattering experiment at an intermediate energy depend on both the vector and axial vector part of the nucleon. In comparison to the SLAC experiment our experiment, however, is by a factor of 6  more sensitive to the axial vector part.Besides the quasi-elastic scattering process the other contribu­tions to the experimental asymmetry like the radiation tail, the dip region scattering process, and the pion production process were taken into account for the theoretical calculation of the asymmetry. By com­parison with the experimental asymmetry we obtained the following linear combination of the coupling constants:0.853 a + 0 + 0.454 y + 0.047 6  = -0.564 ± 0.106 ± 0.031 (6 )Fig. 6 : Region of the axial vector constants 0 and 6  defined by thepresent experiment (90% confidence level).107The SLAC measurement combined with the results from the atomic parity violating experiments allows the determination of a and y in a model independent way . 7 We have included these results into the calculation of the axial vector coupling constants and extracted the linear com­bination0 - 0.032 5 = -0.16 ± 0.18 (7)where the quoted error was deduced from the total error taken as the sum of the statistical and systematic errors. The linear combination restricts the allowed region of the quark coupling constants C2U and C2d to the area shown in figure 6 , where the prediction of the stand­ard model depending on the Weinberg angle is also plotted.In the standard model the coupling constants a, 0, y and 6  can be expressed by sin2^ .  Then, from equation (6 ) we foundsin2©w = 0.218 ± 0.018 ± 0.005 (8 )which is in agreement with the best known value for the Weinberg angle.OUTLOOKFor the first time, a linear combination of the axial vector coupling constants 0 and 6  was found from experiment. Future experi­ments should aim for a considerable improvement of the statistical andsystematical errors. In order to contribute to this ambitious goal theexperience gained in the present experiment will be transferred to thedevelopment of a high intensity polarized electron source, which will be installed at the Mainz continuous wave electron accelerator,MAMI B, with a maximum energy of 840 MeV. At present, MAMI B is under construction and will be ready for operation in spring 1989.REFERENCES1. E. Hoffmann and E. Reya, Phys. Rev. D8 , 3230 (1978)2. C.Y. Prescott et. al., Phys. Lett. 77B, 347 ( 1978)3. C.Y. Prescott et al_., Phys. Lett. 84B, 524 (1979)4. E. Reichert, AIP Conf. Proc. on High Energy Spin Physics - 1982(American Institute of Physics, New York, 1983) p. 5805. P.O. Hung and J.J. Sakurai, Phys. Lett. 63B, 295 (1976)6 . M.A. Bouchiat et al_., Phys. Lett. 134B, 463 (1984)7. S.L. Gilbert et al., Phys. Rev. Lett. 55, 2680 (1985)8 . P.Q. Hung and J.J. Sakurai, Ann. Rev. Nucl. Part. Sci. 31,375(1981)108DISCUSSION:Adelberger:Where does the backgound of PMT's [photomultiplier tubes] in Gas Cherenkov counters come from?NEUHAUSEN:Background can be a lot of different things. There are always hard photons; electrons can produce Cherenkov light in shielding. There are a lot of different processes which lead to Cherenkov light.Adelberger:What determined the choice of the beryllium target?NEUHAUSEN:The first idea was to use neutrons, but they're not available, so we used deuterium. The effect of the neutron is larger than the proton. But to start an experiment like this it was our idea to avoid the problems with a liquid deuterium target: by starting with beryllium which is also a light nucleus with a lot of neutrons In it and later on to use deuterium. But, after years and years, we finished the 9Be experiment, not the deuteron experiment.Adelberger:How about 7 Li?NEUHAUSEN:That's also a nasty target. It doesn't make a difference with 9 Be. 9Be is nice, you can put a lot of current on the target which you cannot do with lithium.van Oers:S o ,  w i t h  M A M I - B , d o  y o u  h a v e  a n  e x p e r i m e n t a l  p r o p o s a l  t o  c o n t i n u e  t h i s  w o r k  a n d  i m p r o v e  t h e  s t a t i s t i c a l  a c c u r a c y ?NEUHAUSEN:MAMI-B is now under construction, but it's too early for proposals. In the field of polarized electrons there is a group, more or less the same group that did this experiment, that will develop a source matched to the capabilities of MAMI-B. A goal is to get around 30 yA, maybe up to 50 yA.van Oers:But, at the moment if you are going to do another experiment what improvements do you really think you should get?NEUHAUSEN:The improvement will be that it's possible to count events with this 100% duty factor accelerator, hence to do coincidences. For example, one can do an experiment to measure the asymmetry effect of the proton, to measure a coincidence between the electron and the proton. The progress is to do coincidences.109Kowolski:What does the coincidence aspect of the experiment buy you? NEUHAUSEN:To suppress the background. You have to change detector systems, but that's what I hope to do.Bowman:You would know that you hit a neutron, you could do a quasi-elastic experiment . . .NEUHAUSEN:But then it would be elastic scattering on the proton. You know the first idea of the theory is to measure elastic scattering of the proton or the neutron. The quasi-elastic is maybe the best choice as long as there are momenta which are higher than the Fermi momentum of the nucleus.Adelberger:Is the idea behind the experiment on the proton to measure sin2 0w accurately at low energies?NEUHAUSEN:I have not looked into this experiment in detail.Haxton:I wonder about the parity violation of the nuclear target. There ought to be a coupling between the electron and the nuclear target from one photon exchange, where the photon couples to the nucleus in a parity violating way. Has that been thought about or does it average out?NEUHAUSEN:I don't know. What we have used in the analysis was just a Fermi gas model for the quasi-elastic peak.Adelberger:You apparently made a source that was reliable for a long period; could you say anything about the technical aspects?NEUHAUSEN:We have started with the GaAs source which was not stable at the time, with a lifetime of 5-20 hours; and the biggest progress was the laser. This laser had so much power that you could throw away a lot of this power, you can shape your pulse and at the same time you can stabilize the total. This keeps the current for the electron pulse constant all the time, so if the source goes down you can raise the laser. You need a very good vacuum in the source, especially since the linac has a vacuum of the order of 1 0 - 6  and the source will be something like 10“ 10. What we use is a distance of approximately two meters between the source and the linac and the110cryopump, which is a rather large cryopump. A good vacuum and good electron optics in the source is also very important. If you lose some electrons near the GaAs cathode you poison the cathode, because there is always some material around.Adelberger:Is the power that the laser puts out into the crystal sufficient to induce outgassing from the crystal just from the power of the laser? The amount of energy you dump into your GaAs comes just from the light. You say that you have enough laser power so that as your source deteriorates you just make up for it with more light. I can imagine that then you start heating the material up.NEUHAUSEN:The cathode sees always the same power. You stabilize the laser light before it hits the GaAs. [See Fig. 1 - ed.].Adelberger:Ah, I misunderstood.Adelberger:What you are doing is that this feedback system, in addition toshaving the pulse, is really stabilizing the laser, it's notstabilizing the source, the source is pretty stable is what you're saying.NEUHAUSEN:The emitted electrons are more or less an image of the laser lightpulse which you bring on to . . .A d e l b e r g e r :A pulse of a certain height and watts or something will always produce a current of so many milliamps?NEUHAUSEN:That's right, yeah!Adelberger:That does not deteriorate with time?NEUHAUSEN:No.McDonald:What is the quantum efficiency of your photo-emitter and how does it vary with time?NEUHAUSEN:That is a question Henley asked me yesterday. I have looked for an answer; haven't found it. I have an idea . . .  of around 2 or "i/o.I l lM c D o n a l d :If you kept the laser constant, how would that current from the GaAs phosphide change with time?NEUHAUSEN:We worked that out 3 or 4 years ago. I do not remember exactly, but I can only show you what we have measured during the production run. This was for a time of 120 hours; the measurements were interrupted once a day for two hours, not to change something at the source, but to change the flash lamps for the dyes for the laser and to change the dye.Bowman:What you actually show here is stabilizing on the electron current. NEUHAUSEN:This is the light here; what you do is you need a voltage here for this Pockel's cell, and the voltage is added from three different sources: from this double pulse, from the function generator, and from a voltage from here [see Fig. 1 - ed.]. The electrons go through here and you peak up the electron signal and use it for the Pockel's cell.McDonald:You are stabilizing the electron beam.NEUHAUSEN:Yes.Bowman:If the GaAs crystal deteriorated, what could happen is it could crank up the light to make more electrons come out.van Oers:How fast is the feedback?NEUHAUSEN:It must be a fraction of the 4 microseconds.Haxton:Is it possible to do an experiment where you bin your data so you look at a certain set of energies of recoil electrons, and then another set?NEUHAUSEN:No. With the gas Cherenkov detector system we take everything from 21 MeV up to the elastic scattering. The inelastic peaks are very small. We have done separate measurements with a single arm magnetic spectrometer to measure the cross-section independent of the energy transfer.112Adelberger:How much of the Cherenkov light actually comes from the inelastic scattering, and how much quasi-elastic scattering and how much comes from the radiative tails . . .NEUHAUSEN:59% of the signal is the quasi-elastic peak. And another point, the electron beam, of course, everywhere is in a vacuum. The target sits inside the vacuum; but between the target and the air there is a very thin foil. You must be careful, you can produce tTq's. It was not one of the backgrounds that we had thought of at the beginning; if you have tt0 ' s  you have immediately two gammas; the two gammas can produce electrons again. One of the ideas we had was to put a 12C absorber between the target and the Cherenkov counters to get a higher threshold; to suppress the low energy part of the spectrum. But this does not work, it is pointless.113BATES PARITY VIOLATION EXPERIMENTS. Kowalski Bates Linear Accelerator Center Laboratory for Nuclear Science and Department of Physics, Massachusetts Institute of Technology,Cambridge, MA 02139 USAABSTRACTAn experiment designed to measure precisely the parity violating interference in the elastic scattering of longitudinally polarized electrons from 12C is under way at the Bates Accelerator Center. The experiment required the construction of a high intensity polarized injector whose successful operation has recently been demonstrated.Present efforts are aimed towards the understanding and control of sources of systematic errors.INTRODUCTIONThe standard SU2 xU1 model of the electroweak interaction is one of the most successful theories in physics. The weak neutral current and the predictions for the W and Z masses have been verified. High precision experiments involving a variety of probes: deep inelastic scattering, i/p elastic scattering, ue scattering, atomic parity violation, e+e’ annihilation, and the W and Z mass measurements are all in impressive agreement with this theory. Taken together, the data yield1 sin20 - 0.230 ± .0048 for the one free parameter in the model. These data”span a r^nge in momentum transfer covering ten orders of magnitude (1 0 ~ 6 < q (GeV/c) < 10 ). Present experimental motivations include tests of the standard model at the level of radiative corrections.Although many neutral current phenomena have been observed, one of the major early experiments which provided a crucial test of the W-S model was the SLAC/Yale measurement of parity violation in the deep inelastic scattering of longitudinally polarized electrons from deuterium. 2 The electron-quark interaction (eq) is described by an amplitude which contains the usual electromagnetic amplitude (virtual photon exchange) together with a weak part (Z° exchange): A = A + A , with A »  A for energies of interest here. Thus in (eq) scattering” the transition probability, proportional to |A|2, contains an interference term -2A A in addition to the dominant term |A | 2 and a negligible term |A |2. ” fi? the weak interation is parity violating, the interference term contains a pseudoscalar piece. Experiments with longitudinally polarized electrons are directly sensitive to the parity-violating part in the interference term. The measured quantity is the asymmetrywhere < = ^ ( < 0  is the cross section for the scattering of electrons with left (right) helicity. Since this asymmetry is manifestly parity114violating, a non-zero result is a direct measure of the parity violating effects due to neutral currents or of parity mixing in the nuclear wave functions. These latter effects can in some cases be neglected. In the case of the Bates 12C experiment, there is no one-photon transition amplitude between the | 0 0 > ground state and the admixed wrong-parity state | 0 0 >.All experiments involving longitudinally polarized electrons as well as atomic physics measurements can be described with a complete set of four fundamental parity-violating neutral current couplings. The effective parity violating Lagrangian can be written3 as:eV s e16■fe^uy u + «?^dy dl + e y e-fe^uy y„u + e^fdy y_d VI[ AV '/j AV n J n L VA n 5 VA >  5 Jjwhere the coupling constants e2v^€VA^’ <l“ U , d  referring to the up anddown quarks, denote an axial (vector) coupling to the electron and a vector (axial) coupling to the quark. These couplings are considered the most likely for the neutral currents.It is often convenient to define an equivalent set4 of couplings more appropriate to experiments. These correspond to the isoscalar and isovector combinations for the hadronic vertex and may be expressed as linear combinations of the Bjorken couplings:Axial Electron Axial Hadronic_ eu ed eu edoc ="€a v + €a v P “ -£VA + eVAeu ed eu ed7 = 'eAV ' £AV6 = *eVA ' eVAThese are appropriate combinations in nuclear physics because of the relationship to charged weak currents (which are necessarily isovector). The objective is to determine these four independent couplings experimentally. In the Standard Model of electroweak interactions these four coupling constants are only a function of sin2 0 :oc = - (1 - 2  sin2 0 .T) w0 = - (1-4 sin2 0w )Isovector7  - 2/3 sin20WIsoscalarOne should note that, for the current best value of sin26 = 0.230, 0 ~ 0.The goal of experiments is to test the standard model predictions with the highest possible precision.115If we consider elastic electron scattering from a 0 nucleus, 12C e.g., one can show that the asymmetry for scattering longitudinally polarized electrons is given by5A - g q2'na\2,(0 )( q 2 )ReThe asymmetry is the ratio of the two form factors characterizing the electromagnetic [F0 ^(q2)] and weak-neutral currents [F(0 )(q2)]. The strengths are characterized by the Fermi coupling constant G=l.02x10 5/m2 and b the strength of the axial-vector coupling of the electron. In  ^principle the two form factors corresponding to the distributions of electromagnetic and weak-neutral charge need not be related. In the Standard Model, however, the weak-neutral current is directly proportional to the electromagnetic current itself.F0 (0 )(q2)^ = -2 sin2 8 F ^ (q2) and b = -1. w o ^The asymmetry then becomesSin 6 W = 3.17 x 10-4MThis is a beautifully precise result. Since the form factors correspond to the Fourier transform of the charge distributions, it says that the distribution of electromagnetic charge and the distribution of the weak neutral charge are exactly the same irrespective of the distance scale being probed. It is important to test this prediction precisely. It is also important to test the q2 dependence.Figure 1 shows the results of the SLAC-Yale e-2H experiment in terms of the neutral current couplings and compared with the predictions of the Standard Model. The SLAC experiment is mainly sensitive to the axial- electron isovector-quark interaction. The bands represent the experimental results with the slope given by neutral current phenomenology and the width corresponds to the experimental error. In Fig. 1 we have also shown the projected hypothetical results for the Bates-Yale experiment on e-12C scattering, intersecting the model at the same values of sin28^ . The band width represents the proiected experimental uncertainty. It is clear that this experiment is sensitive to a different linear combination (nearly orthogonal) of these same two coupling constants and together with the SLAC results should provide very precise measurements for each of them. Not shown in the figure are the results of several atomic physics experiments carried out on heavy atoms. These have similar model sensitivities to the e-12C experiment although at essentially q 2 ~0 .The Bates parity violation experiment is a collaboration6 between physicsts at Yale, Syracuse, CCNY, Harvard and MIT.116Figure 1. Results of the SLAC polarized electron experiment and the proposed Bates e-12C experiment. The predictions of the "Standard Model" are indicatede-12C EXPERIMENTThe Bates parity violation experiment is designed to measure the electroweak interference in the elastic scattering of longitudinally polarized electrons from carbon. For 250 MeV electrons at a momentum transfer of 150 MeV/c the predicted asymmetry, based on the Standard Model, is A=1. 8xl0~ . The goal of the experiment is to measure the effect to approximately 1 0 % of itself.Carbon is a particularly suitable target for weak interaction studies, having zero spin and isospin, it has one elastic form factor which vanishes in the asymmetry. The elastic scattering samples coherently the nuclear charge distribution and is to be contrasted with the incoherent scattering from individual quarks characteristic of the SLAC-Yale deep inelastic experiment. The contribution of the 4.43 MeV(2 ) state, which is not resolved, is only a few percent at this momentum transfer. All other inelastic channels are discriminated against.The choice of momentum transfer is a compromise between maximizing asymmetry and minimizing statistical errors. Large asymmetries are important in controlling systematic errors. Since the asymmetry is proportional to q2 , it is desirable the operate at the highest possible q2. The statistical error, inversely proportional to JctA, is independent of q2 for point-like scattering. We concluded that it was most desirable to operate in the vicinity of unity form factor and asymmetries no smaller than lppm.117Asymmetries in polarized proton scattering have been measured at a level of less than 1 x 1 0  and the projected sensitivity for electron scattering should be achievable.POLARIZED ELECTRON SOURCEA polarized electron source, based on photoemission from a GaAs crystal, has been constructed for the Bates accelerator. Major constraints on the source design include the requirements for high intensity and polarization to achieve the required statistical accuracy. Helicity dependent systematic effects must be reduced to an absolute minimum. The source must be compatible with accelerator requirements for phase space and injection energy. The design goals for the source are summarized in Table I. A schematic of the source is shown in Fig. 2.Table IDesign Goals for Polarized Electron SourceIntensity Duty Factor Polarization Injection Energy Phase Space Intensity Stability20mA peak; 300/^A average 20n sec pulses; 720Hz0.4 365< 1 0  me-cm <0.5^ jitter< 1 0  correlated with helicityThe source design incorporates a gun chamber, which houses the extraction electrodes, an accelerator column, and a preparation chamber.A small extraction airlock permits crystals to be moved in and out of the system while an ultrahigh vacuum is maintained in the gun and preparation chambers. The source is maintained at 365 kV by a Faraday cage at high voltage. The outside of the accelerator column is insulated by a fiberglass bushing filled with six atmospheres of SF6 required to maintain the high gradient.The electrode assembly was designed to be similar to the thermionic gun presently in service at Bates. Space charge effects were an important engineering consideration. The GaAs crystals are flashed, coated with cesium and nitrogen trifluoride, and tested for quantum efficiency in the preparation chamber. This separate chamber protects the critical gun chamber from contamination and also allows for several crystals to be readily available for service.The major components of the optics system are a CW laser, an electro- optical shutter, and a circular polarizer. The Kr-ion laser, operating at a wavelength of 752 nm, produces longitudinally polarized electrons. A Pockels cell chops the light providing ~20/isec long pulses matching the accelerator time structure. A second Pockels cell randomly reverses the helicity of the laser beam (and hence the electron helicity) on a pulse- to-pulse basis. The CW chopped laser produces a much more uniform light intensity than would be possible with a pulsed laser.118ION PUMPSCRYSTALHOLDERMANIPULATOR MANIPULATORACCELERATORp o c k e l IsCELLFigure 2. Schematic of the Bates polarized injector.MOLLER SPECTROMETERThe polarization of the electron beam is measured by a Moller polarimeter. The relativistic longitudinally polarized incident electrons are scattered from longitudinally polarized target electrons. A ferromagnetic foil in a magnetic field serves as the target. The spin dependence in electron-electron scattering is a maximum at 90° in the center-of-mass system. This corresponds to an angle of 3.66° in the lab frame for 250 MeV electrons.A magnetic spectrometer system is used for momentum analysis of the scattered electrons. Tungsten and lead collimators define the beam acceptance. UVT lucite Cerenkov counters coupled to phototubes are the detectors. Substantial shielding limits backgrounds to manageable levels. The dominant remaining background is the nuclear elastic scattering radiative tail. Signal-to-noise is excellent and we see no difficulty in measuring beam polarizations to an accuracy of -5%.119Since the Moller scattering cross section is large, representing approximately 1 0 3 events per beam pulse at our kinematics, the detector signals are integrated on a pulse-by-pulse basis. A very clean Moller scattering peak is observed with a width which is dominated by multiple scattering in the target.BEAM INSTRUMENTATIONOver the past two years, substantial beam-line instrumentation has been developed and tested for the experiment. A set of ten microwave position monitors, which will be used to determine the beam centroid on a pulse-by-pulse basis to much better that 10(V, have been installed. Four of these monitors, which are located just upstream of the carbon target, determine the position and angle of the beam as it strikes the target. Other monitors, located at a point in the beam line where there is dispersion, measure the beam energy at the level of 1 0  each pulse.Several toroid monitors, for measuring beam intensity, are located along the beam line.The beam instrumentation has been designed to provide measurements of any changes in the beam intensity, position, angle, or energy which are correlated with changes in beam helicity at the polarized injector. A change in beam intensity will result in a change in average energy due to beam loading in the accelerator. Since the scattering cross section depends very strongly on energy, such correlations would lead to large false asymmetries. Sensitivity of the spectrometer to measured changes in beam parameters can be studied. This should provide us sufficient information to evaluate and control systematic errors.A Wein filter, which can reverse the helicity of the beam in an independent way, has been fabricated. This will provide additional tests for systematic effects. Specifically, it will allow us to reverse the sign of the physics asymmetry while leaving most systematic errors unchanged.DETECTOR SYSTEMA pair of quadrupole-singlet spectrometers with Cerenkov detectors has been constructed for the experiment. Fixed at a scattering angle of 35°, each provides -lOmsr of acceptance and approximately 10 MeV in momentum resolution. The pair of detectors will be useful in understanding systematic effects -- particularly those involving transverse polarizations.Using a 5gm carbon scattering target, approximately 10s scattered electrons are detected for every machine burst. Signals from the detectors are analog processed and integrated for the 15/i-sec long bursts. They are digitized using 16-bit ADC's and stored to tape on a burst-by- burst basis. Charge normalization is provided by the standard Bates toroids feeding similar ADC's. Extensive preliminary studies with these spectrometers have yielded an acceptable shielding configuration. Noise nearly as small as predicted by statistics alone has been demonstrated.120EXPERIMENT STATUSThe major harware systems required for the experiment have been constructed, tested and are essentially operational. These include the polarized injector, beam-line instrumentation, Moller spectrometer, detector system and data acquisition and control software.In this past year we have achieved several important milestones for the Parity experiment. Most important has been the successful injection and acceleration of polarized electrons in the Bates Linac. The polarization was measured by Moller scattering. For this purpose the accelerator rate was limited to 60Hz at a peak current of 1mA on target. Operating at an energy of 250 MeV, a polarization of 0.36±0.03 was measured. This is the expected value for a GaAs source. Injection capture efficiency was in excess of 40%, as expected. Beam quality, emittance and energy spread, were comparable to those for our standard room temperature gun.Peak intensities at the source of up to 25mA have been achieved. We have accelerated polarized beams with intensities as high as 5mA.Operation at the full accelerator rate of 600Hz has also been demons trated.An important operating characteristic of the source is its lifetime. We have excellent lifetime (>100hrs) at low currents under optimal conditions. The lifetime when operating near design conditions is directly correlated with beam scraping and consequently poor vacuums.Clean injection into the accelerator is an absolute must. In a test this past week the polarized source operated at a peak current of 3mA on target at 600 Hz with a lifetime in excess of 25 hours. The average current was a factor of two less than the design goal while the lifetime was longer than the minimum required for the experiment. We are presently concentrating our efforts on improving further this aspect of source performance.Initial measurements of the parity-violating asymmetry in scattering from carbon have also been performed. During a brief test, operating at 5mA of peak current, the observed asymmetry was consistent with zero with a statistical error of approximately lOppm. Systematic errors were comparable to the statistical errors. The absence of large systematic errors is very encouraging for the goals of the experiment.We have also successfully carried out a test which modulates the incident beam energy on a pulse-to-pulse basis and produces an asymmetry of up to 30ppm. This is accomplished by pulsing the carbon scattering target by a few keV. The results, shown in Fig. 3, have statistical errors of a few ppm and are consistent with theory. This data has provided an important test of the spectrometers, electronics, data acquisition, and analysis. The results support the belief that one can measure small asymmetries.Other tests have addressed sources of systematic errors. One such source of false asymmetries is the Polarization Induced Transport Asymmetry (PITA) effect in which the intensity of the laser beam at the photocathode is correlated with the helicity due to various imperfectios in the optics. Coupled with beam loading effects in the accelerator, the large false asymmetries can mask the physics. We have measured this effect in our transport system and have made significant progress in its control. We plan to null the PITA effect so that beams of either helicity will be identical at the required level.121PULSED V O L T A G E ( V o l t s )Figure 3. Asymmetry produced by high voltage pulsing of a carbon target.Finally, a system has been constructed to control beam line steering in order to measure the effect of beam parameters, e.g. position and angle, on the cross section. It will also allow us to calibrate the beam position monitors. Preliminary tests of this system have been successfully completed.SUM M A RYThe parity experiment has been the primary motivation at Bates for developing a high intensity polarized electron capability. Recent reviews have highlighted the importance of polarized electrons for nuclear physics. Such a capability is not available presently for nuclear structure studies. It is, however, in the future plans at facilities such as CEBAF, MAINZ and the University of Illinois.Fundamental measurements which require polarized electrons include the electric form factor of the neutron and "deformation" of the delta. Specific physics questions can also be isolated in combination with coincidence measurements and scattering from polarized targets. Parity experiments, such as scattering from the proton, could test the Standard Model at the level of 1% or less. The physics potential is exciting and real and continues to motivate us to develop a reliable high intensity source.122REFERENCES1. P. Langacker, private communication and preprint.2. C.Y. Prescott et al., Phys. Lett. 77B, 347 (1978).3. M.A.B. Be£ et al., Phys. Rev. Lett. 33, 606 (1974); J.D. Bjorken, Phys. Rev. D18, 3239 (1978).4. J.J. Sakurai, NCLA/78/TEP/27; T.W. Donnelly and R.D. Peccei, Phys. Reports 50C. 1 (1979).5. G. Feinberg, Phys. Rev. D12, 3575 (1975).6 . Syracuse University: D.H. Kim, K. Kumar, M.E. Schulze, P.A. Souder; CCNY: M.S. Lubell; Harvard: J.S. Patch, J. Glossen, R. Wilson; Yale:G.D. Gates, V.W. Hughes, R. Michaels, R.H. Schaefer; MIT: G.W. Dodson, K.A. Dow, K. Isakovich.123DISCUSSION: van Oers:There is no nuclear structure? That all drops out?KOWALSKI:That is the claim, this is a 12C experiment, no nuclear structure at all.van Oers:This is based on some model?KOWALSKI:This is based on a standard model.Bowman:That must assume isospin purity in the ground state.KOWALSKI:That is correct.McDonald:. . And parity purity in the ground state.KOWALSKI:. . And parity purity in the ground state. It's not too bad in this case, because if you want to go to a 0 ~ state you can't do it with a single photon exchange, you have to have a two photon exchange.Adelberger:Isospin [conservation] is what you are assuming.KOWALSKI:Yes.Adelberger:You are using CVC to say those form factors are equal and that assumes isospin is a good symmetry and that is surely the thing that is going to break down first in all this stuff.Haxton:Just look at beta decays in the sd shell, you get 5% effects all the time.Adelberger:1 2 N, 12B ft values for their beta decays differ by 20%.Blunden:Here it's essentially the difference between the proton and the neutron densities, which is small.Adelberger:I am just telling you, that ft value for 12N beta decay to 12C124differs from the ft value for 12B beta decay to 12C by 20%. That is telling you that the radial wave function overlaps of neutrons and protons are different.Blunden:But that is an excited state; this is the ground state.Adelberger:That is the decay of an excited state to the ground state.Blunden:Well, I can tell you what I have done is to put in reasonable density distribution of the neutrons and the protons and I find a very small effect.Adelberger:Try to reproduce the ft values for the beta decays. That is the test.Bowman:There must be a Coulomb polarization in the ground state.Adelberger:There is a simple way to see if you have got to the basic physics right: Can you account for the ft values of the beta decay in mass1 2 ?Blunden:That's something completely different because those are highly excited states that are nearly unbound, so you have the Coulomb interaction which pushes the proton wave function way out.Holstein:What you have here is the weak neutral current, which is proportional to the electromagnetic current, . . .  so any charge mixing is irrelevant.Adelberger:What about the Ml scattering?KOWALSKI:That's also very weak. I think that the 2+ is the strongest thing at this momentum transfer, so that is even lower.Adelberger:The trouble is of course, it has exactly the other isospin.KOWALSKI:It certainly does, . . . but nothing I've seen would say that the asymmetry that one would predict, taking into account the kind of intensity we have here, would make any significant contribution.125Adelberger:[Barry] Holstein assumed that the isospin was good and, therefore, you don't get the isospin charge rotated.Holstein:My point is that the beta decay is not that relevant. It is true that the 1 = 1  piece can mix into an 1 = 0  isospin, but of course the isoscalar piece is enhanced because it is coherent. So you get a Z associated with the isoscalar. The isovector is suppressed by 1/Z. So again it comes in at that level, but it's suppressed.Adelberger:But if you imagine now, in an ideal world we're going to do what Dirk Walecka always wants to see, you go over this thing and you see it go through the minimum and the maximum and you watch these two things track. Those minima are where this factor of Z has been summed up to zero, because all the cancellation has worked out, and then at that point there's nothing that says the other form factor goes to zero. If you sit on the peak of a form factor, I think its reasonably good, I have no trouble with that. But am I going to see something that just stays perfectly constant as the elastic scattering is going boom, boom, boom. That's what I find stretches my credulity.Blunden:Once you get to about the second or third diffraction minimum then there can be a difference. But at the kind of momentum transfer they're running at it's about a 1 % effect.Bowman:I think the requirement that the beta decay rates be different isn't exactly the problem that faces this experiment, because the beta decay rates could be different just because the charge is different.Adelberger:No, that's all been taken out. That's already removed. That aspect is out. When you get the ft value you've already taken that part out.Bowman:The change isn't in the spatial distribution . . .Adelberger:That's what is seen.Bowman:What I was thinking when I asked my question was that there's probably a fraction of a per cent isospin impurity in the ground state of 1 2 C.126KOWALSKI: You mean of opposite parity?Bowman:No, of different isospin - isospin 1 in the ground state of i2 C. I think that CVC doesn't work for that piece of the 12C ground state wave function.van Oers:What is your objective in terms of error?KOWALSKI:1 0 % of the size of the effect was the original objective which we still hope we can meet ( 2  x 1 0 -7).van Oers:But the Mainz experiment has errors which are smaller.KOWALSKI:Bigger.Neuhausen:15% was the statistical error and 6 or 7% systematic, van Oers:The other question I have is: There was a CEBAF Program AdvisoryCommittee meeting, and a lot was made of the parity experiment. Can you explain, a little bit, that situation?KOWALSKI:I'm not a member of the CEBAF PAC [Program Advisory Committee]. You're just interested in that experiment. Walecka is very interested in parity experiments, and what they'd like to do at CEBAF is a parity experiment on the proton that tests the standard model to 1%. That's a very ambitious goal. I think that what the committee worried about is that you won't get a beam out of CEBAF, Grunder says, until January 1993 at the earliest. That's a long way down the road to make a potentially major committment to an experiment - to which there may be a lot of contributions between now and then: LEP, SLC are among the places that certainly will betesting the standard model to a level that probably approaches that. So I think that the PAC felt it was a little premature and did not take that committment at the moment, for them, in fact, to get ready. They were given encouragement and certainly said that the accelerator should be developed and a source built, because there are a lot of other experiments one wants to do with that machine; including neutron charge form factor, for one; nucleon electric form factor beyond a couple |GeV/c| 2 and other things for which you want polarized electrons. So certainly they were encouraged to build a source; to build into the accelerator all those beautiful things that one needs to be able to do that experiment; but they were asked to wait and evaluate the importance of that experiment over the next127two years in the light of other physics developments. As you all know, if you measure to 1% with a machine like CEBAF one is certainly . . . testing mass scales . . . for deviations from the standard model that would take the SSC to touch. That's an important goal.McDonald:Isn't the point that you want to measure it at a low energy?K O W A L S K I :Sure. Its just like the claim for the 1 2 C, you'd certainly like to sample this thing over a great range in q2. There's no argument about that. Its now been sampled over ten orders of magnitude . .M c D o n a l d :I was referring to your comment about SLC. You want to measure sin2 6 y at low energies relative to the mass of the W. You want to see how it changes as a function of energy.B o w m a n :I think if Walecka were here he would say: Well, those tests are in electron-electron collisions and these tests are where you have the full hadronic mess going on, so you test these neat conservation laws.H a x t o n :But the better the measurement is at high energy the more interesting the measurement is at low energy.A d e l b e r g e r :You mentioned that when the accelerator broke down you could also see it in the source. I assume what's happening is that when the accelerator breaks down the vacuum gets bad and that somehow comes back u p  to . . .K O W A L S K I :They had a recirculator run just before the experiment and at the injection end of the recirculator we have a poor vacuum that gets worse because there's beam dumped and that then takes several days to recover perhaps even a couple weeks, and it was [during] that recovery period, we couldn't even run full pulse length. So every time some window broke, because of that, you'd see the vacuum go up, exactly that, and the source sort of dies by 1 %, 2 %.Adelberger:Does that mean that you would be helped by some more pumping? K O W A L S K I :That's absolutely right. We have a summer shutdown coming, and that's one of the things we want to do. We want to increase differential pumping between the accelerator and the source. Our128accelerator is the same as Mainz, 10-b in the accelerator and you'd like the source to be 1 0 -10.McDonald:When you say lifetime, you mean without the addition of cesium? KOWALSKI:The lifetimes that I have been quoting are a 1/e, no addition ofcesium, no change in the laser intensity.McDonald:But if you do add cesium . . .KOWALSKI:One of the difficulties of the design is that we are not able to addCesium to the source in a simple way.McDonald:I thought that you had added Cesium.KOWALSKI:No.Roy:How much beam time is scheduled?KOWALSKI:With the source operating at design intensity, the statistics for the experiment are reachable in a hundred hours. We will get scheduled as much beam time as the experiment may need, as it goes along, depending what progress it makes. If it doesn't look like it makes much progress, one will cut it off.129A S = 0  W E A K  I N T E R A C T I O N S  A T  T H E  Q UA RK  L E V E LB r u c e  H . J .  M c K e l l a rS c h o o l  o f  P h y s i c s ,  U n i v e r s i t y  o f  M e l b o u r n e ,P a r k v i l l e ,  V i c . ,  3 0 5 2 ,  A u s t r a l i aA B S T R A C TT h e  c a l c u l a t i o n  o f  s h o r t  d i s t a n c e  g l u o n  e x c h a n g e  c o r r e c t i o n s  t o  t h e  A S = 0  w e a k  i n t e r a c t i o n  a t  t h e  q u a r k  l e v e l  i s  d e s c r i b e d .  R e s u l t s  a r e  g i v e n  f o r  t h e  c o e f f i c i e n t s  o f  t h e  3 6  i n d e p e n d e n t  4  q u a r k  o p e r a t o r s  i n v o l v i n g  u d ,  d ,  a n d  s  q u a r k s  e x p l i c i t l y ,  a n d  f o r  t h e  1 6  i n d e p e n d e n t  o p e r a t o r s  w h i c h  r e m a i n  w h e n  s  q u a r k s  a r e  e l i m i n a t e d  a n d  o n l y  u  a n d  d  q u a r k s  e x p l i c i t l y  o c c u r  i n  t h e  o p e r a t o r s .T h e r e  i s  c o n s i d e r a b l e  u n c e r t a i n t y  i n  t h e  i n t e r p r e t a t i o n  o f  p a r i t y  v i o l a t i n g  p h e n o m e n a  i n  n u c l e i  a t  t h e  m o m e n t ,  a s  w i l l  b e  c l e a r  f r o m  t h ep r o c e e d i n g s  o f  t h i s  w o r k s h o p .  T h e r e  i s  o f  c o u r s e  a p o s s i b i l i t y  t h a t  o u rn u c l e a r  s t r u c t u r e  c a l c u l a t i o n s  a r e  i n c o m p l e t e ,  b u t  w h a t  I  w a n t  t oe m p h a s i s e  h e r e  i s  t h e  f a c t  t h a t  t h e  e x i s t i n g  c a l c u l a t i o n s  o f  t h e  w e a kc o u p l i n g  c o n s t a n t s  a t  t h e  h a d r o n i c  l e v e l  h a v e  b e e n  b a s e d  o n  a n  i n a d e q u a t e  q u a r k  l e v e l  d e s c r i p t i o n .L i t t l e  o f  w h a t  I  h a v e  t o  s a y  i s  n e w .  I  r e f e r  y o u  t o  v a r i o u s  p a p e r s  b y  R o b e r  M i l l e r  a n d  m y s e l f  o n  A S = 1  i n t e r a c t i o n s 1 a n d  A S = 0  i n t e r a c t i o n s 2 a t  t h e  q u a r k  l e v e l ,  a n d  t o  o u r  r e v i e w 3 . H o w e v e r  t h e  o n l y  a t t e m p t  t o  c a l c u l a t e  o b s e r v a b l e  e f f e c t s  u s i n g  a q u a r k  l e v e l  H a m i l t o n i a n  w i t h  a l l  o f  t h e  n e c e s s a r y  s t r u c t u r e  i s  t h a t  o f  G o ld m a n  a n d  P r e s t o n 4 a n d  t h e i r  w o r k  w a s  a h i g h  e n e r g y  r a t h e r  t h a n  a l o w  e n e r g y  a p p l i c a t i o n .T h i s  p a p e r  i s  a p e d a g o g i c a l  d i s c u s s i o n  o f  t h e  b a s i c  p h y s i c s  o f  t h i s  A S = 0  w e a k  h a m i l t o n i a n  a t  t h e  q u a r k  l e v e l ,  a n d  i s  d e s i g n e d  t o  e n c o u r a g e  i t s  u s e  i n  f u t u r e  c a l c u l a t i o n s  o f  t h e  w e a k  h a d r o n i c  c o u p l i n g  c o n s t a n t s .  Some p r e v i o u s l y  u n p u b l i s h e d  r e s u l t s 5 o n  a s i m p l i f i e d  a p p r o x i m a t e  f o r m  o f  t h e  A S = 0  H a m i l t o n i a n  a r e  g i v e n  h e r e  t o  f a c i l i t a t e  u s e  o f  t h i s  q u a r k  l e v e l  H a m i l t o n i a n  i n  f u t u r e  c a l c u l a t i o n s .T h e  b a s i c  i d e a  w a s  f i r s t  i n t r o d u c e d  i n t o  t h e  s t u d y  o f  n o n  l e p t o n i c  w e a k  i n t e r a c t i o n s  b y  G a i l l a r d  a n d  L e e 5 -  t h e  s h o r t  d i s t a n c e  QCD i n t e r a c t i o n s  s h o u l d  b e  r e g a r d e d  a s  m o d i f y i n g  t h e  f o u r  f e r m i o n  e f f e c t i v e  w e a k  i n t e r a c t i o n .  T h e s e  QCD c o r r e c t i o n s  i n t r o d u c e  new  o p e r a t o r  s t r u c t u r e s  i n  t h e  w e a k  H a m i l t o n i a n .  A s  a n  e x a m p le  c o n s i d e r  t h e  u d  ■+ u d  w e a k  i n t e r a c t i o n ,  w h i c h  a t  t h e  S U ( 2 )  x  U ( 1 )  g a u g e  t h e o r y  l e v e l  i s  m e d i a t e d  b y  W e x c h a n g e .A s  m^ ” , t h i s  p r o d u c e s  a 4  f e r m i o n  i n t e r a c t i o n  i n  t h e  w e l l  k n o w nw a y$  Iff -7= lui S Y" Mj (1)v 2w h e r e  i  a n d  j  a r e  c o l o u r  i n d i c e s ,  a n d  L  =  - j  (1  +  Y ^ )  i s  t h e  l e f t  h a n d  p r o j e c t i o n  o p e r a t o r .130N ow  we i n t r o d u c e  t h e  QCD i n t e r a c t i o n .  I t  i s  c o n v e n i e n t  t o  d i v i d e  i t  i n t o  l o n g  a n d  s h o r t  d i s t a n c e  e f f e c t s .  T h e  l o n g  d i s t a n c e  e f f e c t s  a r e  i n t i m a t e l y  c o n n e c t e d  w i t h  t h e  c o n f i n e m e n t  r e g i o n ,  a n d  t h e  b e s t  t h a t  c a n  b e  d o n e  a t  t h e  m o m e n t  i s  t o  i n c l u d e  th e m  u s i n g  p h e n o m e n o l o g i c a l  q u a r k  w a v e  f u n c t i o n s .  S h o r t  d i s t a n c e  e f f e c t s  c a n  b e  t r e a t e d  p e r t u r b a t i v e l y  i n  QCD w i t h  som e  d e g r e e  o f  c o n f i d e n c e  b e c a u s e  o f  a s y m p t o t i c  f r e e d o m  o f  QCD.T h e r e  a r e  t w o  c l a s s e s  o f  p e r t u r b a t i v e  c o r r e c t i o n s .  T h e  f i r s t  c l a s s  c o n s i s t s  o f  g l u o n s  l i n k i n g  a l l  p a i r s  o f  q u a r k s  i n  e q u a t i o n  1 .  Some o f  t h e s e  g l u o n  e x c h a n g e s ,  e . g .  t h a t  b e t w e e n  t h e  i n c o m i n g  q u a r k s ,  a l t e r  t h e  c o l o u r  f l o w ,  l e a d i n g  t o  a d d i t i o n a l  o p e r a t o r s  l i k eu .  Y L u 1 d .  L d j  ( 2 )1 U IT h i s  o p e r a t o r  i s  g e n e r a t e d  b e c a u s e  o f  t h e  n o n  A b e l i a n  s t r u c t u r e  o f  t h e  c o l o u r  i n t e r a c t i o n s .T h e  s e c o n d  c l a s s  o f  i n t e r a c t i o n s  i s  g e n e r a t e d  b y  t h e  s o  c a l l e d  " p e n g u i n  d i a g r a m s "  o f  f i g u r e  1 .  T h e s e  w e r e  f i r s t  s t u d i e d  s e r i o u s l y  b y  V a i n s h t e i n ,  Z a k h a r o v  a n d  S h i f m a n 7 i n  t h e  A s = 1  c o n t e x t .  S u p e r f i c i a l l y ,f i g u r e  1 a p p e a r s  t o  r e p r e s e n t  a l o n g  r a n g e  i n t e r a c t i o n  b e c a u s e  o f  t h e  g l u o n  e x c h a n g e .  H o w e v e r ,  g a u g e  i n v a r i a n c e  o f  t h e  g l u o n  c o u p l i n g  f o r c e s  t h e  gd d  v e r t e x  t o  h a v e  a k 2 f a c t o r  ( w h e r e  k  i s  t h e  g l u o n  m o m e n tu m ) ,  a n d  t h i s  c a n c e l s  _2u  H  t h e k  f r o m  t h e  g l u o n  p r o p a g a t o rl e a d i n g  t o  a c o n t a c t  4  f e r m i o n  i n t e r a c t i o n .  B e c a u s e  o f  t h e  g l u o n  F i g  1 .  A p e n g u i n  d i a g r a m .  c o u p l i n g  t h e  p e n g u i n  d i a g r a m s  m i xo p e r a t o r s  o f  l e f t  a n d  r i g h t  c h i r a l i t y .  T h e  n u m e r i c a l  i m p o r t a n c e  o f  p e n g u i n  c o n t r i b u t i o n s  t o  A S = 1  p r o c e s s e s  i s  s t i l l  c o n t r o v e r s i a l .  B u t  i n  A S = 0  i n t e r a c t i o n s  t h e  p e n g u i n s  a r e  n o t  G IM  s u p p r e s s e d ,  a s  c a n  b e  s e e n  f r o m  t h e i r  a p p e a r a n c e  i n  t h e  2 q u a r k  c a l c u l t i o n s  o f  P r e s t o n  a n d  G o l d m a n . 1* Z  e x c h a n g e  p r o c e s s e s  a l s o  c o n t r i b u t e  t o  A S = 0  p r o c e s s e s ,  l e a d i n g  t o  a v e r y  r i c h  s t r u c t u r e  f o r  t h e  b a s i c  w e a k  i n t e r a c t i o n ,  e v e n  b e f o r e  we c o n s i d e r  QCD c o r r e c t i o n s .  I t  i s  t h u s  o b v i o u s  t h a t  t h e  r e s u l t i n g  e f f e c t i v e  H a m i l t o n i a n  i s  g o i n g  t o  h a v e  a n  e v e n  r i c h e r  s t r u c t u r e  a f t e r  t h e  QCD c o r r e c t i o n s ,  a n d  t h i s  i s  i n d e e d  t h e  c a s e .  T h e  f i r s t  a t t e m p t  t o  g e n e r a t e  QCD c o r r e c t i o n s  t o  t h e  A S = 0  i n t e r a c t i o n  w a s  made b y  A l t a r e l l i  e t  a l . 8 .  S h o r t l y  a f t e r  t h e  d e v e l o p m e n t  o f  t h e  p e n g u i n  c o n c e p t s  t h e r e  w e r e  a n u m b e r  o f  a t t e m p t s  t o  a p p l y  th e m  t o  t h e  A S = 0  i n t e r a c t i o n 9 . T h e n  G a l i c  e t  a l . 1 0  a n d  K a r i n o  e t  a l . 11 a t t e m p t e d  a m o re  d e t a i l e d  c a l c u l a t i o n .  H o w e v e r  t h e i r  p u b l i s h e d  r e s u l t s  d o  n o t  i n c l u d e  a l l  1 6  i n d e p e n d e n t  f o u r  f e r m i o n  o p e r a t o r s  f o u n d  t o  b e  n e c e s s a r y  a t  t h e  u , d  f l a v o u r  l e v e l  b y  M i l l e r  a n d  M c K e l l a r 2 a n d  b y  P r e s t o n  a n d  G o ld m a n 1* .I n  t h i s  s e n s e  t h e  o n l y  c o m p l e t e  c a l c u l a t i o n s  a r e  o u r s ,  a n d  t h e  P r e s t o n - G o l d m a n  c a l c u l a t i o n s .  T h e  t w o  c a l c u l a t i o n s  d i f f e r  i np h i l o s o p h y :  P r e s t o n  a n d  G o ld m a n  e x p r e s s  t h e i r  H a m i l t o n i a n  a s  a s e t  o ff o u r  f e r m i o n  o p e r a t o r s  i n v o l v i n g  o n l y  n o n  s t r a n g e  q u a r k s ,  w h e r e  a s  M i l l e r  a n d  M c K e l l a r  i n c l u d e  s t r a n g e  q u a r k s  e x p l i c i t l y  i n  t h e  H a m i l t o n i a n131o p e r a t o r s .T h e  r e a s o n  f o r  r e t a i n i n g  s  q u a r k s  e x p l i c i t l y  i s  t h a t ,  b y  t h e  A p p l e q u i s t  C a r a z z o n e  t h e o r e m * 2 , s  q u a r k s  c a n  b e  d e c o u p l e d  f r o m  t h e  e f f e c t i v e  H a m i l t o n i a n  o n l y  a t  a n  e n e r g y  s c a l e  o n  w h i c h  t h e y  a r e  h e a v y .  B u t  we c a n n o t  r e d u c e  t h e  r e n o r m a l i s a t i o n  p o i n t  y 2 t o o  l o w  i n  e n e r g y  w i t h o u t  e n h a n c i n g  s e c o n d  a n d  h i g h e r  o r d e r  QCD c o r r e c t i o n s  t o  t h e  H a m i l t o n i a n .  T y p i c a l l y  o n e  s h o u l d  e x p e c t  t h e  f i r s t  o r d e r  QCD r e s u l t s  t o  b r e a k  d o w n  a r o u n d  y 2 ~  1 . 3  t o  2 . 0  GeV2 .  T h i s  c l e a r l y  p r e v e n t s  u s  f r o m  c o n s i d e r i n g  t h e  s  q u a r k  a s  a h e a v y  q u a r k  a n d  d e c o u p l i n g  i t  f r o m  t h e  t h e o r y .  W h e n  s  q u a r k s  a r e  r e t a i n e d  t h e  e f f e c t i v e  q u a r k  l e v e l  H a m i l t o n i a n  i s  e x p r e s s e d  i n  t e r m s  o f  3 6  l i n e a r l y  i n d e p e n d e n t  4  q u a r k  o p e r a t o r s ,$ar 36e f f  -  % I C i  3 i  <3 >v 2  1=1T h e  o p e r a t o r s  a n d  c o e f f i c i e n t s  a r e  d i s c u s s e d  i n  d e t a i l  i n  r e f .  4 .  T a b l e1 l i s t s  th e m  f o r  A-^-^ =  2 5 0  MeV a n d  y 2 =  1 . 3  GeV2 . T h e  n o t a t i o n  D ( R )msr e f e r s  t o  t h e  b i l i n e a r  c o v a r i a n t  d  R  d ,  e t c . ,  a n d  t h e  s u f f i x e s  1 a n d8 r e f e r  t o  t h e  c o l o u r  c o u p l i n g s  a .  b 1 c .  d 3 a n d  a .  b 3 c .  d 1i 3 1 3r e s p e c t i v e l y .W h e n  t h e  s  q u a r k  i s  r e t a i n e d  e x p l i c i t l y  i n  t h e  o p e r a t o r  s e t ,  m a t r i x  e l e m e n t s  o f  eff m u s t  o f  c o u r s e  b e  t a k e n  b e t w e e n  h a d r o n i c  w a v ef u n c t i o n s  w h i c h  i n c l u d e  s s  t e r m s  e x p l i c i t l y .  S u c h  w a v e  f u n c t i o n s  a r e  n o t  e a s y  t o  o b t a i n  i n  t h e  l i t e r a t u r e ,  s o  t h e r e  i s  som e p r e s s u r e  t o  t a k e  t h e  n e x t  s t e p  a n d  e l i m i n a t e  t h e  s  q u a r k ,  p u s h i n g  t h e  c a l c u l a t i o n  t o  l o w e r  v a l u e s  o f  y 2 .  I n  r e s p o n s e  t o  t h i s  p r e s s u r e  t a b l e  I I  i n c l u d e s  t h e  c o e f f i c i e n t s  o f  t h e  1 6  o p e r a t o r s  i n v o l v i n g  o n l y  n o n s t r a n g e  q u a r k s ,  c a l c u l a t e d  a t  y 2 =  1 GeV2 , w i t h  t h e  s t r a n g e  q u a r k s  e l i m i n a t e d  b y  f o l l o w i n g  t h e  p r o c e d u r e  o f  r e f . 3 o n e  m o r e  s t e p .  T h e s e  c o e f f i c i e n t s ,  w h i c h  h a v e  n o t  b e e n  p r e v i o u s l y  p u b l i s h e d  a r e  i s s u e d  w i t h  t h e  w a r n i n g  t h a t  o n e  c a n n o t  b e  s u r e  t h a t  h i g h e r  o r d e r  t e r m s  i n  QCD a r e  n o t  i m p o r t a n t  c o r r e c t i o n s ,  a n d  t h a t  t h e  s  q u a r k  i s  s o  c l o s e  t o  t h e  d y n a m i c a l  m a s s  s c a l e  i n v o l v e d  i n  t h e  c a l c u l a t i o n  t h a t  i t  c a n n o t  b e  r e l i a b l y  t r e a t e d  a s  " h e a v y " .P r e s t o n  a n d  G o ld m a n  i n  t h e i r  c a l c u l a t i o n  w o r k e d  d i r e c t l y  w i t h  t h e  t w o  n o n - s t r a n g e  q u a r k s  t h r o u g h o u t  t h e  c a l c u l a t i o n .  T h e y  o f  c o u r s e  f o u n d  t h a t  1 6  l i n e a r l y  i n d e p e n d e n t  o p e r a t o r s  w e r e  r e q u i r e d  t o  r e p r e s e n t  t h e  e f f e c t i v e  H a m i l t o n i a n ,  b u t  made a d i f f e r e n t  c h o i c e  f o r  t h e  o p e r a t o r s  w h i c h  i n h i b i t s  t h e  c o m p a r i s o n  o f  o u r  r e s u l t s .  H o w e v e r ,  o n e  s h o u l d  e x p e c t  d i f f e r e n c e s  f o r  som e o f  t h e  p e n g u i n  o p e r a t o r s ,  a s  t h e y  h a v e  n o t  i n c l u d e d  t h e  c o n t r i b u t i o n s  f r o m  h e a v y  q u a r k s  i n  t h e  l o o p .  N o t w i t h ­s t a n d i n g  t h e s e  o m i s s i o n s ,  t h e  P r e s t o n - G o l d m a n  c a l c u l a t i o n  i s  t h e  o n l y  o n e  t o  d a t e  w h i c h  h a s  a t t e m p t e d  a c o m p a r i s o n  w i t h  e x p e r i m e n t  b a s e d  o n  a c o m p l e t e  s e t  o f  o p e r a t o r s  f o r  t h e  n , d  q u a r k  l e v e l  H a m i l t o n i a n .132T a b l e  1O p e r a t o r  c o e f f i c i e n t s  f o r  t h e  A S = 0  H a m i l t o n i a niif r e e  z e r o t h  o r d e rQCD3 a c t i v e  q u a r k sc o r r e c t e d2 a c t i v e  q i1 D L ) D L ) -1 .1  7 8 6 .1  0 4 0 .1  0 3 32 D L ) U L )  - 1 - . 2 9 1 9 - . 7 8 6 9 - . 8 1 3 13 D L ) U L )  - 8 . 9 5 0 6 1 . 2 2 3 4 1 . 2 4 3 04 D L ) S L )  -1 . 3 5 7 3 . 5 1  2 75 D L ) S L )  - 8 0 . 0 - . 2 9 4 76 U L ) U L )  -1 . 1 1 9 3 . 0 5 9 0 0 . 0 5 5 97 U L ) S L )  -1 - . 2 9 1 9 - . 3 3 5 58 U L ) S L )  - 8 . 0 4 9 3 . 1 0 2 39 S L ) S L ) -1 .1  7 8 6 .1  1 2 21 0 D L ) D R )  - 1 - . 0 6 5 4 - . 0 4 2 4 - 0 . 4 2 911 D L ) D R )  - 8 0 . 0 - . 1 3 6 3 - 0 . 1 4 2 01 2 D L ) U R )  - 1 . 1 3 - 7 . 1 2 8 5 0 . 1 2 7 51 3 D L ) U R )  - 8 0 . 0 . 0 0 9 6 . 0 1 2 91 4 D L ) S R )  -1 - . 0 6 5 4 - . 0 4 2 11 5 D L ) S R )  - 8 0 . 0 - . 1 3 7 21 6 U L ) u R )  - 1 - . 1 0 6 8 - . 0 7 6 8 - . 0 7 5 11 7 U L ) u R )  - 8 0 . 0 - 0 . 1 7 2 3 - . 1 8 7 21 8 U L ) s R )  -1 0 . 0 5 3 4 . 0 6 0 31 9 U L ) s R )  - 8 0 . 0 - . 0 4 2 52 0 S L ) s R )  -1 - . 0 6 5 4 - . 0 4 6 621 S L ) s R )  - 8 0 . 0 - . 1 2 1 52 2 D R ) u L )  —1 . 0 5 3 4 . 0 6 0 5 0 . 0 6 1 02 3 D R ) u L )  - 8 0 . 0 - . 0 4 2 5 - 0 . 0 4 6 72 4 D R ) s L )  -1 - . 0 6 5 4 - . 0 4 6 62 5 D R ) s L )  - 8 0 . 0 - . 1 2 1 52 6 U R ) s L )  —1 .1  3 0 7 .1  21 42 7 U R ) s L )  - 8 0 . 0 . 0 2 4 42 8 D R ) D R )  —1 . 0 0 6 0 . 0 0 8 0 0 . 0 0 8 72 9 D R ) u R )  -1 - . 0 2 3 9 - . 0 3 4 4 - 0 . 0 3 5 13 0 D R ) u R )  - 8 0 . 0 . 0 2 1 4 . 0 2 2 631 D R ) s R )  -1 0 . 0 1 2 0 . 0 0 9 03 2 D R ) s R )  - 8 0 . 0 . 0 0 7 13 3 U R ) u R )  - 1 . 0 2 3 9 . 0 1 9 2 0 . 0 1 7 63 4 U R ) s R )  -1 - . 0 2 3 9 - . 0 3 4 43 5 u R ) s R )  - 8 0 . 0 . 0 2 1 43 6 s R ) s R )  -1 . 0 0 6 0 . 0 0 7 2T o  c o n c l u d e  I  s t r o n g l y  u r g e  t h a t  t h e  f u t u r e  c a l c u l a t i o n s  o f  t h e  h a d r o n i c  l e v e l  p a r i t y  v i o l a t i n g  c o u p l i n g s  o f  t h e  h a d r o n i c  l e v e l  p a r i t y  v i o l a t i n g  c o u p l i n g  w h i c h  m u s t  n o w  b e  a t t e m p t e d  -  a n  u p d a t e  o f  D e s p l a n q u e s ,  D o n o g h u e  a n d  H o l s t e i n 1 8  f o r  e x a m p le  -  s h o u l d  b e  d o n e  o n  t h e  b a s i s  o f  t h e  c o m p l e t e  s e t  o f  o p e r a t o r s  o f  e q n .  ( 2 )  a n d  c o e f f i c i e n t s  o f  t a b l e  1 .  I d e a l l y  s s  t e r m s  i n  t h e  w a v e  f u n c t i o n  s h o u l d  b e  i n c l u d e d ,  a ss h o u l d  r e l a t i v i s t i c  e f f e c t s .133A C K N O W LE D G E M E N TT h i s  w o r k  w a s  s u p p o r t e d  i n  p a r t  b y  t h e  A u s t r a l i a n  R e s e a r c h  G r a n t sC o m m i t t e e .R E F E R E N C E S1 .  R . D . C .  M i l l e r  a n d  B . H . J .  M c K e l l a r ,  A u s t .  J .  P h y s .  3 5 ,  2 3 4  ( 1 9 8 2 ) .2 .  R . D . C .  M i l l e r  a n d  B . H . J .  M c K e l l a r ,  J .  P h y s .  G 1 0 , 11 ( 1 9 8 4 ) .3 .  R . D . C .  M i l l e r  a n d  B . H . J .  M c K e l l a r ,  P h y s .  R e p .  1 0 6 ,  1 6 9  ( 1 9 8 4 ) .4 .  D .  P r e s t o n  a n d  T .  G o ld m a n ,  N u c l .  P h y s .  B 2 1 7 , 31 ( 1 9 8 3 ) ;T .  G o ld m a n  a n d  D .  P r e s t o n ,  N u c l .  P h y s .  B 2 1 7 ,  61 ( 1 9 8 3 ) .5 .  R . D . C .  M i l l e r  a n d  B . H . J .  M c K e l l a r ,  u n p u b l i s h e d  r e s u l t s .6 .  M . K .  G a i l l a r d  a n d  B . W .  L e e ,  P h y s .  R e v .  L e t t .  3 3 , 1 0 8  ( 1 9 7 4 ) .7 .  A . I .  V a i n s h t e i n ,  V .  Z a h k a r o v  a n d  M . A .  S h i f m a n ,  J E T P  L e t t .  2 2 , 5 5( 1 9 7 5 ) .8 .  G. A l t a r e l l i ,  R . K .  E l l i s ,  L .  M a i a n i  a n d  R .  P e t r o n z i o ,  N u c l .  P h y s .B 8 8 ,  2 1 5  ( 1 9 7 5 ) .9 .  J . G .  K b r n e r ,  G. K r a m e r  a n d  J .  W i l l r o d t ,  P h y s .  L e t t .  8 1 B , 3 6 3( 1 9 7 9 ) ;  B .  G u b e r i n a ,  D .  T a d i c  a n d  J .  T r a m p e t i c ' ,  N u c l .  P h y s .  B 1 5 2 , 4 2 9  ( 1 9 7 9 ) ;  F .  B u c e l l a ,  M. L u s i g n o l i ,  L .  M a i a n i ,  a n d  A .  P u g l i e s e ,  N u c l .  P h y s .  B 1 5 2 , 4 4 9  ( 1 9 7 9 ) .1 0 .  H .  G a l i c ,  B .  G u b e r i n a ,  I .  P i c e k ,  D .  T a d i c ^  a n d  J .T r a m p e t i c ' ,  F i z i k a ,  12  1 4 9  ( 1 9 8 0 ) .1 1 .  T .  K a r i n o ,  K .  O h y a  a n d  T .  O k a ,  P r o g .  T h e o r .  P h y s .  6 5 , 6 8 3  ( 1 9 8 1 ) ;6 6 , 1 2 8 9  ( 1 9 8 1 ) .1 2  T .  A p p e l q u i s t  a n d  J .  C a r a z z o n e ,  P h y s .  R e v .  D1 1 ,  2 8 5 6  ( 1 9 7 5 ) ,  K .S y m a n z i k ,  Comm. M a t h .  P h y s .  3 4 , 7 ( 1 9 7 3 ) .1 3 .  B .  D e s p l a n q u e s , J . F .  D o n o g h u e  a n d  B . R .  H o l s t e i n ,  A n n .  P h y s .  1 2 4 j ,4 4 9  ( 1 9 8 0 ) .134DISCUSSION:Iqbal:In terms of understanding things cleanly, would you prefer to see an experiment at 500 MeV or an experiment at Fermi Lab at 100 GeV?McKELLARI would be surprised if one were significantly cleaner than the other.Bowman:Fishbach talked about looking at parity violating effects in transverse momentum, very high p^. Could that help? Presumably that would localize very short distance behavior.McKELLAR:The rationale behind that was the fact that the strong interaction is going down fairly rapidly, the weak interaction can be expected to go down as well. It may help to pick out the two pion interaction.135NOISE FACTORS FOR PARALLEL PLATE IONIZATION CHAMBERSS.A. PageDepartment of Physics, University of Manitoba,Winnipeg, Manitoba R3T 2N2ABSTRACTSources of intrinsic detector noise for ionization chambers operated in d.c. current mode, suitable for parity violation measurements in p-p scattering at intermediate and high energy are discussed. Measurements of the intrinsic noise factor for axial- and transverse-field parallel plate ionization chambers used in high- precision proton beam current measurements at TRIUMF are reported.INTRODUCTIONOne class of experiments which shed light upon parity violating processes involves the measurement of the helicity dependence of the total (or differential) cross-section for elastic scattering from an unpolarized target. Significant effects have been observed for the scattering of longitudinally polarized protons from hydrogen targets and other light nuclei. The parity-violating observable in these experiments is the longitudinal analyzing power, Az, or more generally, Az(6 ) , defined as:da^ Ce) = doMe) (1 ± P A (0)) dfl d£2 z zwhere + and - refer to the incident beam helicity and Pz denotes the magnitude of longitudinal beam polarization. For the case of p-p scattering, there is no nuclear structure enhancement mechanism, and the longitudinal analyzing power is extremely small, of order 10~7. A recent paper by the ETH group1 has demonstrated that remarkably precise measurements of Az in p-p scattering can be achieved: they reportcontrol of statistical and systematic errors in a total cross section measurement at the level ±2 x 1 0 "®, setting the stage for a new class of ultra-high precision experiments.MEASUREMENT OF THE LONGITUDINAL ANALYZING POWER AzThe helicity dependence of the total elastic scattering cross section, Az , can be determined in two ways as illustrated in Figure 1.At low energies, the scattering method is preferable from consideration of the total cross section and effects of energy loss and multiple scattering, which are large at low energy. The aim is to cover the largest solid angle possible with a detector to capture all the scattered particles, and to normalize the scattering yield to the transmitted beam current measured downstream of the target, e.g. with a Faraday cup. At high energy, multiple scattering and energy loss are less severe, and the transmission method is favored. Experiments of the latter type involve highly accurate measurements of the beam current upstream and downstream of the desired target. Note that the asymmetry136Figure 1. Methods of Determlalng k TIonization chambers(a) Transmission method: (12/11)(b) Scattering method: (13/11)observed in a transmission experiment is related to the total scattering asymmetry by A-j. = -SAZ/(1-S)f where S is the total scattering probabi­lity in the target. For small S, this consideration places severe constraints on the accuracy of a transmission apparatus and strongly favors the use of the scattering method, all other factors being equal. In any case, requirements of uniformity and linearity of detector response favor the use of parallel plate ionization chambers to detect the incident, scattered and/or transmitted beam particles. The additional requirement of extremely high statistical precision at the 1 0 -e level implies that the ionization chambers must be operated in d.c. current mode, which introduces a large sensitivity to background and detector noise which must be understood and minimized in the design of a successful parity violation experiment.IONIZATION CHAMBERS —  NOISE SOURCES IN CURRENT MODE OPERATIONAs in any experiment involving the manipulation of analog signals, high precision cross section measurements using ionization chambers in current mode require the elimination of ground loops, minimization of amplifier noise, etc. Sources of noise which are peculiar to parallel plate ionization chambers and enhanced in current mode operation include137chamber gas impurities, pressure fluctuations, microphonics, and temperature fluctuations. In general, these problems can be minimized with careful attention to chamber design. More subtle are the effects of fluctuations in beam parameters which couple to nonlinearities in the chamber response and can lead to large signal fluctuations. For example, at high ionization density, ion pair recombination can couple to fluctuations in the beam current to produce significant noise in the parity violating signal. Similarly, beam position excursions can couple to geometrical asymmetries in the chamber construction to considerably increase the detector noise level.A significant source of noise intrinsic to ionization chambers operated in d.c. current mode is the production of secondary particles in reactions between incoming beam particles and consituents of entrance windows, electrodes, and the ionization medium. These processes give rise to "spallation noise"2; the problem is particularly severe when the secondary fragments have a larger gain in the detector than the primary particles, which is true if the secondaries are heavy fragments from spallation reactions or light, low-energy fragments, e.g. from (p,a) reactions.A rough estimate of the spallation noise contribution can be obtained from the binomial distribution by calculating the detected signal for N primary particles if the probability of a spallation event is 5 (£<<1) and the spallation event has gain GS=RGQ where R>>1 and GQ is the primary gain. One finds a noise to signal ratio given byThe noise factor a allows a convenient assessment of the detector noise contribution relative to "counting statistics" —  e.g. if the detector is used to collect N protons scattered from a target, then the scatter­ing process will introduce relative signal fluctuations of order 1 //N, to which the intrinsic detector noise must be added in quadrature. Clearly, the detector noise contribution is negligible if a<<1, whereas for a>1 , the counting time necessary to perform a high precision experiment may become unreasonably long.To discuss the spallation noise factor further, it is convenient to distinguish two ionization chamber geometries. In an axial field chamber, the collection field is parallel to the beam axis and the particles of interest must pass through the electrodes in order to enter the sense region. In a transverse field chamber, the collection field is perpendicular to the beam axis and the electrodes are not illuminated by the beam; however, it is still necessary for the particles to pass through entrance windows upstream and downstream of the sense region.The spallation noise contribution is determined by the ionization chamber geometry, electrode/window composition, energy and species of the primary particles and the choice of ionization medium. For 800 MeV protons impinging on a narrow gap ( 3  mm) axial field chamber with aluminum electrodes, the Los Alamos group2 found a noise factor a=15, which was prohibitively large. As the beam energy increases, the138spallation noise problem becomes more severe because the primary gain decreases while the yield of secondary fragments in general will increase with increasing beam energy. Spallation can be avoided in the ionization medium by filling the chambers with hydrogen gas. Spallation can also be minimized in the transverse field design by incorporation of a dead region between entrance windows and the active volume of the detector to prevent the majority of the reaction products from contributing to the ionization signal; this is a clear advantage of the transverse field configuration, obtained at the expense of loss of cylindrical symmetry in the detector. In general, the noise factor a cannot be predicted accurately due to lack of inclusive cross-section data for (p,X) reactions on an arbitrary nucleus. However, for a primary proton beam energy in the range 100-500 MeV, the gain ratio R for secondary low energy a particles can be of order 500-1000 depending on the chamber thickness, and estimates of the spallation probability S from existing data indicate that values in the range 1 0 -s - 1 0 -1* are possible for reasonable electrode thicknesses (-25 pm), indicating that even at intermediate energy the noise factor for an axial field chamber could be significantly greater than unity.MEASUREMENTS OF THE NOISE FACTOR a AT 230 MEVA measurement of the longitudinal analyzing power Az and its angular distribution Az(0) in p-p scattering at 230 MeV is in prepara­tion at TRIUMF. As part of a feasibility study for this experiment, a small axial field ionization chamber was constructed for in-beam tests. The chamber was filled with H 2 gas at 1 Atm and contained 2 adjacent 10 cm sense regions (GQ=200). The sense regions were separated by thin electrodes of various materials to study the dependence of the detector noise on electrode composition. A beam of protons (10-30 nA) passed through the detector, and the ionization currents from the two sense regions were amplified by low-noise current-to-voltage preamplifiers, on loan from the Los Alamos 800 MeV collaboration. After matching electronic gains, the noise factor a was deduced from the RMS frequency spectrum of the difference signal, denoted /S(f) , using the expressiona = /S(f)/(/2Ie G0 rg)where I is the beam current, e is the electron charge, GQ is the detector gain, r is the current-to-voltage preamplifier gain and g is the difference amplifier gain. The values of a thus obtained are plotted in Figure 2. The trend of decreasing a with increasing atomic number is consistent with qualitative expectations if the main contribution is from heavy spallation fragments which are suppressed by an increasing Coulomb barrier as Z increases. For the geometry of the test chamber, the theoretical minimum value of a is 0.04, governed by the statistics of the ionization process in hydrogen. For comparison, similar noise measurements were performed with a spallation-minimizing transverse field ionization chamber on loan from Los Alamos2. A noise factor of 0 . 2  was found for that detector, roughly 7 times the theoretical minimum achievable with that geometry.139Figure 2.Detector Noise Factor CL for Various Mo.tcrio.lg in the Axial Chamber, vs. Atomic Number, Z.Further tests were carried out with aluminum electrodes in the axial field test chamber with the aim of determining the relative contributions of spallation and other processes to the detector noise. While the optimum noise factor a = 1.5 was reproducible in several independent test runs, values as high as a = 2 . 3  were observed under conditions of slightly different beam tune for reasons which are not quantitatively understood. Within a 20% error margin, a was found to be insensitive to electrode thickness from 25 pm to 250 pm. The sensiti­vity of a to beam motion was measured by driving the current supply to an upstream aircore steering magnet with a sinusoidal signal at 570 Hz.A sensitivity of 0.005/mm (corresponding to *1x background) was found at this frequency for a set of foils with slightly uneven surfaces, while a second set of electrodes with flatter foils produced by vacuum stretch­ing showed no detectable sensitivity to beam motion. During the noise measurement, beam position excursions were controlled to within several microns with a feedback system, implying a negligible contribution of beam motion to the observed noise factors. The dependence of a on gas pressure was measured for active regions of 1 0  and 1 5 cm in the test chamber and pressures up to 1 Atm. The results are shown in Figure 3;140the detector noise decreases rapidly with increasing pressure, levelling off at a = 1.5 for 10 cm-Atm of H2.Figure 3. Gas Thickness (c m -A tmDetector Noise Factor Ct as a Function of Hp. Gas Pressure in the Axial Field. ChamberSTATISTICAL ACCURACY AND BEAM TIME ESTIMATESThe following expressions for the counting time t(a) required to achieve a fixed statistical accuracy in the measurement of the parity violating longitudinal analyzing power Az are given in terms of the ideal counting time tQ for the case of negligible detector noise. Forthe transmission case:t(a) = tQ(1 ♦ a2 (1+T)/S)and for the scattering asymmetry:t(o) = t (1 + a^/T + a% S/T) o 5 1where S is the total elastic scattering probability, T = (1-S), and for the scattering asymmetry expression, cxg and <jj denote the noise factors141for the scattering detector and in-beam monitor respectively. For the transmission case, note that the detector noise term dominates for S<<1. In particular, for the proposed 230 MeV experiment at TRIUMF where we plan to use a 20 cm LH2 target (S = 0.02), the transmision experiment requires 225 times the noise-free counting time with axial field chambers (a = 1.5), whereas the increase is only a factor of 5 using the low-noise transverse field chambers (a = 0.2). The transmission experiment must therefore be performed with transverse field ionization chambers. For the scattering asymmetry measurement, which we propose to do at TRIUMF using a large axial field chamber, the noise factor of the in-beam normalization detector has negligible effect on the counting time, and the increase over the noise-free time estimate is roughly a factor of 3 -To achieve a statistical precision of ±1 x 10" 8 in the longitudinal analyzing power A z at 230 MeV with the proposed target and detectors at TRIUMF, 200 hr of beamtime would be required for the total scattering method and 300 hr for the transmission method (assuming 1= 1pA, Pz=0.6). By segmenting one collection electrode of the scattering chamber into concentric rings, the angular distribution A z (e) could be measured simultaneously to a statistical precision of ± 1  x 1 0 "® in the worst angular bin (assuming 6 equal bins from 70 -43° lab) in 650 hours. The actual time required to perform the experiment is expected to be significantly longer than the above estimates due to the need for control measurements, optimization and calibration of the apparatus. However, the measured noise factors for parallel plate ionization chambers operated in d.c. current mode at 230 MeV are clearly small enough that attaining the desired statistical precision in a parity violation measurement at TRIUMF is well within reach of established integral counting techniques.REFERENCES1. S. Kistryn et al., Phys. Rev. Lett. 58, 1616 (1987)2. J.D. Bowman et al., Nucl. Instr. Meth. 21_6, 399 (1983)3. J. Birchall et al., TRIUMF Research Proposal E287: "Measurement of Parity Violation in p-p Scattering"142DISCUSSION:McDonald:How much of it is due to each of those two mixings?PAGE:What happens at 230 MeV is that the S-P term contributes almost equally to the P-D term in the angular distribution, in contributing to a shape. However, the S-P term integrates to zero, so it, theoretically, would drop out of the total transmission experiment; whereas it would contribute to the angular distribution. I think Jim [Birchall] has some pictures of that. But I would like to hear what Markus [Simonius] has to say.McDonald:But it's 50-50. The new combination of mixings is about half of that information.PAGE:Yes. Something like that. It's a very significant contribution.. . . 230 MeV is obviously a hard place to do an experiment if the integrated asymmetry is small, on the other hand the physics is different. It's also nice in that we can expect to use a meson exchange model here without being at such a high energy that you can't contribute, and compare it to other proton experiments. So since we are in the situation where we have some very nice experiments, but we don't have a complete set to determine all these couplings, it might be very worthwhile to try to do another one.143Reduction of the effects of transverse polarization in a measurement of parity violation in p-p scattering at 230 MeVJ. BirchallUniversity of Manitoba, Winnipeg, Manitoba, Canada R3T 2N2ABSTRACTAn outline is given of an experiment planned at TRIUMF which will measure an angular distribution of the parity-violating analyzing power Az in proton-proton scattering at 230 MeV. Measurements will be made in six angle bins by a cylindrically symmetric planar ionization chamber. At the same time, a cross-check of the results will be provided by a low- noise ionization detector downstream of the target which will measure the angle-integrated Az. Emphasis is placed on the systematic errors that are expected to be present in this measurement and which are in some cases unlike systematic errors in previous measurements of parity violation in proton scattering. As in other measurements, the major origin of systematic error is the polarization of the beam not being entirely parallel to its momentum. A scanning polarimeter to determine the distribution of these polarization components throughout the beam is sketched.144A measurement of the parity-violating longitudinal analyzing power Az in proton-proton scattering at 230 MeV is planned at TRIUMF. The measurement is twofold -a) measurement of Aztot to a statistical accuracy of 2 xl0 -8,b) measurement of Az(0), with 0 lying between 7° and 43° lab., to aprecision of 2 x 1 0 ~ 8 in each angle bin.The measurements planned will determine the p-p weak coupling constant hp to 0.12xl0- 8  (10% of its DDH "best" value) and h^ to0.15xl0-8 (about 50% of best value).The reason for choosing 230 MeV is that Aztot at that energy is dominated by the weak coupling constant hp, while for previous measurements at 15 and 45 MeV hp and h^ have equal importance. This is because the contribution to the angle-integrated Az due to mixing of S and P partial waves passes through zero near 230 MeV and Az is then due predominantly to mixing in the P and D waves, the amplitude for which is dominated by hp. At lower energy the S-P mixing contribution does not cancel and Aztot is then determined by hp and h^ in about equal measure.One of the main sources of systematic error in such measurements is due to the polarization of the beam not being entirely parallel to its momentum, so that spurious effects from the parity-allowed analyzing power can come into play. Some of these effects will be described in this paper.Ion chamber 2Ion chamber 1Posit ion/size  monitor 1□LH2targetf lPosit ion/size  monitor 2Scanning polarimeter 1Scanning polarimeter 2Ionisat ionchamberFigure 1 Plan view of equipment along the beamline in the region of the target.145A plan view of the equipment is shown in figure 1. The proton beam, incident from the left, passes through a 2 0  cm long liquid hydrogen target (LH2) upstream and downstream of which are two low noise transverse field ionization chambers which measure the fraction of protons transmitted through the target. 1 m downstream of the target is the angular distribution detector, which is a 1 m radius ionization chamber divided into 6 concentric rings spanning the lab angle range from 7° to 43° (90° cm). Each ring is subdivided into 16 segments of equal azimuthal width. By measuring Az(0) from the ratio of scattered current in each segment to the current in the upstream ionization chamber as a function of helicity of the beam and averaging results over the segments in a ring, a number of systematic errors are reduced. At the same time, the magnitude and direction of residual transverse polarization components of the beam can be determined.F ir s t  M oment of  P o la r is a t io nLH2 target s y s te mFigure 2 Illustration of two sources of systematic error.a )  T h e  beam  i s  t r a v e l l i n g  f r o m  l e f t  t o  r i g h t .  A t r a n s v e r s e  c o m p o n e n t  o f  p o l a r i z a t i o n  i s  p o i n t i n g  o u t  o f  t h e  p a g e .b) The beam is travelling into the page. The transverse polarization is pointing up.146Figure 2 shows two sources of systematic error when the beam contains a component of polarization transverse to its momentum. In figure 2 a the proton beam passes through the LH2 target at a distance x from the symmetry axis of the angular distribution detector. Only one segment of the detector is shown. Although the beam is polarized with the spin predominantly along the momentum direction, there is also a small component pointing out of the diagram, at 90° to the segment. Denoting by N± and I± the currents observed in the segment shown and in the upstream ionization chamber for positive and negative beam helicities, respectively, the scattering asymmetry measured by the segment is -e = (N+/I+ - N-/I-)L (N+/I+ + N“/I“ )The apparent parity-violating analyzing power is -and a similar expression can be written for the analyzing power, Ar , measured by the corresponding segment to the right of the beam (not shown). The quantities N+ and N- contain solid angle and cross-section factors for proton scattering from the target and the variation of re­sponse of the ionization chamber with proton energy and angle of incid­ence. When the asymmetry is calculated, these factors cancel, provided the beam does not move on reversal of helicity. Because the beam is dis­placed from the symmetry axis of the detector, A^ and AR are analyzing powers measured at angles differing from the nominal angle by a small amount, 60 -Al = Az(9 - 60) + (PY/P)A(0 - 60),Ar  = AZ(0 + 60) - (PY/P)A(0 + 60).Expanding the parity allowed analyzing power in powers of 60, but keeping Az only to zeroth order, the average measured analyzing power is -Aave = Az(0) - (PY /P)(dA/d0)60= Az(0) - (PY /P)(dA/d0)cos2 (0)(x/L),where L is the distance of the detector downstream from the target (1 m). therefore contains a correction term due to dA/d0, in addition tocL V  6the Az which we want to measure. When averaged over the beamspot, the error in Az is proportional to (dA/d0)< x P y > . The resulting error in the measurement of Az is shown in figure 3a. As seen above, theasymmetry measured by each segment contains a parity allowed asymmetry tofirst order. This can be put to use to monitor the net transverse beam polarization and direction.147Sensitivity to First Moment of Polarization0 lab (degrees)Figure 3 Sensitivity of the angular distribution detector to first moment of polarization.a) The case shown in figure 2a.0lab(degrees)b) The case shown in figure 2b.148Figure 2b shows a second source of systematic error due to transverse polarization components of the beam. The beam is travelling into the page with a small component of polarization, Px , pointing towards a segment of the angular distribution detector. The beam is displaced laterally from the detector symmetry axis. It should be remembered that the segment shown is 1 m downstream from the LH2 target. That there is a false asymmetry due to the parity allowed analyzing power can be seen by considering the count rate in the segment when the spin Px is reversed. To first order, the count rate is proportional to -<t>2N± ~ / (1 ± P Acos<|>)d<j>♦lwhere the limits of integration are -4 > i  ~  J  ~  <t>0  _  <S( t > .<f>2 ~ 1  + <l>0 " 6 <t>> d* ~ LtanQThe false asymmetry is proportional to the parity-allowed analyzing power, A, and is -e ~ Ltan0and the error averaged over the beamspot is proportional to A<yPx>.Errors from these two sources are plotted in figures 3a and 3b.With L = 1 m, a typical error in Az in the range of the angular distribution detector is 2 xl0 ~ 3 times the first moment of polarization expressed in millimetres. An error in Az of 10- 5  is possible if the first moment of transverse polarization of the beam is as high as 0 . 0 1  mm.A first estimate of the error in Az for the transmission measurement gives 6 Ag = 5xl0- 7  x first moment, giving a possible error of about 5xl0- 3  for a first moment of transverse polarization of 0.01 mm.149Figure 4 Preliminary design of the scanning polarimeter.150It Is clearly very Important to be able to monitor transverse polarizations and moments very accurately. Jan Soukup from the University of Alberta has designed a scanning polarimeter operating on the same principle as those at SIN and LANL (figure 4). A graphite blade, 1 mm wide and 3-4 cm thick, scans linearly across the beam (as opposed to the SIN scheme where the blade rotates through the beam).Left, right, up and down detectors measure the current of protons scattered from the blade as the blade moves through the beam. A novel feature is that the detectors are scanned along with the blade, thus keeping the geometry of the detectors fixed with respect to the blades. Systematic errors in the measurement of residual transverse polarization and first moments are thereby reduced by nearly two orders of magnitude. With such an arrangement, the first moment of polarization is measured to a statistical accuracy of better than 10- 3  mm in 1 second for 1 yA of beam incident.Estimates have been made of the relative error of such a scanning polarimeter for a number of beam polarization profiles. The intensity profile, I(x), is taken to be gaussian with 1/e width of 5 mm, while the polarization profile is -a) fully transverse polarized (used for calibrating the sensitivity of the angular distribution detector to transverse polarization),b) transverse polarization proportional to xl(x),c) transverse polarization proportional to x 3 I(x).The so-called "linear" and "cubic" profiles are approximations to the expected actual profile. The predicted relative errors in the measurement of first moment of polarization for beams with these polarization profiles are -a) 1.26x10-5,b) 1.42xl0-5,c) 1.44x10“5#so that a first moment of 0 . 0 1  mm is measured with a systematic error ofabout 10- 7  mm. The relative error is independent of beam position, as itshould be as the detectors scan with the blade.Ideally, the relative error of first moment should be independent of the shape of the polarization profile of the beam. This is so that the scanning polarimeter can be used reliably to correct asymmetries measured by the angular distribution detector, since a transverse-polarized beam is used to calibrate the sensitivity of the parity detector to transverse polarization, while a beam of quite different polarization profile is used in the parity measurement itself. The change in first moment error when one goes from a fully transverse-polarized beam to "linear" or "cubic" is predicted to be about 2 xl0 “ 6 times the first moment. Conse­quently, when one uses a fully transverse polarized beam to calibrate the angular distribution detector, the resulting error in the corrected Az is 4xl0-11, for a first moment of 10- 2  mm and a sensitivity of the angular distribution detector to first moment of 2 xl0 - 3  mm-1.151The first moment has two components -<xPy> = <xPy> intrinsic + x.Py ,where < x P y >  intrinsic is measured with respect to the centre of gravity of the beam. The first contribution may be minimised by careful beam optics, the second by using servo systems to minimise both x  and Py (and y and P ). Typically, the TRIUMF beam wanders by a few hundred microns in a fraction of a second. Taking x  = 0.6 mm and P y  = 0.001, the contribution to the first moment from this source is 6 xl0 -i+ mm and the Az error is of the order of 1.2xl0-6, which is large. Tests with feedback electronics and beam steering elements on loan from LANL show that beam wander can be kept to 2y, thereby reducing the contribution to the Az error to about 4xl0~9.A complication, although not a crucial one, is introduced by the use of a servo system to steer the beam back into position - the spin is precessed out of the momentum direction. Taking an upper limit for the beam wander before feedback is applied as 0 . 6  mm in a distance of 2 . 6  m, then at 230 MeV the beam steering will precess the proton spin by 5xl0_l+ radians with respect to its momentum, thereby generating a transverse component of beam polarization. The corresponding error in Az when the beam is steered back to within 2 y of the detector symmetry axis is 2 xl0 -9.I have attempted to give a flavour of some of the systematic errors in a new measurement of parity violation in p-p scattering. The experiment, although very difficult, is worthwhile and will give new information on the weak p-p coupling constants. Work on systematic errors is continuing, in particular, on the errors to be expected in the transmission part of the experiment, to which little attention has been given by us so far.152DISCUSSION:Haeberli:None of this takes into account that the energies change with angle, that the ionization density changes, that the solid angle changes.Is that correct, that's all neglected here?BIRCHALL:That actually drops out. Because if one has a beam displacement from the symmetry axis and, then reverses the helicity and measures the relative change in count rate in one segment, that cancels out. If one goes down to another segment, then the energy response of the ion chamber is different, but it does cancel out by taking the ratio.Simonius:How large do you expect the modulation in each segment to be? BIRCHALL:If we take a distance of a meter of the detector from the target, then you can see that the false asymmetry is of the order 1 - 2  times 10- 3  times the first moment measured in millimeters. If the first moment is 1 0 “ 2 mm, that means we have 1 0 “5, which is a large effect. One has to calibrate the detector very accurately.Bowman:That's not 10- 5  after you do the azimuthal . . .BIRCHALL:No, that's the uncorrected false asymmetry in each segment. . .Bowman:For some <f>.BIRCHALL:Yes. If you have ciculating polarization then you get the same effect for each segment. If you just have a transverse polarized beam displaced from the axis, that would have a cos(<J>) dependence as you go around each segment of the ring - a different effect.Haeberli:What's a reasonable number for the beam diameter?BIRCHALL:5-10 mm.Haeberli:So the first moment of transverse polarization is a few times 10-2. BIRCHALL:However, the beam position itself can be controlled to a couple of microns.153Haeberli:But that then is rather hard.Simonius:What of the moment due to a displacement and an average transverse beam polarization?BIRCHALL:We can measure the average beam polarization to a few times 10- 5  in a second. We have to have a feedback to a spin precession magnet upstream which precesses the beam in such a way that we have a longitudinally polarized beam.Bowman:<yPx> is a phase space correlation, that could be non-zero even in the case that the beam were perfectly centered and had no average [transverse] polarization.BIRCHALL:That's right, from a circulating polarization, yes. The first moment of polarization has to be measured. The sensitivity of the detector to these moments also has to be measured.McDonald:How big was this circulating polarization component [at SIN]? I realize this changes from polarized source to source, but if you expressed it in mm?Haeberli:There are numbers for the moments in the papers.Simonius:The beam is much smaller than 5 to 10 mm.McDonald:The circulating polarization is an inherent quality of the beam thatwill scale as you blow it up or make it smaller.Mischke:If it doesn't arise at the source but rather it arises at the transport, then it depends on how off-center you go through the fringe fields of bending magnets and then in fact it gets worse the bigger your beam is.McDonald:I'm just trying to get a feel for it. If this circulating polarization is a few per cent then you have to measure what it isto a part in ten to the third or something like that.BIRCHALL:The scanning polarimeter can measure transverse polarization154components to around 4 times 10- 5  in a second.Simonius:You may have trouble with the linearity of your detectors if you want to be sure that this drops out. You say, by taking the difference you go down to 1 0 -9; by taking the difference of two signals of 3 times 10-lt. The linearity of the electronics for these two detectors may not be the same.BIRCHALL:One calibrates the linearity with the fully transverse polarized beam, one measures the non-linearity.Simonius:Whether you can do that is questionable. That's highly demanding for the identity of these two systems.BIRCHALL:It turns out to be identical.Simonius:You want to subtract two measurements of several times 10-1+ to get a result of 1 0 -9?McDonald:Another possible problem is the fact that you are looking at a number of segments with different effective solid angles from your target than the monitoring polarimeter will have. Now if you have some change in the shape of this circulating polarization component, let's say as a function of distance along your detector in the z direction, then they may have different sensitivities. My question is that can you really, with one measurement of that, get sufficient information, that you will be able to correct all your segments.Simonius:You need two at least.BIRCHALL:That's right . . .  at least two measurements.McDonald:Two should be sufficient, right?Bowman:Because in a drift space there's position and angle and that's it.McDonald:Right.Haeberli:The plan will be to have two such polarimeters?155BIRCHALL:Yes.Haeberli:Basically, its a good idea to move the whole thing, because, as you say, you avoid all kinds of complications, but it also imposes some limitations on how rapidly you can sample the beam properties.BIRCHALL:The spin flip rate will be 1 or 2 Hz, so that is the sort of rate that we scan the beam.156MEASUREMENT OF PARITY VIOLATION IN THE PHOTODISINTEGRATION OF DEUTERIUM AND IN THE PRODUCTION OF BREMSSTRAHLUNG ON TANTALUME.D. E a r l e ,  A.B. M c D o n a l d 1 , S.H. K i d n e r ,E . T . H .  C l i f f o r d 2 , J.J. Hil l ,  G.H. Ke e c h ,C h a l k  R i v e r  N u c l e a r  L a b o r a t o r i e s ,  C h a l k  Ri v e r ,  O n t a r i o ,T.E. C h u p p 3 , M.B. S c h n e i d e r 4 ,P h y s i c s  D e p t . , P r i n c e t o n  U n i v e r s i t y ,  P r i n c e t o n ,  N J  0 8 5 4 0ABSTRACTT h e  c i r c u l a r  p o l a r i z a t i o n  d e p e n d e n t  c o m p o n e n t  o f  t h e t o t a l  c r o s s  s e c t i o n  f o r  p h o t o d i s i n t e g r a t i o n  of d e u t e r i u m  h a s  b e e n  m e a s u r e d  to b e  ( 2 . 7 ± 2 . 8 )  x 1 0 " 6 f o r  b r e m s s t r a h l u n g  w i t h  a n  e n d  p o i n t  o f  4 . 1  M e V  a n d  ( 7 . 7 ± 5 . 3 )  x 1 0 “ 6 f o r  an e n d  p o i n t  o f  3.2 MeV. T h e  h e l i c i t y  d e p e n d e n t  c o m p o n e n t  o f  t h e  t o t a l  c r o s s  s e c t i o n  f o r  t h e  p r o d u c t i o n  o f  b r e m s s t r a h l u n g  o n  t a n t a l u m  b y  l o n g i t u d i n a l l y  p o l a r i z e d  e l e c t r o n s  h a s  b e e n  m e a s u r e d  to b e  ( 0 . 6 3 ± 0 . 7 0 )  x 1 0 “ 6 f o r  a n  e l e c t r o n  e n e r g y  of 4 . 1  M e V  a n d  (3.1 ± 1 . 5 )  x 1 0 “ 6 at 3.2 MeV. A l l  m e a s u r e m e n t s  ar e  i n  a g r e e m e n t  w i t h  t h e o r e t i c a l  p r e d i c t i o n s  f o r  t h e s e  p r o c e s s e s .INTRODUCTIONM e a s u r e m e n t s  o f  p a r i t y  v i o l a t i o n  i n  l i g h t  n u c l e a r  s y s t e m s  h a v e  b e e n  p u r s u e d  f o r  m a n y  y e a r s  w i t h  t h e  o b j e c t i v e  o f  d e f i n i n g  t h e  w e a k  n u c l e o n - n u c l e o n  i n t e r a c t i o n  as c o m p l e t e l y  as p o s s i b l e .  T h e s e  e x p e r i m e n t s  h a v e  b e e n  p e r f o r m e d  o n  b a s i c  t w o - n u c l e o n  systems' 1~ 4 ’ a n d  a l s o  o n  m o r e  c o m p l e x  n u c l e i ' 5 ’ , w i t h  e m p h a s i s  o n  t h o s e  n u c l e i  w h e r e  p a r i t y  v i o l a t i o n  e f f e c t s  a r e  e n h a n c e d  a n d  t h e  n u c l e a r  s t r u c t u r e  is w e l l  u n d e r s t o o d .  In a d d i t i o n ,  t h e o r e t i c a l  c a l c u l a t i o n s  h a v e  b e e n  p e r f o r m e d  e x t e n s i v e l y .  S i g n i f i c a n t  p r o g r e s s  h a s  b e e n  m a d e ' 6 ’ t h r o u g h  t h e  i n c l u s i o n  o f  t h e  G l a s h o w - W e i n b e r g - S a l a m  (GWS) S t a n d a r d  M o d e l  w i t h  q u a r k  m o d e l s  o f  t h e  n u c l e o n  a n d  a c c e p t a b l e  r a n g e s  o f  p a r a m e t e r s  f o r  t h e  w e a k  n u c l e o n - n u c l e o n  i n t e r a c t i o n  h a v e  b e e n  d e f i n e d .P a r i t y  v i o l a t i o n  m e a s u r e m e n t s  in t h e  t w o - n u c l e o n  s y s t e m  h a v e  b e e n  p e r f o r m e d  p r e v i o u s l y  i n  o n l y  t h r e e  cases. M e a s u r e m e n t s  h a v e  b e e n  m a d e  f o r  t h e  s c a t t e r i n g  of p o l a r i z e d  p r o t o n s  f r o m  h y d r o g e n  at s e v e r a l  ene r g i e s '  11 , w i t hP r e s e n t  A d d r e s s :  1) P h y s i c s  D e p a r t m e n t ,  P r i n c e t o nU n i v e r s i t y ,  P r i n c e t o n ,  NJ. 2) T R I U M F ,  U n i v e r s i t y  o f  B r i t i s h  C o l u m b i a ,  V a n c o u v e r ,  BC. 3) P h y s i c s  D e p a r t m e n t ,  H a r v a r d  U n i v e r s i t y ,  C a m b r i d g e ,  M a ss. 4) P h y s i c s  D e p a r t m e n t ,  N o r t h  C a r o l i n a  S t a t e  U n i v e r s i t y ,  R a l e i g h ,  NC.157s t a t i s t i c a l l y  s i g n i f i c a n t  r e s u l t s  in a g r e e m e n t  w i t h  t h e o r y .  T h e  p a r i t y  v i o l a t i n g  c o m p o n e n t  in t h e  s c a t t e r i n g  c r o s s  s e c t i o n  w a s  f o u n d  to b e  a b o u t  2 x 10' 7 . M e a s u r e m e n t s  h a v e  a l s o  b e e n  m a d e  f o r  t h e  p a r i t y  v i o l a t i n g  a s y m m e t r y  in g a m m a  r a y  e m i s s i o n  f o l l o w i n g  t h e  c a p t u r e  o f  p o l a r i z e d  c o l d  n e u t r o n s .  T h e  m o s t  r e c e n t  r e s u l t ' 2 ' is A  = (-5±5) x 10'*, to b e  c o m p a r e d  w i t h  a t h e o r e t i c a l  p r e d i c t i o n  o f  0 . 2  x 1 0 ' 7 . O n e  o f  t h e  f i r s t  m e a s u r e m e n t s  in t h e  t w o - n u c l e o n  s y s t e m  w a s  p e r f o r m e d  b y  L o b a s h o v ,  et. a l . ( 3 ) , w h o  m e a s u r e d  t h e  c i r c u l a r  p o l a r i z a t i o n  o f  g a m m a  r a y s  p r o d u c e d  i n  t h e  c a p t u r e  of t h e r m a l  n e u t r o n s  i n  h y d r o g e n  to b e  ( - 1 . 3 0 ±  0 . 45) x 1 0 ' 6 , in d i s a g r e e m e n t  w i t h  m a n y  t h e o r e t i c a l  c a l c u l a t i o n s  p r e d i c t i n g  a v a l u e  l e s s  t h a n  0 . 5  x 1 0 ' 8 . R e c e n t  m e a s u r e m e n t s ' 4 ' b y  the s a m e  g r o u p  h a v e  u n c o v e r e d  p r o b l e m s  i n  t h e  o r i g i n a l  w o r k  a n d  d e f i n e d  a n  u p p e r  l i m i t  o f  0 . 5  x 1 0 ' 7 f o r  t h e  c i r c u l a r  p o l a r i z a t i o n .W e  h a v e  p e r f o r m e d  a m e a s u r e m e n t  o f  t h e  i n v e r s e  r e a c t i o n  [ D( 2f, n)p] a n d  m e a s u r e d  t h e  h e l i c i t y  d e p e n d e n c e  in t h e  c r o s s  s e c t i o n  f o r  t h e  p h o t o d i s i n t e g r a t i o n  o f  d e u t e r i u m  b y  c i r c u l a r l y  p o l a r i z e d  g a m m a  rays. N e a r  t h r e s h o l d  t h i s  r e a c t i o n  is s e n s i t i v e  to t h e  s a m e  c o m p o n e n t s  of t h e  w e a k  n u c l e o n - n u c l e o n  i n t e r a c t i o n  as t h e  t h e r m a l  n e u t r o n  c a p t u r e ,  b u t  at h i g h e r  e n e r g i e s  it is s e n s i t i v e  to a d d i t i o n a lc o m p o n e n t s .  F o r  all e n e r g i e s  w i t h i n  2 M e V  a b o v e  t h r e s h o l d ,  t h e  h e l i c i t y  d e p e n d e n t  p a r t  o f  t h e  c r o s s  s e c t i o n  is c a l c u l a t e d ' 7 • 8 ’ to b e  l e s s  t h a n  5 x 1 0 ' 8 o f  t h e  t o t a l  c r o s s  s e c t i o n .  W e  h a v e  a l s o  p e r f o r m e d  a s e n s i t i v e  m e a s u r e m e n t  of t h e  c h a n g e  in i n t e n s i t y  of b r e m s s t r a h l u n g  p r o d u c e d  b y  b e a m s  o f  p o l a r i z e d  e l e c t r o n s  s t r i k i n g  t a n t a l u m  at e n e r g i e s  o f  3.2 a n d  4.1 MeV. T h i s  is t h e  f i r s t  s e a r c h  f o r  p a r i t y  v i o l a t i o n  i n  t h i s  p r o c e s s .  E x t r a p o l a t i o n s  o f  t h e  c a l c u l a t i o n s  ofK e r i m o v  a n d  S a f i n ' 9 ’ i n d i c a t e  t h a t  e f f e c t s  s m a l l e r  t h a n  a b o u t  1 x 1 0 ' 9 a r e  e x p e c t e d  f r o m  t h e  G W S  S t a n d a r d  m o d e l  at t h e s e  e n e r g i e s .EXPERIMENTA  s c h e m a t i c  d i a g r a m  of t h e  e x p e r i m e n t  is p r e s e n t e d  inFig. 1. P o l a r i z e d  e l e c t r o n s  a r e  g e n e r a t e d  in a g a l l i u ma r s e n i d e  p h o t o e m i s s i o n  s o u r c e  m o d e l l e d  o n  t h e  P e g g y  II s o u r c e  at S L A C ' 1 0 ’ a n d  a r e  a c c e l e r a t e d  i n  t h e  E l e c t r o n  T e s t  A c c e l e r a t o r  (ETA) at C h a l k  R i v e r  N u c l e a r  L a b o r a t o r i e s .  E l e c t r o n s  at e n e r g i e s  of 3.2 or 4.1 M e V  s t r i k e  a w a t e r - c o o l e d  t a n t a l u m  r a d i a t o r ,  p r o d u c i n g  b r e m s s t r a h l u n g  fo r  w h i c h  t h e  h i g h e s t  e n e r g y  g a m m a s  h a v e  a c i r c u l a r  p o l a r i z a t i o n  n e a r l y  e q u a l  to t h e  l o n g i t u d i n a l  p o l a r i z a t i o n  o f  th e  e l e c t r o n s ,  a b o u t  35%. T h e  b r e m s s t r a h l u n g  s t r i k e  a 0.2 c u b i c  m e t e r  D 2 0 t a r g e t  a n d  g e n e r a t e  n e u t r o n s  w h i c h  a r e  t h e r m a l i z e d  b y  t h e  D 2 0  a n d  d e t e c t e d  i n  b o r o n - l i n e d ,  h i g h  e f f i c i e n c y  n e u t r o n  d e t e c t o r s .  T h e  p h o t o d i s i n t e g r a t i o n  m e a s u r e m e n t  s e a r c h e s  f o r  a c h a n g e  i n  t o t a l  n e u t r o n  i n t e n s i t y  c o r r e l a t e d  w i t h  t h e  r e v e r s a l  o f  t h e  g a m m a  r a y  c i r c u l a r  p o l a r i z a t i o n .  To o b s e r v e  a n y  c h a n g e  i n  g a m m a  r a y  i n t e n s i t y  u p o n  r e v e r s a l  of1581. S c h e m a t i c  d i a g r a m  o f  t h e  e x p e r i m e n t .159t h e  e l e c t r o n  h e l i c i t y ,  a l a r g e  area, g a m m a - s e n s i t i v e  d e t e c t o r  w a s  m o u n t e d  d i r e c t l y  o n  t h e  b r e m s s t r a h l u n g  r a d i a t o r .T h e  m a i n  d i f f i c u l t y  i n  s e a r c h i n g  f o r  s m a l l  p a r i t y  v i o l a t i n g  e f f e c t s  c o r r e l a t e d  w i t h  p o l a r i z a t i o n  r e v e r s a l  is e n s u r i n g  t h a t  t h e  o b s e r v e d  i n t e n s i t y  c h a n g e s  h a v e  n o t  b e e n  i n d u c e d  b y  s o m e  o t h e r  s y s t e m a t i c  e f f e c t  u n r e l a t e d  to th e c h a n g e  i n  e l e c t r o n  p o l a r i z a t i o n .  T h e r e f o r e  m a n y  p i e c e s  of s u b s i d i a r y  e q u i p m e n t  w e r e  e m p l o y e d  to m o n i t o r  a w i d e  v a r i e t y  o f  p a r a m e t e r s  w h i c h  c o u l d  c h a n g e  w h e n  t h e  e l e c t r o n  p o l a r i z a t i o n  is r e v e r s e d  a n d  i n d u c e  a g a m m a  r a y  o r  n e u t r o n  i n t e n s i t y  c h a n g e .  A  s e r i e s  o f  m e a s u r e m e n t s  w e r e  p e r f o r m e d  d u r i n g  w h i c h  t h e s e  p a r a m e t e r s  w e r e  a r t i f i c a l l y  v a r i e d  w i t h  l a r g e  a m p l i t u d e  a n d  t h e  s e n s i t i v i t y  o f  t h e  m o n i t o r s  w a s  d e t e r m i n e d .  A t  t h e  s a m e  time, t h e  s e n s i t i v i t y  o f  t h e  t o t a l  g a m m a  r a y  i n t e n s i t y  a n d  t h e  t o t a l  n e u t r o n  i n t e n s i t y  to t h e s e  p a r a m e t e r s  w a s  d e t e r m i n e d .  In t h i s  way, b y  o b s e r v i n g  t h e s e  m o n i t o r s  d u r i n g  t h e  e x t e n d e d  r u n n i n g  t i m e  u s e d  to m e a s u r e  p a r i t y  v i o l a t i o n ,  it w a s  p o s s i b l e  to d e t e r m i n e  c o r r e c t i o n s  to t h e  g a m m a  r a y  a n d  n e u t r o n  i n t e n s i t y  a r i s i n g  f r o m  e f f e c t s  o t h e r  t h a n  p a r i t y  v i o l a t i o n .T h e  p o l a r i z e d  e l e c t r o n  s o u r c e  d e s i g n  w a s  n e a r l y  i d e n t i c a l  to P e g g y  I I <10> w i t h  t h e  a d d i t i o n  o f  a i n t e r n a l  c r y o p u m p  to o b t a i n  p r e s s u r e s  l e s s  t h a n  1 x 1 0 " 1 0  T o r r  m o r e  ea s i l y .  P o l a r i z e d  e l e c t r o n s  w e r e  e m i t t e d  f r o m  a p - t y p e  G a A s  c r y s t a l ,  d o p e d  w i t h  1 x 1 0 " 19 Z n  a t o m s  p e r  c u b i c  c e n t i m e t e r ,  c l e a v e d  in t h e  100 d i r e c t i o n ,  m e c h a n i c a l l y  p o l i s h e d  a n d  c o a t e d  w i t h  a f e w  m o n o l a y e r s  o f  C e s i u m  a n d  o x y g e n .  T o  e n s u r e  s u r f a c e  c l e a n l i n e s s ,  t h e  c r y s t a l s  w e r e  c h e m i c a l l y  c l e a n e d ,  a n o d i z e d  a n d  t h e n  e t c h e d  i m m e d i a t e l y  b e f o r e  i n s t a l l a t i o n  in t h e  v a c u u m  s y stem. C i r c u l a r l y - p o l a r i z e d  l a s e r  l i g h t  w a s  o b t a i n e d  f r o m  a K r - i o n  laser, w h i c h  p r o v i d e d  a c o n t i n u o u s  b e a m  of u p  to 1.5 w a t t s  of 752 n a n o m e t e r  light. T h e  c i r c u l a r  p o l a r i z a t i o n  w a s  p r o d u c e d  b y  a h i g h  q u a l i t y  l i n e a r  p o l a r i z i n g  p r i s m ,  f o l l o w e d  b y  a P o c k e l l s  c e l l  a c t i n g  as a q u a r t e r - w a v e  p l a t e .  T h e  c i r c u l a r  p o l a r i z a t i o n  c o u l d  b e  r a p i d l y  r e v e r s e d  b y  r e v e r s i n g  t h e  2 6 0 0  v o l t  b i a s  o n  th e P o c k e l l s  cell. T h i s  r e v e r s a l  p r o d u c e d  a c o r r e s p o n d i n g  r e v e r s a l  o f  t h e  l o n g i t u d i n a l  p o l a r i z a t i o n  of t h e  e l e c t r o n  b e a m  w i t h  a f r a c t i o n a l  c h a n g e  o f  l e s s  t h a n  7 x 10 5 in e l e c t r o n  b e a m  i n t e n s i t y .  T h e  G a A s  c r y s t a l s  w e r e  c l e a n e d  in p l a c e  b y  h e a t i n g  w i t h  an e l e c t r o n  g u n  in a s e p a r a t e  v a c u u m  c h a m b e r  b e h i n d  t h e  c r y s t a l  m o u n t i n g  p l a t e .  A f t e r  c l e a n i n g  a n d  a c t i v a t i o n  w i t h  c e s i u m  a n d  o x y g e n ,  t h e y  p r o v i d e d  s t a b l e  b e a m  c u r r e n t s  b e t w e e n  3 5 0  a n d  7 0 0  m i c r o a m p e r e s  f o r  1.2 w a t t s  o f  l a s e r  light. T h i s  w a s  a c c e l e r a t e d  w i t h  a b o u t  7 0 %  t r a n s m i s s i o n  e f f i c i e n c y  to t h e  t a r g e t .  S o u r c e  e m i s s i o n  t y p i c a l l y  d e c l i n e d  w i t h  a h a l f  l i f e  o f  a f e w  h o u r s  b u t  c o u l d  b e  r e t u r n e d  to n e a r  t h e  o r i g i n a l  l e v e l  b y  t h e  a d d i t i o n  o f  a s m a l l  a m o u n t  o f  c e s i u m  a n d  o x y g e n .F i g u r e  2 s h o w s  a m o r e  c o m p l e t e  s c h e m a t i c  r e p r e s e n t a t i o n3694 C160!. Schematic drawing of the experimental apparatus161o f  t h e  e x p e r i m e n t ,  s h o w i n g  t h e  p o l a r i z e d  s o u rce, a c c e l e r a t o r  a n d  e x p e r i m e n t  area. T h e  p o l a r i z e d  e l e c t r o n s  w e r e  a c c e l e r a t e d  to 3. 2  o r  4 . 1  M e V  a n d  s t r u c k  a 0 . 0 4  c m  t h i c k  t a n t a l u m  b r e m s s t r a h l u n g  r a d i a t o r ,  b a c k e d  b y  a 1 c m  t h i c k  s t r e a m  o f  f l o w i n g  w a t e r  f o r  c o o l i n g .  T h e  u s e  o f  a t h i n  t a n t a l u m  r a d i a t o r  m a x i m i z e d  t h e  b r e m s s t r a h l u n g  p r o d u c t i o n  a b o v e  t h e  p h o t o d i s i n t e g r a t i o n  t h r e s h o l d .  T o  r e d u c e  th e  e n e r g y  d e n s i t y ,  t h e  b e a m  w a s  r a p i d l y  s w e p t  v e r t i c a l l y  (300 Hz) a n d  h o r i z o n t a l l y  (360 Hz) b y  s i n u s o i d a l  s i g n a l s  a p p l i e d  to a p a i r  o f  d e f l e c t i o n  coils, c r e a t i n g  a 10 c m  b y  10 c m  b e a m  s p o t  o n  t h e  t a n t a l u m .  T h e  b r e m s s t r a h l u n g  r a d i a t o r  w a s  r e e n t r a n t  18 c m  i n t o  a 0 . 6  m  l o n g  b y  0. 5  m  s q u a r e  D 2 0 t a r g e t  can. T h e  g e o m e t r y  o f  t h e  c a n  w a s  c h o s e n  t h r o u g h  M o n t e  C a r l o  c a l c u l a t i o n s  to p r o v i d e  a c o m p r o m i s e  b e t w e e n  e f f i c i e n t  t h e r m a l i z a t i o n  f o r  c a p t u r e  of n e u t r o n s  in t h e  B d e t e c t o r s  a n d  m i n i m a l  s c a t t e r i n g  t i m e  i n  t h e  D 2 0, e n a b l i n g  f a s t  h e l i c i t y  s w i t c h i n g .  A b o u t  6 0 %  of t h e  n e u t r o n s  p r o d u c e d  b y  b r e m s s t r a h l u n g  a b o v e  t h e  2 . 2  M e V  t h r e s h o l d  w e r e  t h e r m a l i z e d  a n d  a b o u t  9 0 %  o f  t h e s e  h a d  l e f t  t h e  D 2 0  v o l u m e  w i t h i n  a t i m e  o f  2 m i l l i s e c o n d s .  E l e c t r o n  c u r r e n t s  c o u l d  b e  m e a s u r e d  f r o m  t h e  t a n t a l u m  r a d i a t o r  a n d  f r o m  a l a r g e  a p e r t u r e  o b s c u r i n g  all b e a m  t r a j e c t o r i e s  o u t s i d e  t h e  t a n t a l u m  p l a t e .E l e c t r o n  p o l a r i z a t i o n  w a s  m e a s u r e d  b y  M o t t  s c a t t e r i n g  f r o m  g o l d  f o i l s  at 60 k e V  in a n  o f f - l i n e  a p p a r a t u s .  T h e s e  m e a s u r e m e n t s  i n d i c a t e d  p o l a r i z a t i o n s  of a b o u t  3 5 %  u n d e r  t y p i c a l  o p e r a t i n g  c o n d i t i o n s .  T h e  2f r a y  p o l a r i z a t i o n  w a s  d e t e r m i n e d  s e v e r a l  t i m e s  d u r i n g  t h e  e x p e r i m e n t  b y  m e a s u r i n g  t h e  c i r c u l a r  p o l a r i z a t i o n  of t h e  b r e m s s t r a h l u n g  at t h e  t a r g e t  e n d  o f  t h e  a c c e l e r a t o r .  F o r  t h e s e  m e a s u r e m e n t s ,  t h e  b r e m s s t r a h l u n g  p a s s e d  t h r o u g h  a 10 c m  long, i r o n  t r a n s m i s s i o n  p o l a r i m e t e r  ( d e v e l o p e d  fo r  a m e a s u r e m e n t  of p a r i t y  v i o l a t i o n  i n  2 1 N e ( 1 1 ) ) a n d  w e r e  d e t e c t e d  i n  a 3 c u b i c  c e n t i m e t e r  i n t r i n s i c  g e r m a n i u m  d e t e c t o r .  T h e  c i r c u l a r  p o l a r i z a t i o n  as a f u n c t i o n  o f  g a m m a  r a y  e n e r g y  w a s  i n f e r r e d  f r o m  t h e  k n o w n  d e t e c t o r  r e s p o n s e  f u n c t i o n  a n d  p o l a r i m e t e r  e f f i c i e n c y .  N e a r  t h e  t i p  o f  t h e  b r e m s s t r a h l u n g  s p e c t r u m  the m e a s u r e d  g a m m a  r a y  c i r c u l a r  p o l a r i z a t i o n  w a s  30%.F i v e  s i d e s  o f  t h e  D 2 0 t a r g e t  w e r e  c o v e r e d  w i t h  a t o t a l  of 20 n e u t r o n  d e t e c t o r s  d e v e l o p e d  at C h a l k  R i v e r  f o r  th e  e x p e r i m e n t ' 1 2 ’ . T h e s e  d e t e c t o r s  c o n t a i n e d  t w o  r e g i o n s  of  h y d r o g e n - f i l l e d  i o n i z a t i o n  c h a m b e r .  T h e  f i r s t  r e g i o n  c o n t a i n e d  9 z i r c a l l o y  p l a t e s ,  0 . 2 5  m m  th i c k ,  c o a t e d  o n  tw o  s i d e s  w i t h  a b o u t  0 . 3  m g / c u b i c  c e n t i m e t e r  o f  b o r o n  to d e t e c t  n e u t r o n s  v i a  t h e  1 0 B ( n , a ) 7 Li r e a c t i o n .  T h e  s e c o n d  r e g i o n  c o n t a i n e d  6 s i m i l a r  p l a t e s  w i t h o u t  b o r o n  c o a t i n g  a n d  w a s  u s e d  to d e t e r m i n e  t h e  g a m m a  r a y  c o n t r i b u t i o n  to t h e  c u r r e n t  in t h e  f i r s t  s e c t i o n .  T h e  b o r o n  t h i c k n e s s  w a s  c h o s e n  to o p t i m i z e  t h e  e f f i c i e n c y  f o r  e s c a p e  o f  t h e  l i t h i u m  a n d  h e l i u m  i o n s  i n t o  t h e  h y d r o g e n  gas, b u t  m a x i m i z e  t h e  n e u t r o n  d e t e c t i o n  e f f i c i e n c y .  T h e  t o t a l  t h i c k n e s s  o f  b o r o n  w a s  s u c h  t h a t  9 0 %  o f  t h e  t h e r m a l  n e u t r o n s  i n c i d e n t  o n  t h e  d e t e c t o r s  w e r e  c a p t u r e d  i n  t h e  b o r o n .  T h e  h y d r o g e n  p r e s s u r e  w a s  1162a t m o s p h e r e  a n d  t h e  d e t e c t o r s  w e r e  o p e r a t e d  at 3 0 0  v o l t s  b i a s .  T h i s  d e s i g n  w o r k e d  v e r y  w e l l  f o r  t h e  d e t e c t i o n  of t h e r m a l  n e u t r o n s  i n  t h e  p r e s e n c e  o f  an i n t e n s e  g a m m a  r a y  b a c k g r o u n d .  T h e  n e u t r o n  s e c t i o n s  o f  t h e  d e t e c t o r  p r o d u c e d  a b o u t  3 x 1 0 “ 13 a m p e r e s  p e r  n e u t r o n  p e r  c m 2 p e r  s e c o n d  a n d  t h e  g a m m a  s e c t i o n s  p r o d u c e d  a b o u t  1 x 1 0 “ 14 a m p e r e s  p e r  r a d  p e r  s e c o n d .  A t  4. 1  MeV, t h i s  m e a n t  t h a t  a b o u t  9 0 %  o f  t h e c u r r e n t  i n  t h e  b o r o n  c o a t e d  s e c t i o n  o f  t h e  d e t e c t o r  w a s  d u e  to t h e r m a l  n e u t r o n  c a p t u r e  i n  t h e  b o r o n .  T h e s e  d e t e c t o r s  w i l l  b e  r e f e r r e d  to as t h e  B I C  ( B o r o n  I o n i z a t i o n  C h a m b e r s )  i n  f u r t h e r  d i s c u s s i o n .F o r  e x p e r i m e n t a l  s e n s i t i v i t i e s  b e t t e r  t h a n  1 x 1 0 “ 6 , it w a s  n e c e s s a r y  to m e a s u r e  c u r r e n t s  f r o m  d e t e c t o r s ,  r a t h e r  t h a n  i n d i v i d u a l  p u l s e s .  T h e  e l e c t r o n i c  s y s t e m  c o n v e r t e d  d e t e c t o r  c u r r e n t s  to f r e q u e n c i e s  o f  a b o u t  5 MHz, w h i c h  w e r e  s c a l e d  w i t h  a C A M A C - b a s e d  L S I - 1 1  a c q u i s i t i o n  s y stem. A  s e p a r a t e  m i c r o p r o c e s s o r  g e n e r a t e d  a s w i t c h i n g  p a t t e r n  w h i c h  c o n t r o l l e d  t h e  r e v e r s a l  o f  t h e  e l e c t r o n  h e l i c i t y ,  t h e  d a t a  a c q u i s i t i o n  t i m e s  a n d  t h e  b e a m  o n - o f f  times. T h e  h e l i c i t y  r e v e r s i n g  p a t t e r n  w a s  s y m m e t r i z e d  to c o m p e n s a t e  fo r  l o n g - t e r m  i n t e n s i t y  d r i f t s  a n d  h a d  a f r e q u e n c y  o f  a b o u t  30 Hz, as r e q u i r e d  to m i n i m i z e  a c c e l e r a t o r  n o i s e .  T o  a v o i d  p i c k u p  i n  t h e  c o u n t i n g  r o o m  o f  s q u a r e  w a v e  s i g n a l s  c o r r e l a t e d  w i t h  h e l i c i t y  r e v e r s a l ,  s h o r t  t r a n s i e n t  p u l s e s  w e r e  g e n e r a t e d  b y  t h e  m i c r o p r o c e s s o r  a n d  s q u a r e  w a v e s  w e r e  s y n t h e s i z e d  at t h e  p o l a r i z e d  e l e c t r o n  s o u r c e  fo r  p o l a r i z a t i o n  r e v e r s a l .  T h e  b e a m  w a s  t u r n e d  o f f  f o r  4 s e c o n d s  i n  e v e r y  10 0  to m e a s u r e  t h e  z e r o - l e v e l s  o f  a l l  d e t e c t o r  s y s t e m s .T h e  m a i n  p a r a m e t e r s  e x p e c t e d  to c a u s e  s y s t e m a t i c  e f f e c t s  i n  t h e  n e u t r o n  a n d  g a m m a  r a y  i n t e n s i t i e s  ar e  th e  e l e c t r o n  b e a m  i n t e n s i t y ,  e n e r g y ,  p o s i t i o n  a n d  size. P r o b l e m s  c a n  a r i s e  if t h e r e  is a c h a n g e  in a n y  o f  t h e s e  p a r a m e t e r s  c o r r e l a t e d  w i t h  t h e  r e v e r s a l  o f  t h e  e l e c t r o n  p o l a r i z a t i o n .  T h e r e  c a n  a l s o  b e  an e f f e c t  o n  t h e  o v e r a l l  a c c u r a c y  o f  t h e  e x p e r i m e n t  if a n y  o f  t h e s e  p a r a m e t e r s  v a r i e s  r a n d o m l y  a n d  i n t r o d u c e s  e x c e s s i v e  n o i s e  in t h e  m e a s u r e m e n t .  F i g u r e  3 s h o w s  a s c h e m a t i c  d i a g r a m  o f  t h e  e x p e r i m e n t a l  a r e a  w i t h  s y s t e m a t i c  m o n i t o r i n g  d e v i c e s  i n  p l a c e .T h e  e l e c t r o n  b e a m  i n t e n s i t y  w a s  m o n i t o r e d  b y  m e a s u r i n g  t h e  c u r r e n t  f r o m  t h e  b r e m s s t r a h l u n g  r a d i a t o r ,  w h i c h  s e r v e d  as a v e r y  g o o d  F a r a d a y  cup. T o  d e t e r m i n e  t h e  p o s i t i o n  a n d  s i z e  of t h e  b e a m  s t r i k i n g  t h e  r a d i a t o r ,  a n  a r r a y  of g a m m a  r a y  d e t e c t o r s  w a s  m o u n t e d  d i r e c t l y  o n  t h e  r a d i a t o r .  T h e s e  d e t e c t o r s  w e r e  m o d e l l e d  o n  " s e l f  p o w e r e d "  d e t e c t o r s  u s e d  fo r r e a c t o r  c o n t r o l ' 1 3 ’ . T h e y  c o n s i s t e d  o f  c o a x i a l  c a b l e s  w i t h  t h e  c e n t r a l  c o n d u c t o r  m a d e  o f  t u n g s t e n  a n d  t h e  o u t e r  s h i e l d  m a d e  o f  a l u m i n u m .  In t h e  i n t e n s e  g a m m a  r a y  f l u x e s  u s ed, a c u r r e n t  o n  t h e  o r d e r  o f  10 p i c o a m p e r e s  w a s  p r o d u c e d  f r o m  th e  d i f f e r e n c e  in C o m p t o n  s c a t t e r i n g  i n  t h e s e  t w o  m a t e r i a l s .  A n  a r r a y  o f  s i x t e e n  0 . 3  c m  t h i c k  b y  13 c m  l o n g  d e t e c t o r s  w a sv/////////:v*//////////a163cro><crivzzzzzzzzzzzzzzzzzzzzm(1}P rHcfl (Qe• po  <u• £H  +JCO -PUa) a)£ -pE-t a>T)• oG ^0•H 73O' <Da) top  G-P m<U PO' a)& *§■P aj£a> oX!■P G0<1-1 -H0  -Pa)e  n(0 -HP gO' Ofl! i h•H-o -aa)T3 ca) -hr H  r H•H(fl G •■P O toa) ra G■0 o 0PQ Pa> ■Pp  a> 0o A a)2  -P Go:UJa.<164u s e d  w i t h  h a l f  o f  t h e m  a l i g n e d  h o r i z o n t a l l y  a n d  h a l f  v e r t i c a l l y  a n d  w i t h  a n  i n t e r - d e t e c t o r  s p a c i n g  o f  1. 4  cm. T h i s  a r r a y  w a s  v e r y  e f f e c t i v e  i n  d e t e r m i n i n g  t h e  c e n t r o i d  a n d  w i d t h  o f  t h e  g a m m a  r a y  d i s t r i b u t i o n  l e a v i n g  t h e  r a d i a t o r .  T o  e x t r a c t  t h i s  i n f o r m a t i o n ,  t h e  c u r r e n t s  f r o m  t h e  d e t e c t o r s  w e r e  a m p l i f i e d  a n d  s e n t  to a n a l o g  c i r c u i t r y  w h i c h  p r o d u c e d  s i g n a l s  p r o p o r t i o n a l  to t h e  t o t a l  c u r r e n t  a n d  t h e  f i r s t  a n d  s e c o n d  m o m e n t s  o f  t h e  g a m m a  r a y  d i s t r i b u t i o n  in e a c h  d i r e c t i o n .  A d d i t i o n a l  p o s i t i o n  i n f o r m a t i o n  w a s  o b t a i n e d  f r o m  t h e  B I C  d e t e c t o r s  w h i c h  p r o v i d e d  t o t a l  c u r r e n t  s i g n a l s  f r o m  t h e  n e u t r o n  a n d  g a m m a - r a y  s e n s i t i v e  r e g i o n s  f o r  t h e  se t o f  d e t e c t o r s  o n  t h e  top, b o t t o m ,  left, r i g h t  a n d  e n d  of t h e  D 2 0  tank.A s  m e n t i o n e d  above, a l a r g e  a r e a  g a m m a  d e t e c t o r  w a s  m o u n t e d  d i r e c t l y  o n  t h e  r a d i a t o r  to d e t e r m i n e  t h e  t o t a l  g a m m a  r a y  f l u x  p r o d u c e d  f o r  e a c h  h e l i c i t y .  T h i s  d e t e c t o r  a l s o  p r o d u c e d  a c u r r e n t  f r o m  t h e  d i f f e r e n c e  i n  C o m p t o n  s c a t t e r i n g  f r o m  d i s s i m i l a r  m a t e r i a l s .  It c o n s i s t e d  of 14.3 c m  d i a m e t e r  p l a t e s  o f  a l u m i n u m  a n d  lead, 0.3 m m  thick. T h i s  c o n f i g u r a t i o n  p r o d u c e d  a b o u t  1 0 0  p i c o c o u l o m b s  p e r  R a d  a n d  t h e r e f o r e  t h e  d e t e c t o r  p r o v i d e d  c u r r e n t s  o f  n a n o a m p e r e s  in a c t u a l  o p e r a t i o n  o n  t h e  b r e m s s t r a h l u n g  r a d i a t o r .  It w i l l  b e  r e f e r r e d  to as t h e  P P S P D  ( P a r a l l e l  P l a t e  S e l f  P o w e r e d  D e t e c t o r ) . T o  d e t e r m i n e  t h e  e n e r g y  v a r i a t i o n  o f  t h e  e l e c t r o n  b e a m  u p o n  h e l i c i t y  r e v e r s a l ,  a s m a l l  v o l u m e  (10 c u b i c  c e n t i m e t e r )  b i s m u t h  g e r m a n a t e  d e t e c t o r  (BGO) w a s  m o u n t e d  o n  t h e  e n d  o f  t h e  D 2 0 t a n k  at z e r o  d e g r e e s .  T h e  v a r i a t i o n  of t h e  b r e m s s t r a h l u n g  a n g u l a r  d i s t r i b u t i o n  a n d  a t t e n u a t i o n  w i t h  e n e r g y  p r o d u c e d  a n  e n e r g y  d e p e n d e n c e  i n  t h e  c u r r e n t  f r o m  t h i s  d e t e c t o r  w h e n  c o m p a r e d  w i t h  t h e  t o t a l  b r e m s s t r a h l u n g  f l u x  m e a s u r e d  b y  t h e  PP S P D .DATA ACQUISITIONT h e  m o n i t o r s  w e r e  c a l i b r a t e d  at 3. 1  a n d  4.2 M e V  in a s e r i e s  o f  m e a s u r e m e n t s  ( r e f e r r e d  to as m o d u l a t i o n  da t a ) , w h e r e i n  t h e  e l e c t r o n  b e a m  i n t e n s i t y ,  e n e r g y ,  p o s i t i o n  a n d  s i z e  w e r e  m o d u l a t e d  w i t h  t h e  s a m e  s w i t c h i n g  p a t t e r n  u s e d  f o r t h e  r e v e r s a l  o f  t h e  e l e c t r o n  h e l i c i t y  in t h e  m a i n  m e a s u r e m e n t .  T h i s  d a t a  w a s  u s e d  to d e t e r m i n e  t h e  s e n s i t i v i t y  of all d e t e c t o r s  to t h e s e  p a r a m e t e r s .  B y  m o d u l a t i n g  t h e s e  p a r a m e t e r s  w i t h  a p a t t e r n  s i m i l a r  to t h e  e l e c t r o n  h e l i c i t y  s w i t c h i n g ,  t h e  e f f e c t s  o f  d e t e c t o r  t i m e  r e s p o n s e  w e r e  a p p r o p r i a t e l y  d e t e r m i n e d  b y  t h e s e  m e a s u r e m e n t s .D a t a  t a k i n g  f o r  t h e  d e t e r m i n a t i o n  o f  p a r i t y  v i o l a t i o n  t h e n  p r o c e e d e d  a n d  t h e  m o n i t o r  d e t e c t o r s  w e r e  u s e d  to m e a s u r e  t h e  v a r i a t i o n  o f  t h e  a b o v e  e l e c t r o n  b e a m  a n d  g a m m a  r a y  f l u x  p a r a m e t e r s  d u r i n g  t h e  m e a s u r e m e n t .B y  a 90 d e g r e e  r o t a t i o n  o f  t h e  p l a n e  o f  p o l a r i z a t i o n  of t h e  i n c i d e n t  l a s e r  l i g h t  p r i o r  to t h e  P o c k e l l s  cell, it w a s  p o s s i b l e  to r e v e r s e  t h e  c i r c u l a r  p o l a r i z a t i o n  of t h e165p h o t o e m i s s i o n  l i g h t  f o r  a g i v e n  v o l t a g e  a p p l i e d  to t h eP o c k e l l s  cell. T h e r e f o r e ,  t h e  o v e r a l l  s e n s e  o f  t h e  e l e c t r o np o l a r i z a t i o n  c o u l d  b e  c h a n g e d  r e l a t i v e  to t h e  e l e c t r o n i c  s w i t c h i n g  p a t t e r n s .  A b o u t  o n e  h a l f  of t h e  p a r i t y  v i o l a t i o n  d a t a  w a s  t a k e n  w i t h  e a c h  o v e r a l l  s e n s e  o f  t h e  e l e c t r o n  p o l a r i z a t i o n .  T r u e  p a r i t y  v i o l a t i o n  e f f e c t s  m u s t  b ec o r r e l a t e d  w i t h  t h i s  o v e r a l l  p o l a r i z a t i o n  r e v e r s a l  a n d  c a n  b e  s o u g h t  as t h e  d i f f e r e n c e  b e t w e e n  h e l i c i t y  c o r r e l a t e d  q u a n t i t i e s  d e t e r m i n e d  f o r  t h e  t w o  o v e r a l l  s e n s e s  o f  th e  e l e c t r o n  p o l a r i z a t i o n .  O n  t h e  o t h e r  h a n d ,  s y s t e m a t i c  e f f e c t s  c o r r e l a t e d  o n l y  w i t h  t h e  e l e c t r o n i c  s w i t c h i n g  p a t t e r n  w i l l  b e  t h e  s a m e  f o r  e a c h  o r i e n t a t i o n  o f  t h e  p r i s m  a n d  w i l l  c a n c e l  if d i f f e r e n c e s  ar e  c a l c u l a t e d  b e t w e e n  t h e  tw o o r i e n t a t i o n s .In a d d i t i o n ,  b y  a 45 d e g r e e  r o t a t i o n  of t h e  p l a n e  of p o l a r i z a t i o n  o f  t h e  i n c i d e n t  l a s e r  light, it w a s  p o s s i b l e  to p r o d u c e  v e r y  s m a l l  l o n g i t u d i n a l  p o l a r i z a t i o n  fo r  t h ee l e c t r o n  b e am. S o m e  d a t a  w a s  a l s o  t a k e n  u n d e r  t h i s  c o n d i t i o n .DATA ANALYSIS General;S c a l e r  v a l u e s  p r o p o r t i o n a l  to t h e  c u r r e n t s  in th e  v a r i o u s  d e t e c t o r s ,  i n t e g r a t e d  o v e r  t h e  p e r i o d  c o r r e s p o n d i n g  to o n e  s e n s e  o f  t h e  e l e c t r o n  h e l i c i t y  w e r e  t h e  p r i m a r y  d a t a  f o r  t h e  m e a s u r e m e n t .  A  t o t a l  of n i n e t e e n  s c a l e r s  w e r e  used:  T e n  c o r r e s p o n d i n g  to c u r r e n t s  f r o m  t h e  n e u t r o n  a n d  g a m m a  s e c t i o n s  o f  t h e  B I C  d e t e c t o r s  o n  t h e  f o u r  s i d e s  a n d  e n d  of  t h e  D 2 0 tank; t h r e e  c o r r e s p o n d i n g  to t h e  t o t a l  c u r r e n t  a n d  t h e  f i r s t  a n d  s e c o n d  m o m e n t s  o f  t h e  g a m m a  r a y  f l u x  s t r i k i n g  t h e  s e l f  p o w e r e d  d e t e c t o r  a r r a y  o n  t h e  b r e m s s t r a h l u n g  r a d i a t o r ;  o n e  e a c h  f o r  t h e  b e a m  c u r r e n t  o n  t h e  r a d i a t o r ,  t h e  b e a m  d e f i n i n g  a p e r t u r e ,  t h e  P P S P D  a n d  t h e  B G O  d e t e c t o r .T h e  o n - l i n e  a n a l y s i s  p r o g r a m  in t h e  C A M A C  c r a t e - b a s e d  L S I - 1 1  c o m p u t e r  w r o t e  t h i s  d a t a  o n  m a g n e t i c  t a p e  in b u f f e r s  c o r r e s p o n d i n g  to 16 s e t s  o f  s c a l a r  r e a d i n g s ,  t o g e t h e r  w i t h  i n f o r m a t i o n  a b o u t  w h e t h e r  t h e  s y m m e t r i c  h e l i c i t y  s w i t c h i n g  p a t t e r n  b e g a n  w i t h  a p o s i t i v e  o r  n e g a t i v e  h e l i c i t y  state. F o r  all d e t e c t o r s  a n d  m o n i t o r s ,  q u a n t i t i e s  o f  t h e  f o r mA  =  (D* - D. ) /  (D+ + D_ )w e r e  d e r i v e d ,  w h e r e  D„ (D. ) r e p r e s e n t s  t h e  a v e r a g e  c u r r e n t  m e a s u r e d  i n  a d e t e c t o r  w h e n  t h e  s t a t e  o f  t h e  e l e c t r o n i c  s w i t c h i n g  p a t t e r n  w a s  p l u s ( m i n u s ) .  In a d d i t i o n ,  t h e  o n - l i n e  p r o g r a m  w a s  c a p a b l e  o f  c a l c u l a t i n g  a v e r a g e s  a n d  v a r i a n c e s  f o r  t h e s e  h e l i c i t y - c o r r e l a t e d  q u a n t i t i e s ,  b u t  w a s  o n l y  f a s t  e n o u g h  to h a n d l e  o n e  s e v e n t h  o f  t h e  data. T h e r e f o r e ,  it w a s  p o s s i b l e  to o b s e r v e  p a r i t y  v i o l a t i o n  s i g n a l s ,  s y s t e m a t i c  m o n i t o r i n g  s i g n a l s  a n d  t h e i r  v a r i a n c e s  o n - l i n e  w i t h166r e s t r i c t e d  a c c u r a c y ,  b u t  a c o m p l e t e  d a t a  a n a l y s i s  h a d  to w a i t  f o r  p l a y b a c k  o f  t h e  m a g n e t i c  t a p e s  w i t h  a m o r e  c o m p l e t e  p r o g r a m  o n  t h e  C h a l k  R i v e r  C D C - 6 6 0 0  c o m p u t e r  s y stem. T h e  f i n a l  d a t a  a n a l y s i s  p r o g r a m  a l s o  i n c o r p o r a t e d  t h e  a b i l i t y  to c o r r e c t  i n d i v i d u a l  p a r a m e t e r s  o n  a b u f f e r  b y  b u f f e r  b a s i s  a n d  to p r o d u c e  c o r r e l a t i o n  p l o t s  a n d  l i n e a r  r e g r e s s i o n s  o n  s e l e c t e d  p a r a m e t e r s .Modulation Data:T h e  r e l a t i v e  m a g n i t u d e s  o f  t h e  v a r i o u s  s y s t e m a t i c  e f f e c t s  w e r e  d e t e r m i n e d  f r o m  t h e  m o d u l a t i o n  data. W h e n  t h e  o v e r a l l  b e a m  i n t e n s i t y  w a s  m o d u l a t e d ,  it w a s  f o u n d  t h a t  all d e t e c t o r s  s h o w e d  s i m i l a r  f r a c t i o n a l  c h a n g e s  in c o u n t i n g  rate, i n d i c a t i n g  e x c e l l e n t  l i n e a r i t y  a n d  s i m i l a r  t i m e  r e s p o n s e .  R e s i d u a l  c o r r e c t i o n s  w e r e  a f e w  p e r c e n t  o f  th e  m o d u l a t i o n  a m p l i t u d e  i n  m o s t  ca s e s .  T h e r e f o r e ,  b e f o r e  f u r t h e r  a n a l y s i s ,  t h e  s c a l e r  v a l u e  f o r  e a c h  d e t e c t o r  w a s  d i v i d e d  b y  t h e  v a l u e  o b t a i n e d  f r o m  t h e  c u r r e n t  m e a s u r e d  o n  t h e  b r e m s s t r a h l u n g  r a d i a t o r .  T h i s  c o m p e n s a t i o n  f o r  r a p i d  b e a m  c u r r e n t  f l u c t u a t i o n s  g r e a t l y  r e d u c e d  t h e  v a r i a n c e  o f  a ll d e t e c t o r  s i g n a l s .  F r o m  h e r e  on, t h e  d i s c u s s i o n  w i l l  b e  i n  t e r m s  o f  s u c h  b e a m  c o r r e c t e d  s i g n a l s .TABLE 1BEAM ENERGY = 4 . 1  MEV FRACTIONAL CHANGES CORRELATED WITH PARAMETER MODULATION (UNITS OF 10'6 )MODULATIONPARAMETERBEAM BIC(N)PPSP(G)BGO(G)NX NY XPOS YP0S XWID YWIDINTENSITY 5025 -14 135 -97 -2 23 69 -46 -66 16ENERGY 58 2323 603 1781 -67 49 -186 -14 -77 -64XPOSITION13 -36 -46 4 -1213 9 -3520 -112 -81 42YPOSITION11 32 10 -431 -120 -1218 -237 3672 -93 9YSIZE28 -14 384 -156 11 89 5 3 117 -1534T a b l e  1 s h o w s  t h e  s e n s i t i v i t y  o f  v a r i o u s  q u a n t i t i e s  to m o d u l a t i o n s  o f  b e a m  i n t e n s i t y ,  e n e r g y ,  p o s i t i o n  a n d  size. B e a m  i n t e n s i t y  w a s  m o d u l a t e d  b y  v a r y i n g  t h e  i n t e n s i t y  o f  t h e  i n c i d e n t  l a s e r  light. B e a m  e n e r g y  w a s  m o d u l a t e d  b y  v a r y i n g  t h e  rf p o w e r  to o n e  o f  t h e  a c c e l e r a t i o n  c a v i t i e s .  B e a m  p o s i t i o n  a n d  s i z e ,r e s p e c t i v e l y , w e r e  m o d u l a t e d  b y  v a r y i n g  t h e  D C  o f f s e t  o r  a m p l i t u d e  o f  t h e  f u n c t i o n  g e n e r a t o r s  p r o d u c i n g  t h e  h i g h  f r e q u e n c y  s w e e p i n g  o f  t h e  b e a m  o n  t a r g e t .  In all c a s e s  t h e  t i m e  s e q u e n c e  o f  t h e  m o d u l a t i o n  w a s  s i m i l a r  t o t h e  h e l i c i t y  r e v e r s a l  p a t t e r n  u s e d  i n  t h e  a c c u m u l a t i o n  of p a r i t y  v i o l a t i o n  data. T h e  BIC, P P S P D  a n d  B G O  q u a n t i t i e s  v a l u e s  i n  t h e  t a b l e  i n c o r p o r a t e  a l i n e a r  c o r r e c t i o n  f o r  b e a m  i n t e n s i t y  as d e s c r i b e d  above.167T y p i c a l  u n c e r t a i n t i e s  in t a b u l a t e d  q u a n t i t i e s  a r e  l e s s t h a n  1 0 “ 5 as d e t e r m i n e d  f r o m  v a r i a n c e s  in t h e  o b s e r v e d  v a l u e s .T h e  q u a n t i t i e s  X P OS, Y P OS, X W I D  a n d  Y W I D  w e r e  o b t a i n e d  f r o m  t h e  p o s i t i o n - s e n s i t i v e  a r r a y  o f  s e l f - p o w e r e d  d e t e c t o r s  a n d  c o r r e s p o n d  to t h e  f i r s t  a n d  s e c o n d  m o m e n t s  o f  th e  b r e m s s t r a h l u n g  a n g u l a r  d i s t r i b u t i o n  at t h e  t a r g e t .  T h e  q u a n t i t i e s  N X  a n d  N Y  w e r e  c a l c u l a t e d  f r o m  t h e  s i g n a l s  o b t a i n e d  f r o m  t h e  B I C  n e u t r o n  d e t e c t o r  array.F o r  e x a m p l e ,  N X  is:N X  = [ N ( l e f t )  - N ( r i g h t ) ]/[N ( l e f t ) + N ( r i g h t ) ]A  s i m i l a r  f o r m u l a  w a s  u s e d  fo r  t h e  v e r t i c a l  d i f f e r e n c e  NY. A s  c a n  b e  s e e n  f r o m  T a b l e  1, t h e s e  q u a n t i t i e s  a r e  a s e n s i t i v e  m e a s u r e  o f  b e a m  m o t i o n  o n  t a r g e t  t h r o u g h  t h e  a s s o c i a t e d  m o t i o n  of t h e  n e u t r o n  f l u x  s t r i k i n g  t h e  B I C  d e t e c t o r s .A  d i s c u s s i o n  o f  t h e  m o d u l a t i o n  d a t a  in T a b l e  1 w i l l  i l l u s t r a t e  t h e  s e n s i t i v i t y  of t h e  v a r i o u s  d e t e c t o r s  to s y s t e m a t i c  v a r i a t i o n  o f  b e a m  p a r a m e t e r s ,  c o r r e l a t e d  w i t h  e l e c t r o n  h e l i c i t y  r e v e r s a l .Fir s t ,  t h e  i n t e n s i t y  m o d u l a t i o n  d a t a  i l l u s t r a t e s  th e  f a c t  t h a t  t h e  n o r m a l i z a t i o n  b y  b e a m  d i v i s i o n  is v e r y  e f f e c t i v e  i n  r e d u c i n g  t h e  s e n s i t i v i t y  to b e a m  i n t e n s i t y .  T h e  r e s i d u a l  s e n s i t i v i t y  i n  t h e  s u m  of t h e  n e u t r o n  d e t e c t o r s / b e a m  is p a r t i c u l a r l y  s m a l l  ( l ess t h a n  3 x 10 3 ), i m p l y i n g  v e r y  l i t t l e  o v e r a l l  e f f e c t  w h e n  c o u p l e d  w i t h  b e a m  i n t e n s i t y  f r a c t i o n a l  c h a n g e s  l e s s  t h a n  7 x 10' 5 d u r i n g  p a r i t y  v i o l a t i o n  d a t a  t a k i n g .T h e  m o d u l a t i o n  o f  e n e r g y  r e v e a l e d  t h a t  t h e  t o t a l  n e u t r o n  f l u x  s h o w s  t h e  l a r g e s t  s e n s i t i v i t y  to b e a m  e n e r g y  v a r i a t i o n s .  T h e  t o t a l  g a m m a  r a y  flux, as m e a s u r e d  b y  th e P P S P D , o n l y  v a r i e s  a b o u t  0 . 2 5  t i m e s  as m u ch. T h i s  is b e c a u s e  o f  t h e  v a r i a t i o n  o f  t h e  p h o t o d i s i n t e g r a t i o n  c r o s s  s e c t i o n  w i t h  e n e r g y ,  w h i c h  e n h a n c e s  t h e  n e u t r o n  f l u x  s e n s i t i v i t y .  H o w e v e r ,  t h e  B G O  d e t e c t o r ,  o b s e r v i n g  o n l y  g a m m a  r a y s  at z e r o  d e g r e e s ,  s h o w e d  a f r a c t i o n a l  v a r i a t i o n  a b o u t  0 . 7  t i m e s  t h a t  o f t h e  B I C  n e u t r o n  d e t e c t o r s .  T h e  e n h a n c e d  e n e r g y  s e n s i t i v i t y  w a s  d u e  to c h a n g e s  i n  t h e  b r e m s s t r a h l u n g  a n g u l a r  d i s t r i b u t i o n  a n d  d i f f e r e n t i a l  a b s o r p t i o n  o f  t h e  g a m m a  r a y s at 0° . T h i s  d e t e c t o r  t h e r e f o r e  c o n s t i t u t e d  o u r  p r i m a r y  e n e r g y  m o n i t o r  a n d  c o u l d  b e  u s e d  to c o r r e c t  t h e  t o t a l  g a m m a  f l u x  (PPSPD) a n d  t o t a l  n e u t r o n  f l u x  (BIC N) s i g n a l s  for s y s t e m a t i c  b e a m  e n e r g y  v a r i a t i o n s .T h e  b e a m  p o s i t i o n  a n d  s i z e  m o d u l a t i o n s  i l l u s t r a t e d  th e  e x c e l l e n t  s e n s i t i v i t y  o f  t h e  s e l f  p o w e r e d  d e t e c t o r  a r r a y s  o n  t h e  b r e m s s t r a h l u n g  r a d i a t o r .  T h e  e f f e c t i v e n e s s  o f  the168e l e c t r o n i c  c i r c u i t r y  i n  e x t r a c t i n g  t h e  f i r s t  a n d  s e c o n d  m o m e n t s  o f  t h e  b e a m  s h a p e  o n  t a r g e t  is a l s o  e v i d e n t  in th e  data. T h e  a c t u a l  X  a n d  Y  m o t i o n s  o f  t h e  b e a m  c e n t r o i d  f o r  t h e  d a t a  i n  T a b l e  1 c o r r e s p o n d  to a b o u t  1 c m  of d i s p l a c e m e n t ,  so it is c l e a r  t h a t  t h e  d e t e c t o r  a r r a y  is v e r y  e f f e c t i v e  i n  t h e  m e a s u r e m e n t  o f  b e a m  m o t i o n .  In a d d i t i o n ,  f o r  m o t i o n  i n  t h e  X  d i r e c t i o n ,  t h e r e  is v e r y  l i t t l e  s p u r i o u s  r e s p o n s e  i n  t h e  Y  d e t e c t o r s  a n d  v e r y  l i t t l e  r e s p o n s e  in X W I D ,  t h e  b e a m  w i d t h  s i g n a l .  F o r  t h e  m o d u l a t i o n  o f  Y  b e a m  size, v e r y  l i t t l e  s p u r i o u s  r e s p o n s e  is o b s e r v e d  in t h e  X  a n d  Y  p o s i t i o n  s i g n a l s ,  X P O S  a n d  Y P O S .In a d d i t i o n ,  it is a p p a r e n t  f r o m  t h e  p o s i t i o n  a n d  s i z e  m o d u l a t i o n  d a t a  t h a t  t h e  NX, N Y  s i g n a l s  a r e  a l s o  s e n s i t i v e  i n d i c a t o r s  o f  t h e  c e n t r o i d s  o f  t h e  n e u t r o n  f l u x  s t r i k i n g  t h e  B I C  d e t e c t o r s .  O n  t h e  o t h e r  h a n d , t h e  s u m  o f  a l l  t h e  n e u t r o n  d e t e c t o r s  is v e r y  i n s e n s i t i v e  to m o v e m e n t s  o f  t h e  c e n t r o i d  o f  t h e  n e u t r o n  f l u x  d i s t r i b u t i o n  o r  c h a n g e s  in i t s  shape. F r o m  a n  e x a m i n a t i o n  o f  t h e  r e s p o n s e s  of i n d i v i d u a l  n e u t r o n  d e t e c t o r s  to t h e s e  m o d u l a t i o n  data, it is c l e a r  t h a t  o p p o s i t e  d e t e c t o r s  e x h i b i t  v e r y  s i m i l a r  f r a c t i o n a l  c h a n g e s  w h e n  t h e  b e a m  c e n t r o i d  is m o v e d  a l o n g  a l i n e  b e t w e e n  them. T o  f i r s t  o r d e r  i n  b e a m  p o s i t i o n ,  t h i s  is e x p e c t e d ,  b u t  th e  c a n c e l l a t i o n  is e v e n  m o r e  e f f e c t i v e  i n  t h i s  e x p e r i m e n t  w i t h  s u c h  a d i f f u s e  n e u t r o n  f l u x  s h a p e  c a u s e d  b y  t h e  w i d e  b e a m  d i s t r i b u t i o n  o n  t a r g e t ,  t h e  g a m m a  r a y  a n g u l a r  d i s t r i b u t i o n  a n d  t h e  t h e r m a l i z a t i o n  p r o c e s s  f o r  t h e  n e u t r o n s .  T h e  c a r e  t a k e n  to o b t a i n  v e r y  s i m i l a r  n e u t r o n  e f f i c i e n c i e s  f o r  t h e B I C  d e t e c t o r s  w a s  a l s o  r e w a r d e d  b y  t h e  c a n c e l l a t i o n s  o c c u r r i n g  h e re.T w o  a d d i t i o n a l  s e n s i t i v i t i e s  a r e  a l s o  i m p o r t a n t  to n o t e  i n  t h e  p o s i t i o n  a n d  s i z e  m o d u l a t i o n  data. Fi r s t ,  t h e  B G O  d e t e c t o r  e x h i b i t e d  a s m a l l  s e n s i t i v i t y  to Y  p o s i t i o n  m o d u l a t i o n  a n d  Y  size, d u e  to i t s  a s y m m e t i c  shape. In a d d i t i o n ,  t h e  P P S P D  e x h i b i t e d  a s m a l l  s e n s i t i v i t y  to b e a m  size. In fact, t h e  s i z e  s e n s i t i v i t i e s  a r e  s m a l l  e n o u g h  t h a t  t h e y  h a v e  l i t t l e  i n f l u e n c e  as c o r r e c t i o n s  b e c a u s e  o f  t h e  v e r y  s m a l l  v a l u e s  o f  X W I D  a n d  Y W I D  o b s e r v e d  f o r  t h e  p a r i t y  v i o l a t i o n  data.Parity Violation Data;H a v i n g  d e t e r m i n e d  t h a t  t h e  v a r i o u s  m o n i t o r s  p e r f o r m e d  t h e i r  f u n c t i o n  s e n s i t i v e l y ,  w i t h  r e l a t i v e l y  l i t t l e  c r o s s  c o r r e l a t i o n  b e t w e e n  them, it w a s  p o s s i b l e  to a c c u m u l a t e  " p a r i t y  v i o l a t i o n "  d a t a  w i t h  e l e c t r o n  h e l i c i t y  r e v e r s i n g  a n d  w i t h  s y s t e m a t i c  e f f e c t s  m i n i m i z e d .  T h e s e  a c t i v e  m o n i t o r s  t h e n  s e r v e d  to d e t e r m i n e  t h e  m a g n i t u d e  of t h e  v a r i a t i o n s  in b e a m  i n t e n s i t y ,  e n e r g y ,  p o s i t i o n  a n d  s i z e  c o r r e l a t e d  w i t h  h e l i c i t y  r e v e r s a l .D u r i n g  t h e  a c c u m u l a t i o n  o f  t h e  p a r i t y  v i o l a t i o n  data, it w a s  o b s e r v e d  t h a t  t h e  P P S P D  a n d  t h e  B G O  d e t e c t o r  s i g n a l s169e x h i b i t e d  c l e a r  c o r r e l a t i o n s  w i t h  t h e  n e u t r o n  s i g n a l s  f r o m  t h e  BIC, w i t h  s l o p e s  c o r r e s p o n d i n g  to t h o s e  e x p e c t e d  f o r  a h e l i c i t y - c o r r e l a t e d  b e a m  e n e r g y  m o d u l a t i o n .  N o n e  of t h e  b e a m  p o s i t i o n  o r  s i z e  m o n i t o r s  s h o w e d  s i g n i f i c a n t  c o r r e l a t i o n s  w i t h  a n y  o f  t h e  a b o v e  t h r e e  p r i m a r y  s i g n a l s ,  as e x p e c t e d  f r o m  t h e  s m a l l  e f f e c t s  o b s e r v e d  i n  t h e  m o d u l a t i o n  data. It w a s  t h e r e f o r e  c o n c l u d e d  t h a t  t h e  l a r g e s t  r e m a i n i n g  s y s t e m a t i c  e f f e c t  w a s  a h e l i c i t y - c o r r e l a t e d  b e a m  e n e r g y  v a r i a t i o n ,  p r o d u c i n g  c o n t r i b u t i o n s  to t h e  t o t a l  n e u t r o n  a n d  g a m m a  r a y  s i g n a l s  o f  a b o u t  3 x 10" 6 a n d  1 x 10 6 r e s p e c t i v e l y .T h e r e f o r e ,  t h e  c o r r e c t i o n s  f o r  s y s t e m a t i c  e f f e c t s  in t h e  p a r i t y  v i o l a t i o n  d a t a  w e r e  c a r r i e d  o u t  as f o l l o w s :  F i r s t  t h e  s i g n a l s  o b t a i n e d  f r o m  t h e  n e u t r o n  s e c t i o n s  of t h e  B I C  d e t e c t o r s  w e r e  c o r r e c t e d  f o r  t h e  s m a l l  g a m m a  c o m p o n e n t  in t h e  s i g n a l  b y  s u b t r a c t i n g  a f r a c t i o n  o f  t h e  s i g n a l  i n  th e  g a m m a  s e c t i o n s  o f  e a c h  d e t e c t o r .  (The r e l a t i v e  s e n s i t i v i t y  to g a m m a  r a y s  w a s  d e t e r m i n e d  f r o m  r u n s  w i t h  H 2 0 r e p l a c i n g  t h e  D 2 0.) T h e n  t h e  s i g n a l s  f r o m  all d e t e c t o r s  a n d  m o n i t o r s  w e r e  d i v i d e d  b y  t h e  s i g n a l s  o b t a i n e d  fo r  t h e  b e a m  c u r r e n t  o n  t h e  b r e m s s t r a h l u n g  r a d i a t o r .  T h e  h e l i c i t y - c o r r e l a t e d  v a l u e s  "A" w e r e  t h e n  c a l c u l a t e d  f o r  all s i g n a l s .  T h e  BIC, P P S P D  a n d  B G O  "A" v a l u e s  w e r e  t h e n  c o r r e c t e d  s l i g h t l y  f o r  s m a l l  (<10-6) b e a m  p o s i t i o n ,  s i z e  a n d  r e s i d u a l  i n t e n s i t y  e f f e c t s ,  u s i n g  t h e  s e n s i t i v i t i e s  m e a s u r e d  f r o m  t h e  m o d u l a t i o n  data. F i n a l l y  t h e  s u m  o f  t h e  B I C  n e u t r o n  d e t e c t o r s  a n d  t h e  P P S P D  w e r e  c o r r e c t e d  f o r  r e s i d u a l  b e a m  e n e r g y  v a r i a t i o n ,  u s i n g  th e  B G O  v a l u e s  as a m o n i t o r  of t h i s  q u a n t i t y .T a b l e  2 s h o w s  t h e  r e s u l t s  at 4. 1  M e V  e l e c t r o n  e n e r g y  f o r  v a r i o u s  s t a g e s  o f  t h e  a n a l y s i s .  T h e  o v e r a l l  h e l i c i t y  v a l u e s ,  plu s ,  m i n u s ,  zero, c o r r e s p o n d  to v a l u e s  o f  t h e  p r i s m  in t h e  l i g h t  p a t h  p r o v i d i n g  e l e c t r o n s  w i t h  p l us, m i n u s ,  or z e r o  h e l i c i t y  f o r  t h e  p o s i t i v e  s t a t e  o f  t h e  s w i t c h i n g  p a t t e r n .  T h e  B I C  N  v a l u e s  c o r r e s p o n d  to th e  s u m  of all B o r o n  I o n i z a t i o n  C h a m b e r  n e u t r o n  s e c t i o n s  a n d  t h e r e f o r e ,  ar e  a m e a s u r e  of t h e  f r a c t i o n a l  c h a n g e  i n  t o t a l  n e u t r o n  f l u x  u p o n  h e l i c i t y  r e v e r s a l .  T h e  P P S P D  v a l u e s  i n d i c a t e  t h e  f r a c t i o n a l  c h a n g e  i n  t o t a l  b r e m s s t r a h l u n g  flux. T h e  n u m b e r s  in b r a c k e t s  i n d i c a t e  o n e  s t a n d a r d  d e v i a t i o n  as d e r i v e d  f r o m  t h e  a v e r a g e  v a r i a n c e  o f  t h e  q u a n t i t y  e v a l u a t e d  f o r  i n d i v i d u a l  b u f f e r s .  N o t e  t h a t  t h e  s t a n d a r d  d e v i a t i o n  f o r  t h e  B I C  N  v a l u e s  is r e d u c e d  at e a c h  s t a g e  o f  t h e  c o r r e c t i o n  p r o c e s s  as th e  i n f l u e n c e  o f  f l u c t u a t i o n s  in i n t e n s i t y  a n d  b e a m  e n e r g y  are r e m o v e d .170TABLE 2 4.1 MEV DATAHELICITY CORRELATED ASYMMETRY VALUES (UNITS OF 10‘ 6 )OVERALL BIC N BEAM BIC N PPSPD BGOHELICITY [UNCORRECTED] CURRENT [DIVIDED BY BEAM](PRISM)PLUS 72.2 70.0 2.2 3.0 1.1(±0.9) (±0.7) (±.46) (±.18) (±.35)MINUS 1.1 4.2 -3.1 .39 -2.8(±1.0) (±0.7) (±.56) (±.22) (±.40)ZERO -117 .8 -115.0 -2.8 -3.9 1.4(±1.6) (±1.3) (±1.2) (±.54) (±.95)OVERALL BIC N PPSPD BIC N PPSPDHELICITY [CORRECTED FOR BEAM.ENERGY,SIZE] [INCLUDING u n c e r t a i n :[IN CORRECTIONS]PLUS -1.4 0 .69 -1.4 0 .69(±0.3) (±.14) (±1.6) (±.38)MINUS 0.30 0.31 0.30 0.31(±0.3) (±.14) (±.60) (±-17)ZERO -.77 - 0.50 -.77 -.50(±1.0) (±.90) (±1.8) (±1.7)T h e  s t a n d a r d  d e v i a t i o n s  i n  t h e  T a b l e  r e p r e s e n t  o n l y  th e r e s i d u a l  f l u c t u a t i o n s  i n  t h e  q u a n t i t i e s  a f t e r  th e c o r r e c t i o n s  h a v e  b e e n  a p p l i e d .  H o w e v e r ,  a d d i t i o n a l  u n c e r t a i n t i e s  a r i s e  b e c a u s e  t h e  c o r r e c t i o n  f a c t o r s  d e t e r m i n e d  f r o m  t h e  m o d u l a t i o n  d a t a  h a v e  i n h e r e n t  u n c e r t a i n t i e s  o n  t h e  o r d e r  o f  15%. F o r  e x a m p l e ,  t h e  c o m p l e t e  f o r m u l a  f o r  c o r r e c t i o n  o f  t h e  B I C  N  r e s u l t s  isA  ( B I C N ) = A  ( B I C N / B E A M ) - 0 . 0 2 7  A  ( B E A M ) - 1 . 3 9  A 1 (B G O / B E A M )  w h e r e  A 1 ( B G O / B E A M )  = A  ( B G O / B E A M )  - 0 . 3 5  A ( N Y )+ 0 . 0 4  A (B E A M )C o r r e c t i o n s  f o r  o t h e r  b e a m  s i z e  a n d  p o s i t i o n  e f f e c t s  w e r e  n e g l i g i b l e .  A l t h o u g h  all q u a n t i t i e s  o t h e r  t h a n  b e a m  i n t e n s i t y  e x h i b i t e d  A  v a l u e s  l e s s  t h a n  ~ 4 x 1 0 " 6 ,u n c e r t a i n t i e s  o f  1 5 %  i n  t h e  c o e f f i c i e n t s  i n c r e a s e  t h e  f i n a l  u n c e r t a i n t y  s i g n i f i c a n t l y .  T h e  f i n a l  c o l u m n s  i n  T a b l e  2 i n d i c a t e  t h e  o v e r a l l  u n c e r t a i n t i e s ,  w h i c h  a r e  d o m i n a t e d  in171m o s t  c a s e s  b y  u n c e r t a i n t i e s  in t h e  c o r r e c t i o n s .F i n a l  v a l u e s  of t h e  p a r i t y - v i o l a t i n g  o b s e r v a b l e s  w e r e  o b t a i n e d  asA P v ( N E U T R O N )  =  [A. (BIC N) - A_ (BIC N ) ] /  2A PV (GAMMA) =  [A* (PPSPD) - A. (PPSPD)] /  2w h e r e  +(-) r e f e r s  to t h e  o v e r a l l  h e l i c i t y  v a l u e .  T h e s e  a s y m m e t r i e s  m u s t  b e  d i v i d e d  b y  t h e  a v e r a g e  g a m m a  r a y  or e l e c t r o n  p o l a r i z a t i o n  to d e t e r m i n e  t h e  f i n a l  c r o s s  s e c t i o n  a s y m m e t r y .  T h e s e  p o l a r i z a t i o n s  w e r e  t a k e n  to b e  0 . 3 0  ± 0 . 0 5as d e t e r m i n e d  b y  t h e  m e a s u r e m e n t s  o f  b r e m s s t r a h l u n g  c i r c u l a r  p o l a r i z a t i o n .TABLE 3PARITY VIOLATING OBSERVABLES (UNITS OF 10 6)BEAM ENERGY4.1 MEV3.2 MEVQUANTITYOBSERVEDNEUTRONSGAMMASNEUTRONSGAMMASMEASUREDASYMMETRY0 .8 5 ± 0 .860. 1 9 ± 0 .212.3 ± 1 .60.9 4 ± 0 .46CROSS SECTION ASYMMETRY2.7+2.80.6 3 ± 0 .707 . 7 ± 5 .33.1 ± 1 .5T a b l e  3 l i s t s  t h e  f i n a l  r e s u l t s  at t h e  t w o  e n e r g i e s  s t u d i e d .  T h e  r e s u l t s  at 3.2 M e V  a r e  l e s s  a c c u r a t e  b e c a u s e  of l o w e r  y i e l d s ,  c o u p l e d  w i t h  l e s s  r u n n i n g  t i m e  a n d  l o w e r  i n t e n s i t y  c a u s e d  b y  l a s e r  d i f f i c u l t i e s  n e a r  t h e  e n d  o f  the r u n n i n g  p e r i o d .  S u b s e q u e n t  to t h i s  r u n n i n g  p e r i o d ,  t h e  E T A  f a c i l i t y  w a s  s h u t  down, p r e c l u d i n g  f u r t h e r  m e a s u r e m e n t s .A s  c a n  b e  s e e n  f r o m  T a b l e  2, f i n a l  u n c e r t a i n t i e s  w e r e  d o m i n a t e d  b y  u n c e r t a i n t i e s  i n  t h e  c o r r e c t i o n s  m a d e  for s y s t e m a t i c  e f f e c t s .  A s  j u d g e d  b y  t h e  r e s i d u a l  f l u c t u a t i o n s  i n  t h e  B I C  N  a n d  P P S P D  v a l u e s ,  t h e  a c c u r a c y  o f  t h e  f i n a l  r e s u l t  c o u l d  b e  a b o u t  t w o  to t h r e e  t i m e s  b e t t e r  if s y s t e m a t i c  c h a n g e s  i n  b e a m  i n t e n s i t y  a n d  e n e r g y  w e r e  r e d u c e d .  In fact, m e a s u r e m e n t s  o f  t h e r m a l  n e u t r o n  f l u x e s  b y  t h e  a c t i v a t i o n  o f  g o l d  f o i l s  i n d i c a t e d  t h a t  t h e  r e s i d u a l  f l u c t u a t i o n s  i n  t h e  n e u t r o n  d e t e c t o r  s i g n a l s  w e r e  o n l y  a b o u t1.4 t i m e s   th e ______i d e a l  s t a t i s t i c a l  v a l u e  of1\ /  n u m b e r  of n e u t r o n s  d e t e c t e d .N o n e  o f  t h e  r e s u l t s  i n  T a b l e  3 e x h i b i t  a s t a t i s t i c a l l y172s i g n i f i c a n t  p a r i t y  v i o l a t i n g  e f f e c t .  T h e  r e s u l t s  fo r  p h o t o d i s i n t e g r a t i o n  o f  d e u t e r i u m  ar e  in a g r e e m e n t  w i t h  t h e o r e t i c a l  c a l c u l a t i o n s *  7 ■8 ’ t h a t  i n d i c a t e  t h a t  e x p e c t e d  e f f e c t s  a r e  l e s s  t h a n  a b o u t  5 x 1 0 ' 8 at t h e s e  e n e r g i e s .  T h e  c a l c u l a t i o n s  o f  O k a ' 8 ’ s h o w  t h a t  at 4.2 MeV, the p h o t o d i s i n t e g r a t i o n  m e a s u r e m e n t  is e x p e c t e d  to b e  d o m i n a t e d  b y  w e a k  n e x c h a n g e  te r m s .  A t  t h r e s h o l d ,  b o t h  c a l c u l a t i o n s 7 • 8 ’ i n d i c a t e  t h a t  tt e x c h a n g e  t e r m s  ar e  small, so t h a t  t h e  p a r i t y  v i o l a t i o n  e f f e c t s  a r e  d o m i n a t e d  b y  w e a k  p a n d  w e x c h a n g e .  T h e  p r e s e n t  m e a s u r e m e n t s  a n d  t h e  m e a s u r e m e n t s ' 4 ’ o f  t h e  p ( n , Y ) d  i n v e r s e  r e a c t i o n  at t h r e s h o l d  h a v e  n o t  r e a c h e d  t h e  l e v e l  o f  s e n s i t i v i t y  n e c e s s a r y  to d e f i n e  p a r a m e t e r s  o f  t h e  w e a k  n u c l e o n - n u c l e o n  i n t e r a c t i o n .  H o w e v e r ,  t h e  r e m o v a l  o f  t h e  m a j o r  d i s c r e p a n c y  w i t h  t h e o r y  p r e s e n t e d  b y  t h e  o r i g i n a l  p(n,JT)d m e a s u r e m e n t ' 3 ’ m e a n s  t h a t  e s s e n t i a l l y  all m e a s u r e m e n t s  o f  p a r i t y  v i o l a t i o n  e f f e c t s  i n  t w o  n u c l e o n  a n d  l i g h t  n u c l e a r  s y s t e m s  ar e  c o n s i s t e n t  w i t h  t h e o r y . '5 ’T h e  p r e s e n t  e x p e r i m e n t  is th e  f i r s t  m e a s u r e m e n t  of p a r i t y  v i o l a t i o n  i n  t h e  p r o d u c t i o n  o f  b r e m s s t r a h l u n g  b y  l o n g i t u d i n a l l y  p o l a r i z e d  e l e c t r o n s .  F o r  t h e  e n e r g i e s  u s e d  in  t h e  p r e s e n t  m e a s u r e m e n t s ,  a s y m m e t r i e s  ar e  e x p e c t e d ' 9 ’ to be  v e r y  s m a l l  (< 1 x  1 0 ' 9 ). E f f e c t s  f r o m  d i r e c te l e c t r o n - n u c l e o n  n e u t r a l  c u r r e n t  w e a k  i n t e r a c t i o n s  a r e  s u p p r e s s e d  b y  t h e  v e r y  l o w  m o m e n t u m  t r a n s f e r s  u s e d  i n  t h e  p r e s e n t  m e a s u r e m e n t .  E f f e c t s  f r o m  t h e  w e a k  n u c l e o n - n u c l e o n  i n t e r a c t i o n  a r e  a l s o  e x p e c t e d  to b e  v e r y  small. T h e y  w o u l d  c o n t r i b u t e  t h r o u g h  i n e l a s t i c  s c a t t e r i n g  to p a r i t y - m i x e d  n u c l e a r  s t a t e s  a n d  ar e  s u p p r e s s e d  b e c a u s e  t h e  i n e l a s t i c  s c a t t e r i n g  is s u c h  a s m a l l  p a r t  o f  t h e  t o t a l  c r o s s  s e c t i o n  at t h e s e  e n e r g i e s '  9 ’ .It is i n t e r e s t i n g  to c o n s i d e r  f u t u r e  p o s s i b i l i t i e s  f o r  i m p r o v e m e n t s  to a m e a s u r e m e n t  o f  t h i s  type. First, m o r e  s t r i n g e n t  c o n t r o l s  c o u l d  b e  i m p o s e d  o n  i n t e n s i t y  c h a n g e s  u p o n  p o l a r i z a t i o n  r e v e r s a l .  W e  h a d  d e v e l o p e d  t w o  f e e d b a c k  s y s t e m s  w h i c h  c o n t r o l l e d  i o n  l a s e r  i n t e n s i t y  or P o c k e l l s  c e l l  v o l t a g e .  T h e s e  s y s t e m s  r e d u c e d  A  v a l u e s  f o r  b e a m  i n t e n s i t y  to l e s s  t h a n  1 x 1 0 ' 5 , b u t  t h e y  i n t r o d u c e d  s o m e  b e a m  m o t i o n  a n d  t h e r e f o r e  w e r e  n o t  u s e d  d u r i n g  f i n a l  r u n n i n g .  W e  w e r e  r e s t r i c t e d  to o p e r a t i o n  at a b e a m  l o c a t i o n  w i t h  n o  e n e r g y  a n a l y z i n g  m a g n e t  a n d  t h e r e f o r e  h a d  to r e s o r t  to a n  i n d e p e n d e n t  d e t e c t o r  to m e a s u r e  e n e r g y  c h a n g e s .  T h e  s t a t i s t i c a l  a n d  s y s t e m a t i c  a c c u r a c y  i n  d e t e r m i n i n g  b e a m  e n e r g y  c o u l d  t h e r e f o r e  b e  i m p r o v e d  in a f u t u r e  m e a s u r e m e n t .  A l t h o u g h  w e  o p e r a t e d  w i t h  a v e r a g e  b e a m  c u r r e n t s  a b o u t  f i v e  t i m e s  h i g h e r  t h a n  o t h e r  p u l s e d  p o l a r i z e d  e l e c t r o n  s o u r c e s ,  o u r  q u a n t u m  e f f i c i e n c y  f o r  t h e  g a l l i u m  a r s e n i d e  p h o t o e m i t t e r  w a s  t y p i c a l l y  a b o u t  1 x 1 0 " 3 . E x t e n s i v e  d e v e l o p m e n t  w o r k  o n  b e a m  o p t i c s  i n  t h e  s o u r c e  w o u l d  p r o b a b l y  e n a b l e  o p e r a t i o n  at c u r r e n t s  o f  t e n s  o f  m i l l i a m p e r e s  as w e  h a d  o r i g i n a l l y  h o p e d .  T o  date, d e v e l o p m e n t  w o r k  o n  o t h e r  p h o t o e m i s s i o n  m a t e r i a l s  f o r  1 0 0 %  e l e c t r o n  p o l a r i z a t i o n  h a s  b e e n  u n s u c c e s s f u l .173H o w e v e r ,  s e v e r a l  a p p r o a c h e s  a r e  s t i l l  u n d e r  i n v e s t i g a t i o n .  T h e r e f o r e ,  it is p o s s i b l e  to c o n s i d e r  f u t u r e  m e a s u r e m e n t s  w i t h  t e c h n i q u e s  s i m i l a r  to t h o s e  u s e d  i n  t h e  p r e s e n t  m e a s u r e m e n t  b u t  i m p r o v e d  i n t e n s i t y ,  p o l a r i z a t i o n  a n d  c o n t r o l  o f  s y s t e m a t i c s  t h a t  c o u l d  i m p r o v e  t h e  m e a s u r e m e n t  a c c u r a c y  b y  a s u b s t a n t i a l  a m o u n t .  W i t h  a m a j o r  e f f o r t ,  it m i g h t  b e  p o s s i b l e  to r e a c h  a s e n s i t i v i t y  c o m p a r a b l e  w i t h  t h e  p r e s e n t  t h e o r e t i c a l  p r e d i c t i o n s  o f  p a r i t y  v i o l a t i n g  e f f e c t s  < 5 x1 0 “ 8 i n  t h e  p h o t o d i s i n t e g r a t i o n  of d e u t e r i u m .ACKNOWLEDGEMENTSW e  w o u l d  l i k e  to t h a n k  R.H. T o o n e  a n d  J.G.V. T a y l o r  f o r  t h e i r  e x t e n s i v e  w o r k  o n  t h e  B. I . C .  d e t e c t o r s  a n d  C h a r l e s  S i n c l a i r  a n d  D a n  P i e r c e  f o r  t h e i r  g u i d a n c e  in th e  d e v e l o p m e n t  o f  t h e  p o l a r i z e d  e l e c t r o n  source.REFERENCES1. J.M. P o t t e r ,  et. a l ., Phys. Rev. Lett. 33 (1974) 1307,R. B a l y e r ,  et. al., P h ys. Rev. C 30 (1984) 1409,V. Y u an, et. al., Phys. Rev. Lett. 57 (1986) 1680.2. R. W i l s o n ,  et. al., N B S  S p e c i a l  P u b l i c a t i o n  711 (1986) 85.3. V.M. L o b a s h o v ,  et. al., Nucl. P h ys. A 1 9 7  (1972) 241.4. V.A. K n y a z k o v ,  et. al., N u cl. Phys. A 4 1 7  (1984) 209.5. F o r  a s u m m a r y ,  see E.G. A d e l b e r g e r  a n d  W.C. H a x t o n ,  Ann. Rev. Nucl. Part. S c i . 35 (1985) 501.6. B. D e s p l a n q u e s ,  et. al., Ann. Phys. 1 2 4  (1980) 449.7. H.C. Lee, Phys. Rev. Lett. 41 (1978) 843.8. T. Oka, Phys. Rev. D 2 7  (1983) 523.9. B.K. K e r i m o v ,  M.Y. Safin, Sov. J. N u cl. Phys. 42 (1985) 433, M.Y. Safin, p r i v a t e  c o m m u n i c a t i o n .10. C.Y. P r e s c o t t ,  et. al., Phys. Lett. B 7 7  (1978) 347.11. E.D. E a r l e ,  A.B. M c D o n a l d ,  E.G. A d e l b e r g e r ,  K.A. S n o ver,H.E. S w a n s o n ,  R.D. V o n  L i n t i g ,  H.B. Mak, C.A. B a r n e s ,  N u cl. Phys. A 3 9 6  (1983) 221.12. E.D. E a r l e ,  et. al., to b e  p u b l i s h e d .13. J.F. Ly n c h ,  et. al., I E E E  T r a n s .  N u cl. Sci. N S 2 4  (1977) 692.174DISCUSSION:McDonald commented that the error on the first Lobashov experiment was probably due to improper shielding o f the apparatus from gammas.Wilson: Our Monte Carlo program for their shielding shows that they shouldhave gotten the result they did. They did a bad calculation for their shielding. They also did a measurement with graphite in place o f water that was wrong. When they did it again in their last runs in 1979 they found more nearly the right answer. They put in more shielding the second time.McDonald was pointing out that calculations by Lee using different wave functions were lOx larger than Oka's calculations near the breakup threshold.Adelberger: I am surprised at the factor o f 10 disagreement in the calculations.There must be a mistake somewhere.McDONALD: Part of it is in using more modem values o f /zp(0) and hpO-), but partof it is also due to differences in the wave function. I think there were variations o f a factor o f 5 among different people in calculating the Lobashov result.McKellar: More like a factor o f 100.Adelberger: But there were mistakes too!McDONALD: That may be, but is is also possible that there were differences in thewave functions.Adelberger: That is hard to understand. Are you telling me you couldn't calculatenp —> dy, given an interaction, to better than a factor o f 5? That is the slop you have in the choice o f the wave functions? There is something wrong.McKellar: Things have settled down.McDONALD : Oka is probably the current value ( <10'8) in these cases.During the description o f the experiment.Adelberger: What were the materials in the self-powered detectors?McDONALD: Aluminum and tungsten.Discussion at the end o f the talk.Woloshyn: Are the errors quoted only statistical?M cD O N A L D : They are a combination o f statistical and systematic errors, but thesystematic errors dominate by a factor of 3-4.Adelberger: Some people obtain 42% polarization from the source. Did you use adifferent substrate?M cD O N A L D : Yes. People seldom get >35% polarization from GaAs. ReinerNeuhausen used a GaAs phosphite source. The quantum efficiency is about lOx lower175but the tunable laser wavelengths are more favourable in that region so there is a trade­off.van Oers: What is the polarization reversal frequency that can practically beapplied?McDONALD: The times from the equation in the plot are the spin-exchange time.Ultimately one can reach a 3He polarization equal to the polarization o f the alkali in the volume of the pump. The reversal takes place by applying an RF pulse to essentially flip over the 3He spin if you simply reverse the circular polarization at the same time. You could do reversal frequencies o f a few Hz if you want to. The question is how much polarization is lost in flipping from one state to the other. This depends on how uniform the fields are. This is probably not a problem at a few hertz, but it takes a couple of hours to spin it up in the first place.Kowalski: The 3He is held in a weak holding field, so you can also adiabaticallyreverse it by field ramping.McDONALD : Yes, there are various ways to do it.Adelberger: That sounds not quite as good.McDONALD : You are moving your magnetic fields around, so you have systematicproblems, whereas the adiabatic passage just implies the RF picture.176STUDIES OF PARITY VIOLATION USING POLARIZED SLOW NEUTRON BEAMS*J. Alberi, R. Hart, E. Jeenicke, R. Ost, and Richard Wilson Harvard University, Cambridge, MA 02138I.G. ShroderNational Bureau of Standards, Gaithersberg, MD 20899A. Avenier, G. Bagieu, H. Benkoula, J.F. Cavaignac,A. Idrissi, D.H. Koang and B. VignonInstitut des Sciences Nucleaires, IN2P3, 38044 Grenoble, FranceABSTRACTIn this paper I review the work that has been done on studies of parity violation in nucleon systems using polarized beams of slow neutrons, first describing an early attempt by Haas, et al.. and ending with some suggestions and predictions for the future.INTRODUCTIONSoon after the discovery of parity violation in weak interactions, attempts were made to find it in hadron-hadron interactions. One such attempt, presaging the future, was made by Haas, et al. .^ who attempted to find an asymmetry (A^) of the 7  ray in capture of polarized neutrons by Cd^^. They found that the asymmetry was less than 10'^. A more sensitive experiment was performed by Abov, et al. . ^  who found an asymmetry of 3 x 10'^. This was the first successful measurement of parity violation in hadron-hadron interactions.Subsequently, measurements have been made on capture in Cl^ -*, Sn^® and attempts have been made to find asymmetries in np capture and nd capture.A closely related set of experiments is the measurement of the polarization of the capture gamma ray. In the ground state transition from the neutron capture state (1 +) to the ground state 0 + , only one parameter can be determined, the mixing of the electric and magnetic dipole transitions. This is determined either by the asymmetry of the 7  ray from capture of polarized neutrons or the polarization of the 7  ray from capture of unpolarized neutrons, P^ = 2A^. The polarization of the capture gamma ray from neutrons on cadmium was measured^ and agrees with the asymmetry after a sign error was corrected. A search has also been made for polarization of that in np capture.®A third set of measurements was first proposed by Michels^ and has been carried out by Forte, et al.^  This set involves the rotation of neutron spin in passing through a material. Parity violating effects have been found in Sn ® , and Pb^®®. An experiment is in progress to measure spin rotation in na interactions.This third set of measurements led to a fourth. It was soon realized that the amplitude for spin rotation is closely related to the difference in capture cross section for neutrons with longitudinal spin. One is the real part of an amplitude, the other, the imaginary. The measurement of this*presented by Richard Wilson.177cross sectional difference has been carried out for a number of nuclei,1 1 19particularly as Gatchina•LJ- and at ITEP, Moscow. ^The measurements on nuclei are comparatively simple. Polarization and asymmetries of a few times 1 0 "^ are common and using resonance neutrons, the ITEP group measured a cross section difference of 2 x 1 0 for Sn^-^ -*■ ySnU®. However, these are hard to interpret. Accordingly, much effort has been expended on the nucleon system - np -*■ dy; nd -*• ty. Here, however, predicted polarizations and asymmetries are 1 0 '^ to 1 0 "®.For np capture from the state to the state, two parameters can be determined. P^ and are almost independent.EXPERIMENTAL PROCEDURESTwo great advantages of neutron beams in the study of small asymmetries are that the neutrons are uncharged and not greatly influenced by stray fields. The only effect of importance is the bending of the neutrons by interactions of the magnetic moment with the inhomogeneous edges of a guide field. A second advantage is that the fluctuation in neutron intensity from a reactor is small - typically 0.1%. Accordingly, all these experiments can be performed with very good control of systematic errors.Disadvantages of neutron beams are that they are not as intense ascharged particle beams, and the energy of the polarized beam is not as easy to vary. The parity violating effects arise from an interference between a strong parity conserving amplitude and a weak parity violating one. The latter is roughly proportional to energy, and at zero energy there is no parity violating effect. The "effective energy" of a neutron beam for this discussion is the energy of the capturing state, 8 Mev for Cd; 2 Mev for deuterium. It is not possible to increase this to 15 Mev and 45 Mev to increase the parity violating parameters, as is the case for pp and painteractions.At present, most experiments on the np system are intensity-limited. Increases of 10- to 100-fold in intensity are needed to find the predicted effects.NEUTRON BEAMSIt is straightforward to obtain a reasonably intense beam of slow, polarized neutrons. The neutrons are slowed by moderation in a liquiddeuterium source inside a high flux reactor. The slow neutrons will scatterexternally off several metal surfaces, and can be guided from the reactorinto an experimental hall. The last section of such a guide can be amagnetized polarizer: only the neutrons of one spin are externallyreflected; the others are transmitted and then absorbed. A particularly useful design is the supermirror of Mezei who showed that a set of partially reflecting layers carefully spaced can give almost total reflection even below the cutoff wavelength.The total unpolarized neutron intensity that was available at the endof beam H14 at ILL was 3 x 10^ n/sec over an area 3 cm x 5 cm. This has nowbeen increased to about 5 x lO^/sec by a small modification in the cold1 1source. At a beam from a new cold source, we anticipate 3 x lO^Vsec over an area 6 cm x 17 cm. The transmission of a Mezei supermirror is about 25%, leading to possible polarized neutron beams of nearly lO^/sec.The layout of beams at Institut Latte-Langevin (ILL) is shown in Fig. the arrangement for in np -*• dy in Fig. 2.ContainmentH14 ECExperimental HallPN7CS * cold source (liq. D2) HS ■ hot source C ■ CoreE C 'e x te rn a l  cabih  Various beamsFig. 1. Arrangement of beams at Institut Latte-Langevin showing the beam H14 and the experiment position PN7BG P _AFI F2BGPABC:I)B C'4 _ A >4 Di, D jPol o riz e rPermanent m agnetic fie ld  Reversible m agnetic f ie ldF1.F1TPMPMD 0H i © ' 1\ J W0  uPMDetectors' 4 0 0  1 o f liqu id  sc in tilla to r NE-235 Current s tr ip s  (see fig . 3)Hydrogen ta rg e t P ho tom u ltip lie rsFig. 2. Arrangement of neutron guides for np dy experiment.179C a n c e l l a t i o n  o f  S p u r i o u s  E f f e c t sAlthough, in principle, all that is required is to compare the capture7  ray intensities along, and against, the direction of the neutron spin, nodetector has a detection efficiency and solid angle known to the required accuracy of 10"® or less. Accordingly all experiments are done by comparing the effects of changing the direction of the neutron spin. The neutron spin is "guided" by a small magnetic field of a few gauss, and the spin of a slow neutron follows the field. When it passes a thin current strip, the field suddently reverses, the spin cannot follow, and the spin is subsequentlyguided against the field. This is shown in Fig. 3.Fig. 3. Arrangement of "Dabbs" current strip for flipping the spin.Then the experiment is performed by flipping the neutron spin as rapidly as possible (once per second is usual), and comparing the rates for the two different directions of spin. The following possible errors can creep into the data.1. Beam fluctuations can be greater in the one second than the statistics in the number of neutrons collected. This is compensated by using two almost identical detectors. The average of the measurements gives the signal we want; the difference tells us how big the effect of beam fluctuations can be, and ensures that a second order effect does not occur.2. The spin flipping can itself produce a systematic error from one of the following causes:a) The current can cause a magnetic field change at thei) scintillator ii) phototubesb) There can be spurious electrical effects on the electronic circuits.c) There can be a displacement of the neutron beam because of the interaction of the magnetic moment with inhomogenous fields.We guard against these by three independent methods. I describe the final setup used in ref. 5.1. We designed the apparatus so that these errors were negligibly small.180a) We have wires carrying a compensation current of opposite sign on either side of the current strip spin flipper. This was both calculated and measured to reduce the change in magnetic field at the photomultiplier tube and scintillator to less than 1 0 ' 5 gauss. Magnetic shielding, particularly of the photomultiplier reduced this to negligible proportions. The effect of magnetic fields on the scintillator liquid was measured to be quadratic with field, and reach 1 % at about 1 0  gauss.b) The circuits were arranged to avoid current loops. The photo­multiplier tubes were grounded at the electronics rack. The power supply for the current strip was located on a separate rack, controlled by an optically coupled LED. The ac for this was taken from a separate ac line, and for the np -» dy experiment the power supply was switched into a dummy load. Measurements showed noshift of discriminator levels on switching the current strip evenwithout these precautions (Fig. 4).c) We calculated the effect of the inhomogeneous magnetic fields on bending the beam. It was zero if the beam was directed down the center of the guide fields, and only non-zero if it were displaced.We calculated the effect to be small.Fig. 4. Layout of the electronic circuits for the nd experiment (ref. 6 ) showing ground loop breaking. At this time the current in the spin flipper was not balanced by a dummy load when switched.2. We took auxiliary measurements with a pulser (which did not intro­duce statistical fluctuations) with a Co^O source, and for the nd -*■ ty experiment with capture on np -+ dy. These showed that spurious effects were small as calculated in (1) above. We also measured background from captures on other materials using a Ge(Li) detector and made small corrections, with the errors, for the effects on the asymmetry.3. We took data in a number of configurations so that these effectscancel in the average and can be measured in the difference. For example, asecond spin flipper was used which was reversed every 27 seconds. This enabled the whole experiment to be repeated with identical spurious effects, but the direction of the spin changed. In no case did the difference show an181effect larger than that allowed by (1) and (2) above. The cancellation was adequate, and verified that second order effects did not have to be con­sidered. Finally, each set of spin reversals corresponded to a separate experiment, and the distributions of asymmetries gave a gaussian as expected.In all our experiments we ended up with very few errors. In 1970, in in an early test of the experiment in reference 7, the polarimeter power supply and the detector electronic circuits were on the same rack. When this polarimeter was run asymmetrically (to measure the analysing power) a spurious asymmetry of 10 " 3 was observed. This was avoided by using separate racks and taking the power supply from a separate line transformer. It also vanished when the polarimeter was run symmetrically.In the first publication of the nd experiment^ we made an error. For each minute we measuredNLt n r ir = ---------- 1 + 46 (1 )NL1 NRtThen we averaged all values of R to get the final result. This was wrong. The distribution of R is asymmetric and careful thought shows that the correct result is given byRfinal = " 1/2 (variance of single measure) (2 )This was corrected in the final result.In the np -*• dy data of the 1984 experiment, a plot of R - <R> forvarious measures gives a good gaussian (see Fig. 5), but there remains a set of data 4a from the mean. These have been recently re-examined and there are good reasons for discarding them. This changes the result from A„(np) = -(4.7 ± 4.7) x 10'® to A7 (np) = -(1.5 ± 4.8) x 10"®.Fig. 5. Histogram of ^1 ~ \i in np -►dyCT1182Figure 6 shows a Ge(Li) detector spectrum of the captures in D2 O and surrounding target material. This shows that nd capture was dominant even for this low capture cross section. For np capture this applies with greater force.CHANNEL NUMBERFig. 6 . Ge(Li) detector spectrum of the captures in a D 2 O targetand holder showing dominance of nd captureRESULTSThe data can be expressed in the usual DDH-^ formalism by noting that A 7 (nd) = 0.92^ - 0.5hp°- 0.16hw°+ O.lOt^ 1 - 0.002hw1+ 0 . n J J h p (3)= (4.2 ± 3.8) x 1CT6A7 (np) - -0.107 f „  + O.OOSh^1- 0.001h^1 + 0.02(hp 1 ) 1 (4)= (-1.5 ± 4.7) x 1(T8The second of these may be compared with other experiments on the pp and pa system, and F-*-8 , F and F*^, by noting that the principal sensitivity is to fn , and that the others involve both fn and the combination h = h^ + 0 .6 hw .In Fig. 7, this is superimposed on an early plot. More recent values of fn against h^ can be found in the review of Adelberger and Haxton . -*-8 It will be noted that the sensitivity to f^ still does not approach that from measuring P7  in F-*-8 .The value for A7 (nd) also has too large an error to be useful, and the formula is too complex to make any prediction except that it is in the rangeA7 (nd) theoretical - 5 x 10"^ to 10 ‘ 8 (5)183Fig. 7. Plot of hp vs. f^ from various experiments.FUTURE POSSIBILITIESA-y (np)The np experiments were intensity limited and not limited by systematic errors. In the last, we used an intensity of neutrons, before the polarizer, of 3 x 109/sec over an area of 3 cm x 5 cm. This has now risen to 5 x 109/sec over the same area, by m a k i n g  a  hollow liquid deuterium moderator as a source for the cold neutrons.For the future the intensity could be increased as follows.1. A beam of 3 x lO^/sec over 6 cm x 17 cm has now been made at ILL using a new cold source from a 20 cm diameter through tube. It will be used during CY1989 and CY1990 for a neutron-antineutron oscillation experiment. Since the old set of experiments is complete, a new collaboraton would have to be performed for using this. An error of ±3 x 10 " 9 seems possible.2. The much delayed 200 MW research reactor at Garchina near Leningrad is now expected to operate in 1990, with full intensity by 1992. A beam of lO^/sec is possible.3. The existing accelerator spallation sources do not compete. It is possible that future ones will.F 7 ( n p )This was limited both by intensity and systematic errors. There must be adequate shielding to avoid detecting any of the bremsstrahlung y rays from the fission products in the core, which have circular polarizations of about 10"3. The polarimeter must be well constructed so that the magnetostriction is small and symmetric.184I believe the only hope for improvement is the use for a year or two of a 20 cm diameter through tube in a high flux reactor as proposed in 1974 and illustrated in Fig. 8 . ” A polarimeter at each side balances fluctuations. Lead (high thermal conductivity) and bismuth (low capture cross section) alternate absorbers will alternate polarized 7  rays from bremsstrahlung in the reactor core without producing too many 7  rays of their own.y  frombremsstrahlung //// Bismuth low cr (ny)PbhighthermalconductivityMagnetized iron ' polarimeterFig. 8 . Schematic for P^(np) experiment in 20 cm diameter through tube.So far, I have not found a reactor owner with enthusiasm for this idea.R(np)In this meeting, Adelberger described the experiment on neutron spin rotation in hydrogen that he and Blayne Heckel are building. I am surprised that it is more sensitive than A^(np) but it seems to be. I note that thefreedom from systematic error of an A^(np) experiment applies to this also.It is a surprise to me that Heckel and Adelberger believe they can geta 4cr effect with the present beam (H14) whereas A^(np) got a la effect. The reasons probably include: no loss of solid angle due to 7  counting; no back­ground subtraction; and no waste of beam in the spin flipping arrangement.( a .  - q+ ) / ( q +  +  a . )The experiment (a. - a+)/(a+ + a.) is believed to be very small for np capture because it depends on S,P interference. However, there is a Russian claim that it is of the order of P^, which I do not now understand.A^(nd)This was limited by count rate in the 7  ray detector. The requirement is to have a detector that can separate background and we chose to count pulses. Two possibilities exist for improvement.1. Find a detector that has a shorter scintillation decay constant (e.g. Nal without T1 activation at liquid nitrogen temperatures).2. Integrate and correct for backgrounds which are measured at lower rates.185P-y (nd)This also seems hard, but would be done similarly to P_(np). It would be impossible to count pulses because the polarimeter efficiency of 3% demands a count rate which is 1 0 0 0  times higher than that for the comparable experiment.R(nd)This seems a natural follow on from R(np).L i g h t  N u c l e iNone of the nuclei near A = 20 are easily accesible for neutron capture. However, since measurements in many nuclei area easy and reliable, I urge continual thought on which ones might give useful information. In Fig. 9 I show the level scheme of Cl^. I note the closeness of the 2 + level to the 2 ' capturing state, which might lead to a simple understanding of the measured asymmetry (1.57 ± 0.53) x 10 ' 4  of the ground state of the 7  ray. I note that there is a close lying opposite parity state also in Fe5 5 (ny)Fe , as well as a state of opposite parity close to the ground state.8.580 393eVA C K N O W LE D E G E M E N TSThe work of Richard Wilson and Harvard colleagues was supported until 1981 by grants from the National Science Foundation (award nos. PHY-777-1115 and PHY72-05122). The work of the Grenoble authors was supported by the Institut des Sciences Nucleaires (IN2P3).In addition to the authors listed, many others have helped in this work over the years. We would like to thank G. Scharff-Goldhaber, R. Steinberg and R. Liaud for their assistance and Drs. Hayter (Rutherford), 0. Schaerf and B. Heckel for making the polarizers and helping set up the H14 beam.Finally, we would like to thank the reactor staffs of the Institut Laue-Langevin and the National Bureau of Standards for their assistance.186REFERENCES1. R. Hass, L.B. Leipuner, R.K. Adair, Phys. Rev. 116, 1211 (1959).2. Yu. G. Abov, et al.. Sov. J. Nucl. Phys. .16, 670 (1973).3. M. Avenier, et al.. Nucl. Phys. A436. 8 (1985).4. H. Benkoula, et al, Phys. Lett. 71B. 287 (1979).G.A. Von Danilyan, et al.. JETP Lett. 24, 344 (1976).5. J.F. Cavaignac, et al.. Phys. Lett. 67B. 148 (1977).E. Jeenicke, Ph.D. Thesis, Harvard University (1976).E.A. Idrissi, Ph.D. Thesis, University of Grenoble (1985).6 . M. Avenier, et al.. Phys. Lett. 137B. 125 (1984).M. Avenier, et al.. Nucl. Phys. A459. 335 (1986).7. J. Alberi, et al.. Phys. Rev. Lett. 29, 518 (1972). Note that the signof the result in this paper is incorrect.8 . V.A. Kaniaskov, et al.. Nucl. Phys. A396. 221 (1983).9. F.C. Michels, Phys. Rev. B133. B329 (1964).10. M. Forte, et al.. Phys. Rev. Lett. 45, 2088 (1980).11. V.P. Alfimenkov, JEPT Lett. 3JL. 313 (1972).12. V.A. Verna, et al.. JETP Lett. 36, 59 (1982).13. See 0. Shaerf, Annual Report, Institut Latie-Langevin, Grenoble, France, p. 33 (1981).14. E. Jeenicke, et al.. Nucl. Inst. Meth. 126. 459 (1975).15. B. Deplanques, et al.. Ann. of Phys. 124. 449 (1980).16. E.C. Adelberger and W.C. Haxton, Ann. Rev. Nucl. Part. Sci. 35., 501(1985).187DISCUSSION:Wilson presented a histogram o f asymmetries obtained over one period and results with and without data points beyond 4 a . The plot showed only 1 point beyond 4 a . Bowman: (paraphrased slightly to include sense o f several other commentsfrom the audience) Are the data shown from one data period? One data point should not cause such a large effect.Wilson: There are in fact several 4a  data points. It appears that these werecaused by readout errors o f the data and should be removed. I do not have more details at the moment.Wilson showed the histogram o f the asymmetry parameter R = N h * N ld/ ( N]d* N m) = 1 + 4  8 and commented on the fact <R> = 1 + 112 * variance, ifh  = 0.Adelberger: The problem of a skew distribution can be avoided if one uses(R-1)/(R+1) as the variable instead.Discussion after the presentation.McDonald: Lobashov had a nice symmetric system, but what about thedistribution o f flux across the tube? (paraphrased)WILSON: The flux was non-uniform.Bowman: Can you explain how the non-uniformity produces a fake effect?WILSON: It depends on the type of polari