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A study of background radiation distributions for the SLAC B factory drift chamber Goodenough, C. R. Cherie 1996

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A STUDY OF BACKGROUND  RADIATION  FOR T H E SLAC B FACTORY  DRIFT  DISTRIBUTIONS CHAMBER  By C.R. Cherie Goodenough B.Sc. (Hon), University of Victoria, 1994  A THESIS SUBMITTED IN PARTIAL F U L F I L L M E N T OF T H E REQUIREMENTS F O R T H E D E G R E E OF M A S T E R OF S C I E N C E  IN T H E F A C U L T Y OF G R A D U A T E STUDIES D E P A R T M E N T OF PHYSICS  W E A C C E P T THIS THESIS AS CONFORMING T O T H E REQUIRED STANDARD  T H E UNIVERSITY OF BRITISH COLUMBIA AUGUST ©  1996  C . R . C H E R I E G O O D E N O U G H , 1996  In presenting this thesis in partial fulfillment of the requirements for an advanced degree at the University of British Columbia, I agree that the Library shall make it freely available for reference and study. I further agree that permission for extensive copying of this thesis for scholarly purposes may be granted by the head of my department or by his or her representatives. It is understood that copying or publication of this thesis for financial gain shall not be allowed without my written permission.  Department of Physics The University of British Columbia 6224 Agricultural Road Vancouver, Canada V6T 1W5  Date:  OCT.  IO  X  Abstract The distribution of lost-particle and bremsstrahlung background radiation in the  BABAR  Drift Chamber at the Stanford Linear Accelerator Center (SLAC) has been modeled. Simulations show that occupancies are well within manageable levels (peaked at 0.55% in the inner layers) at nominal rates and that radical changes in the endplate design are unnecessary. Chamber aging is also shown to occur at an acceptable level, with no significant damage to the chamber wires within the first five years.  11  Table of Contents  Abstract  ii  List of Figures  v  Acknowledgments  vii  Foreword  ix  1 Introduction  1  1.1  The Standard Model and the C K M Mixing Matrix  2  1.2  The Matter/Antimatter Puzzle  6  1.3  C P Violation  10  1.3.1  The K Mesons  10  1.3.2  The B Mesons  14  2 The PEP-II Machine and  BABAR  Detector  27  2.1  The Linac  27  2.2  The PEP-II Storage Rings  27  2.2.1  29  2.3  The Interaction Region  The BABAR Detector  33  2.3.1  Silicon Vertex Detector  34  2.3.2  Drift Chamber  36  2.3.3  Particle Identification  40  2.3.4  Electromagnetic Calorimeter  42  in  2.3.5  Instrumented Flux Return Magnet  3 Detector and Background Radiation Simulation 3.1  3.2  4  5  44 47  Background Radiation  47  3.1.1  Synchrotron Radiation . . . •  47  3.1.2  Residual Gas Collisions  48  3.1.3  Electromagnetic Showers  49  3.1.4  Other Background Sources  50  Detector and Background Simulation  50  3.2.1  Tools  50  3.2.2  Decay T U R T L E  51  3.2.3  GEANT  51  3.2.4  OBJEGS  61  Discussion of Results  62  4.1  Endplate Design Studies with the O B J E G S Simulation Package  62  4.2  Occupancy Studies  66  4.3  Aging Studies  72  Conclusions  73  A OBJEGS Input File  74  B  BBSIM Input File  81  C  BABAR  Author List  90  Bibliography  99  iv  List of Figures  1.1  The Feynman diagram of a flavour-changing charged weak interaction.  1.2  The decay of both the A" and K° particles to two and three pions.  1.3  a)/\~°'s oscillate into K°'s via the exchange of W bosons and, b) via a  0  .  . . .  4 11  ±  real intermediate state of pions  12  1.4  The "unitary triangle"  15  1.5  B° - B° mixing via two virtual top quarks  18  1.6  Three decay modes of B°to a C P eigenstate  22  2.7  Schematic view of PEP-II complex on S L A C site  28  2.8  Plan view of the PEP-II interaction region  30  2.9  Support barrel for IR components inside the detector  31  2.10 Synchrotron radiation beams for the low-energy and high-energy beams. .  32  2.11 A cross-sectional view of the detector  34  2.12 Three-dimensional cutaway view of the silicon vertex detector  35  2.13 Cross-sectional view in a plane containing the beam axis  36  2.14 The drift chamber  37  2.15 Schematic of the DIRC including support structures  41  2.16 Side view showing dimensions (in mm) of the calorimeter barrel and forward endcap  43  2.17 The Instrumented Flux Return detector  45  2.18 Schematic representation of the Resistive Plate Chamber components. . .  46  3.19 G E A N T simulation of beam line (IR) components  55  v  4.20 z distribution intensity of beam-gas photon radiation in photons/4 ^sec. 4.21 z distribution intensity of beam-gas electron radiation in 1 0  - 3  67  electrons/4  fisec  68  4.22 Simulated background occupancies for each drift chamber layer due to converted photons from showers produced by lost particle interactions.  .  69  4.23 The background distribution in (j>  70  4.24 Energy distribution per layer in M e V  71  vi  Acknowledgments  Working at S L A C on the  BABAR  drift chamber project has been an exciting, educational  and rewarding experience. This would not have been the case, however, were it not for the many wonderful people who have given me their support and friendship over the past four years, first as an undergraduate and later as a U B C student. I am glad to have this chance to thank some of the people who have been the most influential to me. It is undoubtedly true that without Dale Pitman as a mentor and friend I would not only never have had the opportunity to work at S L A C , but I would also not have pursued my graduate degree in particle physics. I have been very lucky to have her as a teacher, advocate and role model. Thank you for everything, Dale. My good fortune continued with having Janis McKenna as my supervisor. Janis has been an incredibly supportive, understanding and patient person to work for and I can't imagine having completed my degree without her help. Thanks, Janis, I should be so lucky as to have another boss like you. There are many people at S L A C who have provided a great deal of help and encouragement. They are too many to name them all, so perhaps I will simply thank all of Group C. Requiring specific thanks, though are Art Snyder, Dave Coupal and Tom Mattison. I would also like to express my thanks to the second reader on my thesis, Garth Jones. I appreciate the time and effort he has expended both as a teacher and a reader. The wonderful friends that I have made during my days at S L A C have been a source of support and even, occasionally, actual information. The S L A C Spinors softball team includes almost everyone in this category. If it weren't for the Bridge Club (Jim ' Q ' vii  Quigley, Dave Reyna, Greg Mitchell and Suzanne Komili) I might have finished this thesis months ago, but I would have had a lot less fun. And last, but not least, two people with whom it has been my honour to be acquainted with over the last few years, Jaret Heise and Mark McDougald, thanks for everything.  vin  Foreword  A new storage ring (PEP-II) and detector  (BABAR)  are currently under construction  at the Stanford Linear Accelerator Center. The physics goal for this new machine and detector is to measure the parameters of C P violation using the B meson system. Many small studies are required to optimize the design and performance of both the machine and the detector. A number of Canadian institutions, including the University of British Columbia, are contributing to the design and construction of the central tracking drift chamber component of the  BABAR  detector. This thesis is a detailed discussion of one of  the studies carried out for the drift chamber group, namely modeling the distribution of background radiation in the drift chamber. The results of these studies were published in the  BABAR  Letter of Intent [11] and the  BABAR  ix  Technical Design Report [12].  Chapter 1 Introduction  Why does the universe look the way it does? This complex question remains thus far unanswered. How did galaxies form? Is the universe going to expand forever from the Big Bang or will it all end with a Big Crunch? Why are there three generations of quarks  and  leptons  with very different masses? Why is the universe made almost entirely  of matter, not equal parts matter and antimatter? BABAR  It is this last question that the  collaboration at Stanford Linear Accelerator Center (SLAC) hopes to answer, at  least in part. To that end, S L A C and a large group of other institutions (including the University of British Columbia) are currently designing and constructing an asymmetric, high-luminosity B-meson facility at S L A C . The central topic of this thesis is a detailed study of the distribution of background radiation in a long central tracking chamber now under construction for  BABAR.  Specific  design considerations will be addressed from the point of view of backgrounds.  This  study was undertaken using two Monte Carlo programs in work done at S L A C from 1993 to 1995. Since the PEP-II e /e~ +  collider used by  BABAR  has several unique aspects to its  design that seriously affect background radiation, there is a chapter discussing the machine design. These accelerator innovations are likewise motivated by the B physics to be studied at  BABAR.  Consequently, this introductory section will discuss the physics of  C P violation and its study using the B system. There will also be a brief section defining the types of background radiation studied.  1  Chapter 1. Introduction  2  1.1 The Standard Model and the C K M Mixing Matrix The Standard Model of particle physics is a very successful model for describing particles and their interactions via three of the four fundamental forces. These forces are the strong, the electromagnetic, and the weak. The theory of quantum chromodynamics (QCD) describes strong interactions and the electroweak theory describes a unification of the electromagnetic and weak interactions. For a more complete discussion of the Standard Model see [1, 2]. There are two types of particles described by the Standard Model, fermions (which make up matter) and gauge bosons (which mediate forces). Fermions are likewise separated into two distinct groups, quarks and leptons. Both quarks and leptons are spin 1/2, pointlike particles. One way in which quarks differ from leptons is that they carry colour charge, allowing them to interact via the strong interaction. A consequence of this colour charge is that quarks are never seen in unbound states, but only in three-quark bound states (baryons) and quark/antiquark bound states (mesons). This is a result of the fact that only colourless objects are seen in nature. There are six flavours of quarks in the Standard Model, which are grouped into three generations:  The upper quark in each generation has charge +2/3e and the lower -l/3e. There are also six leptons, grouped into three generations. Each generation contains a massive particle with charge -le and a corresponding massless, chargeless neutrino (denoted by  Chapter 1.  Introduction  3  Each quark and lepton lias a corresponding antiparticle, denoted by a bar (ie. the antiparticle of a c quark is c). Fermions can interact via the weak force. This force is mediated by the Z° and W  ±  gauge bosons. Weak interactions involving a Z° exchange are called neutral currents and those involving  are called charged currents. The charged weak interaction is said to  be flavour changing as a result of the fact that when a quark emits or absorbs a W  ±  boson, the particle flavour changes to another quark flavour. In the case of leptons, the massive particle changes to the neutrino in its generation (or vise versa). For example, a u quark emits a W and changes flavour to a d. This process is shown diagrammatically in +  Figure 1.1, an example of a Feynman diagram. Feynman diagrams are not only useful for depicting particle interactions, but are also powerful theoretical tools for the calculation of the probability that an interaction will occur. Throughout this thesis, the time axis in Feynman diagrams will point to the right. Not only is the charged weak interaction flavour changing, it also allows "crossgenerational mixing" of quarks, according to the Standard Model.  This means that  while a c quark can decay (ie. interact by emitting or absorbing a gauge boson) into an s quark, it might also decay into a d quark. This phenomenon is accommodated in the Standard Model by saying that the s quark is not an eigenstate of the weak interaction. The particle participating in the weak interaction (the weak eigenstate) is not the s quark, but rather a linear combination of the d, s, and b quarks (which are referred to as mass or flavour eigenstates). The charged weak interactions are incorporated into the Standard Model through the Cabibbo-Kobayashi-Maskawa ( C K M ) matrix. The weak  Chapter 1.  Introduction  Figure 1.1: The u quark emits a W changing its flavour to d. The W then decays int +  a positron and an electron neutrino.  +  Chapter 1.  Introduction  5  eigenstates, denoted by a primed symbol, are given by: V i  V  Vh  Vd c  v  Vb  td  V  U(  \ v  us  u  cs  V  ts  (1.1)  s  c  j  tb  w here C12C13  •S12C13  V  C12C23 -  \  512523 -  C 2C 35i e'" ' 1  1  2  3  5  -Ci S 2  2 3  •Si25 35l3e*  5 1 3  2  ~  5 i 2 C 3 S l eJ$13 2  3  (1.2)  •S23C13  C23C13  /  Here, C;J = cos^j and Sij = sm0;j with i and j being "generation" labels 1, 2 and 3. C P violation enters in non-zero values of the phase 5i [3] (more on this in Section 1.3.2). The 3  convention of assigning the +2/3 mass eigenstates u, c, and t to also be weak eigenstates and the -1/3 weak eigenstates to be linear combinations of the -1/3 mass eigenstates is arbitrary. It should also be noted that the parameterization of V given by Equation 1.2 is valid only if there are exactly three quark families (as in the Standard Model). A 3 x 3 unitary matrix contains 3 free rotation angles (%) and one physical phase (<5i3)[3]. The magnitudes of the elements of the C K M matrix have been determined experimentally. These are shown below, with 90% confidence limits [3]: ^ 0.9747 - 0.9759 \V\ =  ^  0.218 - 0.224  0.002 - 0.005  0.218 - 0.224  0.9738 - 0.9752  0.032 - 0.048  0.004 - 0.015  0.030 - 0.048  ^ (1.3)  0.9988 - 0.9995 j  Note that the diagonal elements are all approximately unity and that the elements 14(, and V d are the smallest in the matrix. This implies that while the charged current t  weak interaction allows cross-generational mixing, this mixing is suppressed compared to flavour changing within a generation. In other words, the flavour changing charged current interaction c —>• d is suppressed relative to the c —> s interaction. The c —> d  Chapter 1.  Introduction  6  interaction is said to be Cabibbo suppressed. By the same argument, the interaction b —> u is highly Cabibbo suppressed, owing to the very small magnitude of the V b element of u  the quark mixing matrix. Finally, it is observed in charged current weak interactions that the W  ±  couples only  to lefthanded fermions and righthanded antifermions. A particle is said to be righthanded if its spin vector is aligned with its momentum vector, lefthanded if antialigned. Charged current weak interactions, the C K M matrix and how C P violation is incorporated into the Standard Model will be discussed further in Sections 1.3 and 1.3.2.  1.2  The Matter/Antimatter Puzzle  As mentioned in Section 1.1, every particle has an antiparticle (although some neutral particles, like the photon, are their own antiparticles). In fact, particles and antiparticles are routinely created in collisions using particle accelerators. It is very interesting to note that matter and antimatter seem to be created in equal quantities by these high-energy collisions. This observation is in direct contradiction to the evidence of the everyday world. Antimatter is rarely encountered outside of the particle accelerator. In fact, evidence suggests that the entire universe is composed of matter, with very little antimatter. Why is this? Did such an asymmetry exist from the moment of the Big Bang, or did this imbalance develop sometime later? If the latter, what is the mechanism for this imbalance to exist? But first, what is the evidence for the dominance of matter in the universe? It is easily shown that matter and antimatter cannot exist in close proximity. A star made of half matter, half antimatter would annihilate immediately, all of its matter converted to energy. However, the possibility still exists that matter and antimatter  Chapter 1.  Introduction  7  might coexist if confined to isolated regions in the universe. Simply looking at objects in the universe does not provide any answers about their composition. Since photons are their own antiparticles, photons created in interactions in a matter galaxy look identical to photons created in antimatter galaxies. The same is not true for neutrinos, which have distinct antiparticles. While these certainly would mark whether the originating galaxy was composed of matter or antimatter, neutrinos are very difficult to detect since they are massless, chargeless and interact very weakly with matter. Cosmic rays provide the first clue. These are high-energy particles that arrive at earth from space, mostly from objects within the Milky Way. Cosmic rays are composed almost entirely of particles, not antiparticles, indicating the the Milky Way is composed predominantly of matter.  This argument also holds for the local group, a cluster of  galaxies of which the Milky Way is a member. For more distant galaxies, the evidence consists more of what is not observed. If a matter and antimatter galaxy were to collide, this would appear as a very strong source of gamma-ray emissions. Such sources are not observed, although several sources of gamma radiation are under investigation. The most compelling argument, however, is that it is very difficult to imagine how matter and antimatter could have been created in equal quantities in the Big Bang and become segregated into distinct regions of the universe. It seems much more likely that they would have been created homogeneously and annihilated everywhere. Given that the universe is dominated by matter, how did this imbalance arise? One answer could be that the asymmetry was built in. However, this is not a satisfying conclusion. Virtually any composition of the universe could be explained in this way, and it requires the imposition of initial conditions with no apparent rationale. The most attractive explanation of the matter/antimatter imbalance lies in an argument that  Chapter 1.  Introduction  8  starts with a symmetric universe that develops an imbalance through some observable mechanism some time after the Big Bang [7]. The Sakharov Criteria state the minimum conditions necessary for this asymmetry to develop from an initially symmetric universe. These criteria are listed below [5, 6]. 1. Baryon non-conserving interactions occur. 2. C P invariance must be violated. 3. Statistical thermal equilibrium does not exist at the time that conditions 1 and 2 were able to operate. It is beyond the scope of this thesis to discuss the Sakharov Criteria in detail. However, a very brief discussion of conditions 1 and 3 is in order here. The following sections will address only C P invariance violation. The properties (except mass) of any particle can be completely described, according to the Standard Model, by the values of their quantum numbers. A n example of a quantum number is electric charge. A n electron has charge quantum number -1, a proton +1, a photon 0, a u quark +2/3, and so on. One fundamental law of interactions is that electric charge is conserved. The total charge of a system before interaction must be equal to the total charge after. Another quantum number is baryon number. Baryons are three-quark bound states. Each baryon has baryon number +1, and antibaryons have baryon number -1. A l l other particles have baryon number 0. Baryon number has long been thought to be conserved in interactions, just like charge. The reason this appears to be the case is that if baryon number was not conserved, the proton (the least massive particle with baryon number +1) could decay into lighter particles with baryon number 0 (say a positron and a neutrino). The consequence of this decay would be to make impossible the existence of stable atoms. A l l of the protons in the universe would have long ago decayed away to lighter particles.  Chapter 1.  Introduction  9  The dominance of matter over antimatter in the universe is tantamount to the universe having a positive baryon number. Again, the following arguments will be based on the assumption that the universe began with baryon number 0 and developed an imbalance as seen today. This implies that baryon non-conserving interactions must have taken place at some point in the evolution of the universe. It is postulated that very early in the history of the universe, the temperature being in excess of 10 K , the weak and 28  strong interactions were unified and this unified force was mediated by another gauge boson (call it X , for convenience). These X bosons were very massive and could decay via baryon number violating interactions. Once the temperature of the universe dropped below this threshold, the strong and weak interactions decoupled and no more of these X's were created. Eventually, all of the X ' s decayed away in their baryon non-conserving interactions. This requires that statistical thermal equilibrium did not apply at this point in the history of the universe. Postulating the existence of the X boson is not sufficient to explain the baryon asymmetry of the universe, however. Symmetry arguments made previously would predict that equal numbers of its antiparticle would be produced and decay via non-antibaryon conserving interactions. Later, it will be shown that C P violation allows asymmetry in the decays of particles and their antiparticles. These two mechanisms, baryon non-conserving interactions and C P violation, acting together in a non-equilibrium situation might have been sufficient to cause the dominance of matter over antimatter in the universe. It should be noted that in fact the asymmetry need only be small. A one in one billion excess of baryons over antibaryons in the early universe would be sufficient to result in the matter dominated universe seen today.  Chapter 1.  Introduction  10  C P Violation  1.3  The charge operator, C, which takes particles into antiparticles and the parity operator, P, which results in space inversion, are invariant in both the strong and electromagnetic interactions. It was assumed until the late 1950's that C, P, and T (the time reversal operator) were individually conserved in all interactions.  In the case of parity, this  symmetry would mean that if a particular interaction takes place, its mirror image must also take place with equal likelihood. However, large violations of P were seen in weak interactions, most notably by C.S. Wu in the famous  6 0  C o experiment [8]. The electrons  created in the /? decay of C o are emitted in a preferential direction. In fact, it is now 6 0  said that charged current weak interactions violate parity maximally, due to the coupling of the W  ±  exclusively to lefthanded fermions (there is no mirror symmetry) as stated in  Section 1.1. Next, it was postulated that C and P operating together were conserved. This transformation takes a physical particle state into a physical antiparticle state. Consequently invariance under C P implies a particle-antiparticle symmetry in nature. As mentioned in Section 1.2, however, the universe is dominated by matter. This observation requires that C P violating interactions take place.  1.3.1  The K Mesons  C P violation has been observed in the neutral K system. Both K° (quark content: sd) and K° (TT  +  (sd) are seen to decay into  7r 7r +  -  and into 7r 7r 7r° as shown in Figure 1.2 +  _  — ud, TT" = ud and 7T° = a mixture of uu and dd). Since they share decay channels  they can mix through the weak interaction. In other words, iv*°'s can change into K°'s via an exchange of W  ±  bosons. The lowest order diagram of this oscillation through  pions is shown in Figure 1.3a. This "box diagram" is called a virtual process while the  Chapter 1.  Introduction  Figure 1.2: The decay of both the K° and K°  11  particles to two and three pions.  oscillation shown in Figure 1.3b is called a real process since the the intermediate state of pions can be seen in nature (remember that a single u quark is never seen due its colour charge). The result is that through weak decays K s 0,  and K° 's cease to be orthogonal  states (ie. they cease to be distinct particles). While they are eigenstates of the strong interaction, the same cannot be said for the weak [9].  Chapter 1.  Introduction  Fi gure 1.3: a)A^°'s oscillate into K°'s via the exchange of intermediate state of pions.  12  bosons and, b) via a real  Chapter 1.  Introduction  13  The neutral kaons can be considered as linear combinations of two weak eigenstates, K° and K° as follows: 2  \K°) = ^ ( l * i ° > + 1 ^ ° ) ) I* )  - 1^2°))  =  5  ( 1 4 )  The states \K®) and \K®) would be C P eigenstates with eigenvalues +1 and -1 respectively. Consequently, each of these states would be responsible for specific decays of neutral kaons, for example: K?  - > 7T+ + 7T-  (1.5) since  7r 7r~ +  is a C P eigenstate with eigenvalue +1, while 7 r 7 r 7 r has eigenvalue -1. The +  _  0  phase space of the two-body decay would be much larger than that of the three body decay so the state l-f^)  w o u l  d have a longer lifetime than \Ki). Most importantly, since  these weak eigenstates are orthogonal, they would not have common decay channels. If C P is conserved in weak interactions,  should never be seen to decay into a C P  eigenstate with eigenvalue +1. (1.6) K%  7T+ + 7T-  Two components of K° are indeed observed, one long-lived and one short-lived. However, the long-lived component is seen to decay into two pions with a branching ratio on the order of 2.3 x 1 0 . So, instead of K° and K%, one constructs the following eigenstates - 3  of the neutral K system: \K$) = p\K°) + q\K5) \K° ) L  C P violation in K°-K°  = p\K°)  -  q\K5)  gives: q p  1 +e  '1.  Chapter 1.  Introduction  14  Here, e is a measure of C P violation in the K system. If C P violation did not occur in this system, e = 0 and p = q = To summarize, C P violation is manifested in the neutral K system by the existence of two physical particles (Kg and K^) with very similar masses, but much different decay rates, which both decay into two pion and three pion C P eigenstates (although not in equal proportions). It is shown that C P violation occurs, but more detail on its possible inclusion in the Standard Model requires further information. This information can be obtained through the study of the B meson system, which is shown in the next section. 1.3.2  The B Mesons  In order to understand why the B meson system is expected to be an excellent source for the study of C P violation, it is necessary to look at the C K M matrix defined in Section 1.1 in more detail. The C K M matrix is unitary (ie. VV^ = 1). Applying this constraint to the first and third columns of the matrix 1.2 gives the relationship:  v v: ud  b  + v v; cd  b  + v vi td  t  =o  (i.9)  The unitary triangle, shown in Figure 1.4a [10], is simply a geometric representation of Equation 1.9 in the complex plane. It should be noted that while the triangle shown here is referred to as the unitary triangle, the constraint of unitarity can be applied to all the other rows and columns of V resulting in a total of 6 unitary triangles. The experimental measurements of the magnitudes of the elements of the C K M matrix given in Equation 1.3 show that Si ~ 0.22, 523 ~ s\ w 0.05 and 5 2  2  I 3  ~ s\  2  0.01 which  Chapter 1.  Introduction  Figure 1.4: The "unitary triangle"  Chapter 1.  Introduction  16  allows V to be represented, to a very good approximation, by: (  1  V  s  Sl2  V  12  s e~  1  523  iS  13  iS  5 e ° 2  •S12-523 -  - 5  13  2  1  3  ^  (1.10) /  Note that in the above parameterization, all of the elements of V are real except V and ub  V. td  Using the parameterization 1.10, equation 1.9 becomes: v: + v ^-v; v b  td  b  (l.n)  cd  Since, according to the parameterization above, the elements V and V cb  cd  are real (or  very nearly), the unitary triangle will have one side along the real axis, as shown in Figure 1.4b. The angles a, /3, and 7 result from a non-zero phase, 6, in the C K M matrix. This phase incorporates C P violation into the Standard Model. If C P violation did not occur, all of the angles of the unitary triangle would be zero. If the angles do not add up to TT radians, interactions are occuring that are not explained by the Standard Model. If one notes that Im(f^) = sin (26) and z = Re , it follows from Figure 1.4a that: %e  fy^Mf\  = sin2a  Im  \v: v v v j b  I  m  ud  td  (U2) 1  { ^ V M )  =  -  s  ub  cd  ^  m  \v: v v v; ) d  1  tb  1  (  1  L  1  3  )  ;  b  The quantities 2a, 2/3 and 2j can be measured through the study of time-dependent, C P violating asymmetries in decays of neutral B mesons [4]. It is useful to define several quantities for the discussion of time-dependent, C P violating asymmetries. The B particles produced in the e /e~ collision with a specific quark +  Chapter 1.  Introduction  17  content are denoted by B°= bd and B°= bd. These are not necessarily mass eigenstates, denoted by Bi and B , nor are they C P eigenstates, B = B° + B° and B- = B° - B°. 2  B  +  +  and 5_ are somewhat analogous to K\ and K i n Section 1.3.1 as C P eigenstates 2  with eigenvalues +1 and -1 respectively. The Hamiltonian for the neutral B system is given by: (  H =  I  M  M  1  2  \  i (  r  j { r;  r  N  1 2  (1.15)  2  M*  M  2  2  r  ,  where the first matrix is the mass matrix and the second matrix is the decay matrix [10]. The mass matrix describes processes involving virtual intermediate states that do not lead to the decay of the meson. The off-diagonal elements are dominated (in the Standard Model) by B°*-> B° mixing via two virtual top quarks. The diagram for such mixing is shown in Figure 1.5. The off diagonal elements are therefore proportional to the quark mixing matrix elements at the four vertices: M  L2  cx [V V; f TB  d  (1.16)  for the Bd system. A similar argument results in: M  S  « (V V; )  2  12  TB  S  (1.17)  for the B° system (B° = Is). s  s  The decay matrix describes processes with real intermediate states which can result in the decay of the meson. Here again, the off-diagonal elements are due to  B°  mixing. The dominant decays of B° are to final states containing a c (B° —*• Xlv\, branching ratio = 9.5 ± 1.6% where X is dominated by anticharm) and the B° to final states containing a c quark. This means that, unlike the K system mixing via real pions, the real states accessible to both B° and B° are highly Cabibbo suppressed. Therefore,  Chapter 1.  Introduction  a)  Vb  b  t  d  IF d  <  1  v«  t <—  1  <—  b  v,  b)  B°  Figure 1.5: B° - B° mixing via two virtual top quarks.  Chapter 1.  Introduction  19  it is expected that: | r | < |M |  (1.18)  12  1 2  The eigenvectors of H are B\ and B , the mass eigenstates: 2  IB,) = p\B°) + q\W) \B ) 2  (1.19)  q\W)  = p\B°) -  (compare to Equation 1.7) and  Mh - »n/2  ;i.20)  2  p  \| M  1  -  2  iT /2 12  Using the approximation in Equation 1.18, IM*  2  VM  P  £1  (1.21)  12  The eigenvalues corresponding to the eigenvectors B, and B are m — | T i and 2  m — |T 2  2  x  where: m  = M±  1>2  ReQ,  (1.22)  T =T±ImQ h2  Q =  yf(M? - | n ) ( M 2  2  12  -  ir) ia  From Equation 1.18 it is clear that: AT < A m  (1.23)  These quantities ( A T and A m ) are measurable [4, 10]. They are simply the difference in the decay rates and the masses of the physical B mesons. Note that the B meson system is opposite in this respect to the K meson system. In the case of the K mesons, the two physical particles  and K$ have almost identical masses, but very different lifetimes,  while in the B system the masses are different and the lifetimes are expected to be very similar.  Chapter 1.  Introduction  20  Since the physical states B° and B° produced at t = 0 can mix, these states evolve in time. A n initially pure 5 ° develops a B° component. To further the discussion of C P violation, only decay to final states which are C P eigenstates (fcp) accessible to both physical states, will be considered here. As previously mentioned, these decays are highly Cabibbo suppressed and therefore have extremely small branching ratios ( 1 0  -3  to 10~ ). 5  The amplitude for a time-evolved B° or B° to decay to fcp is given by: (fcp\B° (t))  = U(t)(fcp\B°)  phys  (fcp\Bt (t)}  = *f-{t)(f p\B°)  phys  where /  +  +  lf-(t)(fcp\B°)  (1.24)  f (t)(fc m  +  C  +  P  and / _ are defined as follows: f (t) = e- e- ' mt  cos  Tt 2  +  = r-  i m t  Ami  e - ' i sin T t  2  2  (1.25)  Ami  and m and T are the mean mass and decay rate of the physical B mesons respectively. The time-dependent rate for an initially pure physical state to decay into a final C P eigenstate, then, is found by multiplying the equations in 1.24 by their complex conjugates:  r(# VW -* M P  T(B* (t) pkys  - f ) CP  = \(fcp\B°)\ e2  x {cos  rt  = \(f p\B*)\ e2  + |A| sin ^  2  2  x {cos ^  rt  2  C  +  2  sin ^ 2  - 2/mAsin ^  cos  - 2 / m A " sin ^  cos  1  (1.26) where A is defined as: _q(f p\B°)  x  C  p(fcp\B°)  (1.27)  Using Equation 1.21 and noting that if a single weak-decay diagram dominates: \ifcp\B* (fcp\B°)  (1.28)  and it follows that  |A| =  (fop\B°) (fcp\B°)  (1.29)  } }  Chapter  1.  Introduction  21  In other words, in the B meson system, A is pure phase (A = e^) and Im\ = —7mA . 1  In the end, the equations in 1.26 simplify to (for A m —> 0): nB° s(t)  -  phy  T(W (t)  fop) = \(fcp\B°)\^{l -  phys  f p)  - 7mA sin Ami}  = | ( / a p | ^ ) | e - { l + 7mAsin A m / } 2  C  r <  The time-evolved, CP-violating asymmetry in the decay rate of an initially pure B° state to a C P eigenstate, compared to that of an initially pure B° is given by: A(7> —> / C P ) = —==  :  :  ,  r = 7mA s i n A m /  n  1.31  Recalling Equations 1.16, 1.21 and 1.27, A can be written as:  n  -^W (fcp\Bf,  Ad  d  \  -  VM.  (fcp\B°)  [  3  2 6  )  '  It is now possible to see how the study of specific decays of neutral B mesons to C P eigenstates will enable the precise measurement of the angles of the unitary triangle. The decay B proportional to  —• 7 r 7 r +  V* V dh  u  -  is shown in Figure 1.6a. The amplitude of this decay is  Therefore:  VtbVtd ub -ud V  V  Since Vtb is nearly unity as is V d, and from 1.12: u  7mA = 7  m  tbV  v  td  ^  ^ = sin2c  ub  ud  It follows that the time-dependent asymmetry for B A(B  —>• 7 r 7 r ) +  _  (1.34)  V V  —>  7r 7r~ +  is:  = 7mA sin A m / = sin 2a sin A m /  Next, Figure 1.6b shows the decay B —> J/ibK$.  (1.35)  This is a more complicated decay  in that the final state is a C P eigenstate only to the extent that 7f° is a C P eigenstate.  Chapter 1.  Introduction  a)  b)  c)  Figure 1.6: Three decay modes of B° to a C P eigenstate.  Chapter 1.  Introduction  23  A factor £ arising from K°- K° mixing is included (see Figure 1.3) in the equation for A:  v v; ib  v; v  d  b  v v*  cs  v v;  cd  ib  v; v  d  b  { cd  • >  Using the parameterization 1.10 this simplifies so that:  = Wl±Y^Yk  ImX  Im  _  =  /3  sin2  (1.37)  and the time-dependent asymmetry for B —»• J ftp Kg is: A(B -»• J/ipK°s)  = - sin 2/3 sin A m i  (1.38)  The last decay shown in Figure 1.6 is B —> /a-K^. Again a term for K°- / ^ m i x i n g is s  included in A giving: , _ VttVtd VubVud V*d cs  / , QQ\  v  K*&Kd  1  •  ;  and therefore: / m A  =  /  r  ^  a  tbV  V  td  ^  ^  =  V Vud V V ub  cd  _  s i n 2 7  ( L 4 0 )  cs  and A(B  S  -> p / ^ ) = - s i n 2 7 s i n A m i  (1.41)  Other decay modes of interest in the study of time-evolved CP-violating asymmetries are listed in Table 1.1. The decays of the neutral B meson system into final C P eigenstates allow precise measurement of the angles of the unitary triangle (and therefore good measure of C P violation) by measuring the time-dependent asymmetries. A l l that remains is the measurements themselves. There are three basic experimental requirements to measure the time-dependent asymmetries: 1. To produce B mesons. 2. To reconstruct B meson decays to C P eigenstates.  Chapter 1.  Introduction  24  Table 1.1: More decay modes of the neutral B mesons. Quark subprocess 6  —» c + cs, c + cd, s  Decay Mode B  -» V ^ 5 , X#s,  d  uK ,  pK ,  s  L  b b —>  u + ud  T/J-C , 5  (j)K ,  pK ,  L  L  —» 7 r 7 r , pp, pir°,  B  +  d  B  c + cs, c + cd  s  _  -> pK , s  - sin (2/3)  D+D-,D°D°,  s  ibK , b —> u + ud  ImX  LOK ,  0  pK ,  s  L  — sin (27)  toK  L  B -> ^(^, 77 0, ^/^5 C  s  sin ( 2 a )  LOT: , 7r°7r°  KsK6  2  s  i  n  7  3. To determine the bottom-flavour (quark content b or 6) of the decaying B meson at t = 0 and to measure the decay time t. The first requirement can be fulfilled i n many ways. Large numbers of B mesons are produced at hadron colliders and fixed-target experiments. While the cross-section for production of B°/B° e /e~ +  pairs is much higher for these types of experiments than at an  collider, it is a much smaller percentage of the total cross-section, so a very low  signal-to-noise ratio results. The difficulty of picking the signal out of the background makes the ability to fulfill the other two requirements questionable. This leads to the conclusion that the best machine for B meson experiments is an e /e~ collider. +  Using an e /e~ collider, the highest cross-sections for B°/B° +  production are found  at the T(45') resonance and at the Z° resonance. The T ( 4 5 ) has quark content 66. A resonance occurs when the energy of the colliding beams is equivalent to the mass of the particle produced, in this case 9.46 GeV. While the cross-section is highest for the Z° resonance, the T(4S') resonance is more attractive since the average number of charged tracks per e e~ —> T(45') —» B°B° +  event (ie. e e~ —> qq continuum) is much lower than +  for the Z° resonance. While there are other events produced at the T(iS) resonance,  Chapter 1.  Introduction  25  the signal-to-noise ratio of approximately one-to-three is better than at any other energy. Therefore, requirements 1 and 2 are best met by producing B mesons at an e /e~ collider +  running at the T(45') resonance. The branching ratio of the T(45)  —>  B°B°  events as well as the lower resonance re-  quire that the e /e~ machine used have a very high luminosity. This is further reason to +  carefully consider the background rates. The machine and detector must be designed in such a way as to minimize the impact of luminosity related backgrounds, specifically synchrotron radiation, with serious attention paid to radiation hardness in a high-luminosity environment. The necessity of identifying the flavour of the B meson at time t = 0 requires that the vertices of both the B° and B° mesons can be resolved. This so-called "tagging" involves reconstructing the decay of one of the mesons produced in the T(AS) decay in order to determine whether it was a B° or B°. Once this is accomplished, the precise quark content of the other meson at t = 0 is known. This knowledge, combined with good time resolution for the decay and time-evolution of mixing, enables the decay of the second meson to be reconstructed and the C P violating parameters to be measured. This criterion leads again to design considerations for both the machine and the detector. The PEP-II B-Factory has been designed asymmetrically with a 9 GeV electron beam and a 3.1 GeV positron beam. Particles produced by the colliding beams will therefore not be at rest in the lab frame. The result will be spatial separation of the decay vertices allowing for good resolution. Detector requirements for good event tagging include excellent spatial resolution especially close to the interaction point (IP) to resolve vertices, as well as good particle identification and momentum resolution to separate out backgrounds from non-B decays. Further, multiple scattering from material in the detector must be minimized to attain the necessary momentum resolution. Finally, the asymmetry in beam energies results in  Chapter 1.  Introduction  26  the need to have solid angle coverage for tracking, particle identification and calorimetry to small angles in the forward (electron beam) direction. The PEP-II B-Factory machine and  BABAR  detector at S L A C are currently being  designed with the requirements of measuring the parameters of C P violation using the B°/B°  system as benchmarks.  Further discussion of the design considerations and  specifics of the detector is given in later chapters.  \  Chapter 2 The PEP-II Machine and  BABAR  Detector  The B-Factory consists of three major components: the linear accelerator (Linac), the PEP-II Collider and the  BABAR  detector. This section is intended as a very brief overview  of the various components. For a more complete discussion see [11, 12].  2.1 The Linac Electrons produced by a heated filament are accelerated in SLAC's two-mile Linac to 9 GeV and injected into one of the PEP-II storage rings in a clockwise direction. The electron beam in the Linac is then aimed at a tungsten target to produce positrons. The positrons are accelerated to 3.1 GeV and injected into the other ring in a counterclockwise direction. The charged particles are accelerated to velocities very close to the speed of light using electromagnetic fields in cavities along the accelerator.  2.2 The PEP-II Storage Rings The storage rings are stacked in a single tunnel and cross at the interaction point (IP). After the IP the beams are redirected back into their respective rings. The result is that the beams actually cross at two points: the IP and a point 62 cm away from the IP. However, they do not collide at both points since they are separated horizontally with a bending magnet at the 62 cm point. Figure 2.7 shows the Linac and PEP-II storage rings.  27  Chapter 2.  The PEP-II  Machine and BABAR  Detector  28  Figure 2.7: Schematic view of PEP-II complex on S L A C site. The low branching ratios of the benchmark decays to be studied at the B-Factory require very high luminosity beams. The bunch crossing separation in PEP-II will be 4.3 ns. The beams must also be strongly focused near the IP to ensure a maximum collision rate and thus a high luminosity. This strong focusing is achieved by the placement of magnets very close to the IP in the interaction region (IR). The following section will discuss the IR in more detail as its design significantly impacts the study of background  Chapter 2.  The PEP-II  Machine and BABAR  Detector  29  distributions in the drift chamber.  2.2.1  The Interaction Region  Figure 2.8 shows the PEP-II interaction region in detail. The low energy beam (LEB) enters from the lower right and exits through the upper left. The high energy beam (HEB) enters left and exits right. It is important to note that the scale of this figure is highly exaggerated in the vertical direction. Also note that the quadrupole magnet, Q l , and the dipole magnet, B l , are within the detector with all of B l and some of Q l to the inside of the drift chamber volume. The magnet B l separates the beams horizontally before the second crossing point 62 cm downstream of the IP. Q l focuses both the H E B and L E B . However, the L E B is off-axis in Q l to maximize beam separation. Q2 focuses only the L E B with the H E B traversing a field free-region. Likewise, Q4 and Q5 focus only the H E B . The Q l and B l magnets are within a 1.5 T magnetic field and therefore must be permanent or superconducting. It is possible for Q l to be a superconducting magnet. However, space constraints would not allow the placement of a cryostat for B l . Consequently, both Q l and B l will be permanent, samarium cobalt magnets. The beam pipe near the IP is a double-walled beryllium structure. The inner tube is 800 pm thick with a radius of 25 mm. The outer tube in 400 pm thick with a radius of 27 mm. Beryllium is chosen as the beam pipe material in order to reduce multiple scattering, and the tube is coated with 10 pm of gold to attenuate synchrotron radiation. The total beam pipe represents 0.6% of a radiation length. There is also a support barrel within the detector volume on which the Q l and B l magnets, the IP beam pipe, and the vertex detector are assembled. The support barrel will be assembled and internally aligned before being inserted into the drift chamber. The end sections of the support barrel are made from stainless steel to support the Q l magnet  Chapter 2.  The PEP-II  Machine and BABAR  Detector  30  Figure 2.8: Plan view of the PEP-II interaction region. The vertical scale is highly exaggerated. The dashed lines represent the 300 mr detector acceptance cutoff. assemblies. The barrel portion is constructed of a carbon fiber composite representing 0.005 radiation lengths (Xo). Figure 2.9 shows the support barrel and masks on the B l magnet which are discussed later. The location of the Q l and B l magnets so close to the IP is necessary for the separation and strong focusing of the beams. However, bending the beams with these magnets is also the source of a large synchrotron radiation flux (see Chapter 3). For this reason there are water-cooled beryllium masks inside the B l magnet to prevent synchrotron radiation from striking the beam pipe. Figure 2.10 shows the synchrotron radiation fans in the IR region. Aside from the synchrotron flux generated by beam separation and strong focusing within the detector  Chapter 2.  The  Carbon-fiber support tube 0.5% X  PEP-II  Machine  and  BABAR  Detector  Vertex detector Distributed ion pump  0  2.5 cm radius Be beam pipe (cooled)  31  Magnet support  43 cm OD  Inboard and outboard L E B masks (water cooled)  Figure 2.9: Support barrel for IR components inside the detector. Only one end is shown. volume, other sources of radiation are: photons scattering off a mask tip, synchrotron photons generated far upstream of the IP and backscattered photons from downstream surfaces. With the addition of the masks to the IR design, the impact of synchrotron radiation backgrounds is minimized. In fact, while the high luminosity environment and bending within the detector volume would indicate greater difficulty with synchrotron radiation than in more conventional e / e +  _  machines, the masks eliminate the major  sources of synchrotron radiation flux. The result is that synchrotron radiation is so small that it may be neglected in further background studies. The IR having been designed to minimize synchrotron radiation backgrounds, the major source of backgrounds are Bremsstrahlung and Coulomb scattering of beam particles from residual gas molecules in the beam pipe. These can result in high energy beam particles and photons striking both the masks and the beam pipe near the IP. Electromagnetic showers caused by these collisions will result in both high detector occupancies and radiation damage. This effect is minimized by maintaining a very high vacuum in the beam pipe close to the IR. Studies have shown that it is possible to maintain pressures  Chapter 2.  The PEP-II  Machine and BABAR  Detector  32  below 1 nTorr in regions from 3 to 40 m from the IP in either direction. However, due to space constraints, it is impossible to put pumps within the IR. For this reason, the pressure 3 m to either side of the IP is likely to exceed 1 nTorr.  Figure 2.10:  Synchrotron radiation beams for the low-energy (upper diagram) and  high-energy (lower diagram) beams. The density of shading gives an indication of relative photon intensity from the various radiation fans.  Chapter 2.  2.3  The  The  BABAR  The PEP-II  BABAR  Machine and BABAR  Detector  33  Detector  detector is composed of five major components designed for precise mea-  surements of parameters specific to C P violation in the B meson system. The major elements are a silicon strip vertex tracker, a drift chamber, a Cherenkov particle identification detector, a Cesium Iodide (Csl) electromagnetic calorimeter, and a magnet with instrumented flux return for muon detection. The components are concentric around the beam pipe and are offset in the forward direction. Figure 2.11 shows a cross-sectional view of the  BABAR  detector as the design was in 1995. Note that minor modifications  continue to be made. For example, the endplates of the drift chamber are shown here as conical, but are now planned to be flat. There are several issues that arise from factors discussed in Chapter 1 that must be considered in detector design. First, since the B system weak eigenstates have very similar (and short) lifetimes, the detector must have excellent spatial resolution near the interaction point in order to separate the vertices of B° and B° decays. Second, low branching ratios for B decays result in background difficulties, both from non-B decays and from synchrotron and lost beam-particle backgrounds arising from the high luminosity required to produce enough events. The non-B decay backgrounds are excluded with excellent particle identification and momentum resolution. The synchrotron and lost beam-particle backgrounds must be minimized in the detector and accelerator design as discussed in the previous section. The need for precise momentum resolution, particularly for low momentum particles, necessitates minimization of multiple scattering. Third, the asymmetry in beam energies results in the need to have solid angle coverage for tracking, particle identification and calorimetry down to very near the beam pipe in the forward direction. Fourth, the electromagnetic calorimetry must be excellent particularly for reconstructing 7r°'s in B° decays. And finally, all detectors must be radiation  Chapter 2.  The  PEP-II  Machine and BABAR  Detector  34  I  1730  uoo  3-95 7857A21  Figure 2.11: A cross-sectional view of the detector. hard for long life in the high luminosity environment at  BABAR.  While it is beyond the scope of this paper to discuss in detail the design of each detector component, a brief overview of each is presented here. A detailed treatment of the design of each component can be found in [12].  2.3.1  Silicon Vertex Detector  The silicon vertex tracker is located within the 20 cm radius of the carbon fiber support barrel. It and the drift chamber provide all the momentum and most of the tracking information for the detector. The vertex tracker must have excellent spatial resolution  Chapter 2.  The PEP-II  Machine and BABAR  Detector  35  to determine the difference in decay times of the B° mesons. Additionally, it provides measurements of production angles. Also, low momentum particles loop in the 1.5 T magnetic field so tightly that they never enter the drift chamber. Such particles, with PT (transverse momentum) between ~ 40 MeV/c  and ~ 100 MeV/c  are tracked only  within the vertex tracker. The location of the vertex tracker immediately surrounding the IP requires that the detector be reasonably radiation hard. Also, since it is located within the volume of other detectors, it requires low multiple scattering. The vertex tracker is constructed from double-sided silicon strip detectors, arranged in five layers. The inner three layers have a barrel layout with the detectors parallel to the beam pipe. The outer two layers have a barrel layout in the central region, with wedge detectors in the forward and backward regions. Figures 2.12 and 2.13 show both a three-dimensional and cross-sectional view of the silicon vertex tracker.  Figure 2.12: Three-dimensional cutaway view of the silicon vertex detector.  Chapter 2.  The PEP-II  Machine and BABAR  Detector  36  B0080CA  Figure 2.13: Cross-sectional view in a plane containing the beam axis. The vertex tracker is somewhat larger than conventional silicon vertex detectors, with strips ranging in size from 95 m m x 39 mm ( in <j> and z respectively) in layer 1 to 241 mm X (35 —» 52) mm in layer 5. The size is required in order for it to perform not only as a vertex tracker but also as a tracking device for low momentum particles. The tracker achieves an intrinsic resolution of 25 ^ m at normal incidence, and a typical vertex (spatial) resolution of 60 —> 75 pro. depending on the species and momentum of the particle used to tag an event.  2.3.2  Drift Chamber  The second detector component is the drift chamber. Since measuring the parameters of C P violation in the B meson system requires that exclusive final states from B° and B° decays be reconstructed efficiently and with high resolution, the drift chamber must provide good momentum resolution at all momenta, efficient reconstruction of even low momentum tracks, and good solid angle coverage. Its location inside the calorimeter requires that it not contribute significantly to multiple scattering. Additionally, it must provide particle identification through ionization loss for low momentum particles that  Chapter 2.  The PEP-II  Machine  and BABAR  Detector  37  don't reach the exterior particle identification component of the detector.  Finally, the  drift chamber provides one of the two major triggers of the experiment. In order to achieve all of the above goals, a low-mass small-cell drift chamber has been designed for the  BABAR  detector. The small-cell arrangement will provide excellent  tracking and energy loss resolution. The low-mass construction will minimize multiple scattering to improve momentum resolution while minimally degrading the performance of the particle identification and electromagnetic calorimeter components of the detector. Figure 2.14 shows a cross-sectional view of the 1995 drift chamber design.  |-  -p  1273  1717  -i  Figure 2.14: The drift chamber. The drift chamber has an inner radius of 22.5 cm and an outer radius of 80 cm. It is 280 cm in length and bounded at the inner radius by the support tube, the outer radius by the particle identification detector, and in the forward region by the forward endcap of the electromagnetic calorimeter. The volume of the drift chamber is filled with forty concentric layers of low-mass  Chapter 2.  The PEP-II  Machine and BABAR  Detector  38  wires arranged in a small-cell configuration. Each cell consists of a gold plated tungsten wire anode (diameter 20 pm) surrounded by 80 pm and 120 pm gold plated aluminum field wires. The anode wires are also called sense wires as they provide the readout for the detector. The cells are rectangular in shape, 13 mm x 19 mm along the radial and azimuthal directions respectively. A voltage of +1.8 k V is applied to the sense wires while the field wires are maintained at ground and shape the electric field in the cell. This cell shape was chosen for the uniformity of the field produced in the cell while adding little mass to the system. W i t h +1.8 k V applied, the avalanche gain is approximately 5 x l O . 4  The chamber is filled with a low atomic number gas mixture. The gas mixture is helium-based with additives to increase primary ionization and for quenching. The baseline gas mixture for the drift chamber is He:isobutane in a 80:20 combination. Other helium-based mixtures still under consideration include He:C02:isobutane (83:10:7). A basic description of the operation of the drift chamber follows: as a charged particle traverses the drift chamber it ionizes the gas mixture molecules in its path. The ionized electrons then drift toward the anode. During this drift time, further gas molecules are ionized by the drifting electrons producing more electrons which drift toward the anode. This is known as the avalanche effect. As the electrons are accelerated in the field, more and more ionization occurs, producing more electrons. In the high field region near the sense wire, the avalanche effect occurs wherein a large number of electrons in a localized space are produced. The electrons are collected at the anode producing a pulse which is read out. Knowledge of which sense wire was struck, the drift velocity of electrons in the gas mixture, and the drift time of the electrons in the gas (start time determined by external trigger) allows for track reconstruction. In the  BABAR  drift chamber, the layers of wires are arranged in superlayers. Super-  layers consist of 4 layers which are all either axial or stereo. Axial superlayers run parallel  Chapter 2.  The PEP-II  Machine and BABAR  Detector  39  to the beam line (defined as the z direction) and provide x, y information. Stereo superlayers are offset by a small angle (up to 3 degrees from end to end, depending on which layer is being considered). The stereo superlayers provide resolution in the z direction. The total numbers of wires in the drift chamber are approximately 7104 sense wires and 28,768 field wires. A l l of the wires are strung tensed with a weight of 50g to reduce sag and better define the electric field produced in the cells. The maximum deflection due to gravity is 120/zm at mid-length. The result is an axial load on the endplates of approximately 24,000kg. Consequently, the endplates must be manufactured in such a way as to sustain this load without flexing. However, since the endplates will be located within the calorimeter, they must also be of low mass. In order to maximize strength and minimize mass and construction complexity, a flat, aluminum endplate configuration is to be used. These represent approximately 0.14Xo. Due to the energy asymmetry of the beams, the drift chamber is offset in the forward (downstream for the electron beam) direction with a length of 166cm in the forward direction and 114cm in the backward direction. The inner wall of the drift chamber is constructed of 1 mm of beryllium and carries 40% of the wire load. It also provides a gas seal and R F shield for the chamber. As such it must be able to support the pressure differential between the inside and outside of the chamber and it is designed to withstand 30mbar of overpressure. The outer wall must be sufficiently strong to carry 60% of the axial load of the wires between the endplates. However, its large circumference allows this mechanical strength to be achieved with a 3.2 mm thick carbon fiber tube. This exceeds the nominal strength required to support the wire load and includes a safety factor for impact resistance. A l l of the readout electronics and high voltage service boards are located on the backward (upstream) end of the drift chamber. Since there is no particle identification and  Chapter 2.  The PEP-II  Machine and BABAR  Detector  40  limited calorimetry in this direction the significant number of radiation lengths represented by the electronics cards and cables is not problematic. 2.3.3  Particle Identification  Particle identification of both hadrons and leptons is crucial for the measurement of C P violation parameters in the B meson system. In particular, exclusive final states must be reconstructed and the quark content of the other B in the event must be tagged. Particle identification can be carried out for the most part by the drift chamber, electromagnetic calorimeter and instrumented flux return. However, a dedicated particle identification system is required to distinguish charged pions from kaons with momenta above 0.7 GeV/c  and protons above 1.3 GeV/c.  To this end, a ring-imaging Cherenkov detector  has been designed for the barrel region of the detector immediately outside of the drift chamber. A l l Cherenkov detectors operate on the same fundamental concept. Particles enter the detector medium, which has a different index of refraction than that of the medium outside the detector. The changed index of refraction results in the particle's speed exceeding that of light in the detector medium and the particle radiates photons in a characteristic cone distribution. The opening angle of the cone depends on the speed of the particle. Given the momentum of the particle (in  BABAR  this is determined in the  drift chamber), the mass of the particle can be determined. The DIRC (Detection of Internally Reflected Cherenkov light) is composed of 4.7 m long quartz bars oriented parallel to the z direction of the detector. The passage of a particle through the bars produces Cherenkov radiation. The photons produced are reflected internally and exit the bar at the backward end of the DIRC. The Cherenkov image is expanded from the end of the bar through a water-filled standoff region and is detected on a toroidal surface of photomultiplier tubes (PMT's). The P M T ' s are 1.125  Chapter 2.  The PEP-II  Machine and BABAR  41  Detector  in. in diameter and arranged in a close packed array 120cm from the ends of the quartz bars. There are mirrors at the forward end of each bar to reflect forward-going light back to the P M T ' s . The high optical quality of both the quartz bars and the forward mirrors preserve all angle information of the emitted Cherenkov light. Figure 2.15 shows a cross-sectional view of the DIRC.  -Beam Line  IP-X  Figure 2.15: Schematic of the DIRC including support structures. There is no dedicated particle identification system in the forward region, where the drift chamber, calorimeter and instrumented flux return will fulfill the particle identification requirements.  Inclusion of a forward aerogel type of detector was proposed,  but it was abandoned in the interest of minimizing the degradation of calorimetry for low-energy photons. No particle identification is needed in the backward region. The DIRC system is ideal for the  BABAR  detector. The asymmetric energies of the  beams produce a forward boost for high momentum particles. Since the angle of incidence is significantly off-normal, the particles have a longer path length in the quartz, increasing the light production and allowing the thickness of the quartz to be reduced. With this  Chapter 2.  The PEP-II  Machine and BABAR  Detector  minimization of mass, the DIRC represents 0.18 X  0  42  for normal incidence. The small  radial dimension also reduces the volume of the electromagnetic calorimeter [12].  2.3.4 The  Electromagnetic Calorimeter  BABAR  detector requires excellent calorimetry for several reasons. It is necessary to  reconstruct C P eigenstates containing one or more ir° decays. The small branching ratios of these C P eigenstates require high efficiency in detecting low-energy photons to make it possible to reconstruct final states containing several 7r°'s. Also, the calorimeter must provide lepton identification, again for tagging decays of C P eigenstates. A cesium iodide (Csl) crystal calorimeter has been designed to provide the necessary high resolution and efficiency. The C s l calorimeter consists of a cylindrical barrel section, offset in the forward direction, with a forward conical endcap.  The barrel has an inner radius of 90 cm  and outer radius of 135.6 cm. The barrel is constructed of 250 fim thick carbon fiber composite compartments that house individual crystals. Each crystal is wrapped with a diffuse reflecting material on its sides and a reflector on its front face. The crystals are tapered along their lengths with the average area of the front faces being 4.8cm x 4.7cm and the back faces 6.1cm x 6.0cm. They vary in length in 0.5X  0  the forward part of the barrel and the endcap to l6.0X  o  steps from 17.5X in 0  in the backward part of the  barrel. Photodiodes and preamplifier packages provide readout and are located at the outer end of each compartment.  The choice of photodiodes as readout devices was made  since readout must be accomplished in the 1.5 T magnetic field. The forward endcap is segmented vertically into two pieces, each of which can be removed separately allowing relatively easy access to the barrel end region. Figure 2.16 shows a cross-sectional view of the C s l calorimeter.  Chapter 2.  The PEP-II  Machine and BABAR  Detector  43  INTERACTION POINT  Figure 2.16: Side view showing dimensions (in mm) of the calorimeter barrel and forward endcap. The calorimeter is expected to achieve a mass resolution for 7r°'s of 4.5 to 8.3  MeV/c . 2  Since C s l crystals were chosen for the electromagnetic calorimeter, ensuring the necessary high resolution and efficiency, the primary concerns are minimizing the cost and the amount of material in front of the calorimeter. In the interest of cost, the size of all internal components has been restricted and there is no backward endcap. Material between the IP and the calorimeter has been minimized in all internal components. The largest contributor to radiation lengths in front of the calorimeter is the DIRC (T ~ 0.18 XQ). The total amount of material including the beampipe, vertex tracker, drift chamber and DIRC represents T ~0.23 Xo at normal incidence. The major source of material in front of the endcap is the drift chamber endplate (T ~0.14 XQ) [12].  Chapter 2.  2.3.5  The PEP-II  Machine and BABAR  Detector  44  Instrumented Flux Return Magnet  The excellent momentum resolution necessary for the physics studies proposed at  BABAR  requires that the detector operate in a 1.5 T magnetic field. For this purpose, a large solenoidal magnet is located immediately outside of the electromagnetic calorimeter. Since approximately 18% of all B decays contain at least one muon, an instrumented flux return (IFR) muon detector has been designed to make use of the large iron structure needed for the magnet return yoke [12]. The goal is to achieve the highest muon tagging efficiency that is practically possible. Additionally, the I F R constitutes a neutral hadron calorimeter. The I F R design consists of active detectors "sandwiched" between iron plates. Since access to the detectors will be difficult or impossible after construction, the choice of simple, reliable and low-cost detectors was imperative. The active detectors in the I F R are Resistive Plate Counters (RPCs) [12]. RPCs consist of a gas gap at atmospheric pressure enclosed between two 2 mm thick phenolic polymer (Bakelite) resistive plates. The plates are coated on their outer surfaces with graphite layers, with one plate connected to an 8 k V high voltage and the other to ground. As in the drift chamber, a charged particle traversing the gas gap produces ions. In the R P C s however, the result is a quenched spark that produces signals on external pick-up electrodes which provide x and y information. Neutral hadrons are detected by observing the characteristic length of the hadron showers (primarily 7r°'s and TT^) produced in the IFR. The selection of a gas mixture for the R P C s has not yet been made. Figure 2.17 shows a three dimensional view of the I F R component of the detector. Figure 2.18 shows a schematic of an R P C component.  Chapter 2.  The PEP-II  Machine and BABAR  Detector  Figure 2.17: The Instrumented Flux Return detector.  45  Chapter 2.  The PEP-II  Machine and BABAR  Detector  46  Aluminum  jlpJ^X  strips  BOB ^/Insulator — Graphite 2  mm  2  mm  ! I  mm " Graphite •Insulator P V C spacers  2  Y strips Aluminum  IT 1 1 cm  mm  0  Figure 2.18: Schematic representation of the Resistive Plate Chamber components.  Chapter 3  Detector and Background Radiation Simulation  3.1  Background  Radiation  As described in Sections 2.1 and 2.2, the B-Factory will produce T(AS) particles by colliding beams of electrons, e~, and positrons, e , at energies of 9 GeV and 3.1 GeV +  respectively. The e /e~ are accelerated in the S L A C two-mile linear accelerator using +  klystrons and injected into the PEP-II storage rings where they are guided and focused by magnets for collision. In PEP-II and  the main sources of background radiation are synchrotron  BABAR,  radiation and residual gas collisions in the beampipe. These can result in photons or beam particles striking the beampipe, magnets, masks and other components in the IR producing electromagnetic showers. The particles produced in these showers can enter the detector where they are observed as background noise. If backgrounds are sufficiently high they can obscure the "real" signal and also cause radiation damage. 3.1.1  Synchrotron  Radiation  Electrons and positrons radiate as they are accelerated and guided around bends in the accelerator. The resulting photons are referred to as synchrotron radiation. The energy of photons emitted in this process is distributed from near zero up to the energy of the primary particle, peaked at low energies and falling off rapidly to zero at the primary energy [3].  .  ,  47  Chapter 3.  Detector and Background  Radiation  Simulation  48  Since the storage rings necessarily pass through the detector region, the flux of synchrotron photons produced as the beams traverse the magnets is usually a primary concern in studies of background radiation. 3.1.2  Residual Gas Collisions  Further production of background radiation occurs as a result of the interaction of the particles in the beam with residual gas molecules in the beampipe.  While the beam  line is maintained in a very high vacuum condition, residual molecules remain in the path of the accelerated beams. Beam particles interact with molecules in the beampipe either via Coulomb scattering or bremsstrahlung.  Particles which undergo either of these  interactions and strike an aperture or the beam pipe are referred to as lost beam particles. Coulomb Scattering Coulomb scattering is the completely elastic scattering of a particle by the heavy nucleus of an atom. In this case, a beam particle is scattered by the nucleus of an atom in a residual molecule in the beampipe. There is no resulting change in energy of the beam particle, and therefore no resulting emitted photon.  However, the path of the beam  particle can change dramatically, making it incident on material in the IR. Bremsstrahlung Bremsstrahlung, or "braking radiation," takes the form of photons emitted as the e /e~ +  are severely accelerated by the Coulomb field of a nucleus. At high energies, the bremsstrahlung process dominates over Coulomb scattering. Like the synchrotron energy distribution, the bremsstrahlung energy distribution ranges from low energies up to the energy of the primary. However, this distribution is much flatter than that of synchrotron radiation  Chapter 3.  Detector and Background  Radiation  Simulation  49  with the probability of producing a photon of a given energy falling off as the inverse of that energy [13]. Bremsstrahlung interactions change the energy of the primary particle, often significantly. In PEP-II, this results in "off-energy" beam particles that can be swept out by the magnets in the IR, thus impinging on material in the beampipe, masks, and other components causing electromagnetic showers. 3.1.3  Electromagnetic Showers  The photons and lost beam particles arising from synchrotron radiation and bremsstrahlung can impinge on the material in the IR and the detector itself. In the case of photons, there are three photo-processes that take place that depend on the energy of the photon involved. At high energies pair-production process. This is the materialization of the photon into an e /e~ +  is the dominant  pair in the high field  region of a nucleus. At low energies Compton scattering (or photon-electron scattering) is more prevalent. In the lab frame, this is the absorption of the incident photon by a free electron and consequent re-emission of a photon at a different energy. A third photoprocess is the photoelectric  effect. Here, photons impinging on matter are absorbed by  bound electrons which are then emitted with some kinetic energy [1]. A l l of these processes result in e /e~ +  and photons which can then in turn interact  with the material in the detector via bremsstrahlung, Coulomb scattering and photoprocesses which in turn produce more photons and e /e~ +  processes and so on.  which can interact via these  The resulting multiple interaction is called an  electromagnetic  cascade shower. It should be noted that the net effect of these showers is an increase in the number of particles and a decrease in their average energy in the forward direction with each iteration of the shower process. Hence, a photon or beam particle striking some material in the IR can result in a large number of low-energy particles in the detector, deadening readout channels and causing radiation damage (also called aging).  Chapter 3.  Detector and Background  Radiation  Simulation  50  The electromagnetic shower process ends when the average energy of the particles drops below the critical energy where ionization is the dominant process.  3.1.4  Other Background Sources  There remain other sources of background radiation that will not be discussed in this thesis. These include: e e~ (Bhabha) scattering, e~e~ (M0ller) scattering and cosmic +  rays. A l l of these sources are small compared to those discussed above.  3.2  Detector and Background Simulation  The PEP-II machine and all of the components of the BABAR  detector have been simu-  lated for the purpose of design and optimization, development and testing of the reconstruction and analysis software and, eventually, interpretation of the experimental data. Simulation is carried out using several Monte Carlo packages, each of which allows the machine and detector components to be defined by their geometries and materials. The Monte Carlo packages propagate particles through defined volumes by the generation of random numbers combined with the known behavior of the particles in the components' materials. The simulation packages used differ in the detail of their implementation, but all use random sampling of cross-sections (interaction probability distributions for a specific particle and process in a given material). The tools used specifically for this study of background radiation distribution i n the BABAR  drift chamber are discussed here in  varying degrees of detail.  3.2.1  Tools  The steps taken in simulating background radiation in the drift chamber for this thesis were: first producing rays for input (Decay T U R T L E [14]), and then tracking charged  Chapter 3.  Detector and Background  Radiation  Simulation  51  particles and photons through the detector (OBJEGS [15], G E A N T [16]). Both of the latter Monte Carlo packages use similar methods for transporting particles through matter, but they differ in their capability to represent the complicated geometry of the detector and interaction region. For this reason, the more sophisticated G E A N T simulation is discussed in greater detail. 3.2.2  Decay T U R T L E  Decay T U R T L E simulates the geometry of the PEP-II machine from the middle of the preceding arc to several meters downstream of the IP and includes all of the magnets, collimators and machine elements that will be present in the final machine. Each primary beam particle is tracked through these elements. A scattering point is chosen randomly along the beam line depending on the probability for an interaction to occur and a beam particle is transported to that point. The scattered secondary particle is then propagated along the beam line until it hits an aperture (magnet, mask or other element) or the end of the beam line (as defined by the geometry represented i n Decay T U R T L E ) . In the case of bremsstrahlung, the emitted photon is propagated as well. Particles striking an aperture near the IP are written to disk as rays for input into an O B J E G S or G E A N T simulation of the detector. It should be noted that Decay T U R T L E does not simulate particle interactions with matter in the detector, but rather particle motions through the magnetic fields and apertures of the storage rings near the IR. The version of Decay T U R T L E used in this study has been modified to also simulate bremsstrahlung and Coulomb scattering i n the beamline [14].  3.2.3  GEANT  G E A N T is a Monte Carlo simulation package developed and maintained at C E R N specifically for high energy physics implementation. The primary tool for simulation of the  Chapter 3.  BABAR  Detector and Background  Radiation  Simulation  52  detector is a G E A N T front-end interface called B B S I M . The following sec-  tions discuss the BABAR  implementation, but are by no means a complete discussion  of G E A N T ' s capabilities. The G E A N T manual provides a detailed description of the package [16].  Method of Computing the Occurrence of a Process Simulating the processes which occur during the propagation of a particle through matter is accomplished somewhat differently for the various Monte Carlo packages. The G E A N T method, however, provides a good general description of how these packages work and a brief outline of the principles of Monte Carlo simulation is presented here [16]. First, a new particle to be tracked is fetched. In the B B S I M implementation, the new particles are rays generated by Decay T U R T L E . Once the particle is fetched the number of interaction  lengths-  that the particle will travel before interacting via one of  the processes available is sampled. Next, the distances that the particle may travel in the current medium before any of the processes occurs is calculated. The smallest of these distances is the step over which the particle will be transported.  The particle is then  transported either in a straight line (in the absence of a magnetic field or in the case of a neutral particle) or along a helical path (for charged particles in a magnetic field). The energy loss of the particle is then calculated using the Bethe-Bloch formula [3]:  dE — =  ^ N  2  A  v  r  o  e  m  2 e  Z1  (3.42)  c z - -  for a particle with charge ze passing through an element with atomic number Z and atomic weight A. The mass and classical radius of an electron are m and r , respectively, e  e  NAV is Avogadro's number, and c is the speed of light. The value I PS 16Z°- eV for Z > 1 9  and is referred to as the ionization constant. The 7 is the relavistic boost of the particle,  Chapter 3.  Detector and Background  Radiation  Simulation  53  and (3 = v/c, where v is the velocity. Finally, the shielding of the nucleus by the charge density of atomic electrons is accounted for by 8 which approaches 2ln^ plus a constant for very energetic particles [3]. Once the energy loss is calculated, and if a physical discrete process has occurred, the final state of the interaction is generated. In the case where the incident particle survives the interaction (for example, in bremsstrahlung) the number of interaction lengths for the surviving particle is calculated. These steps are repeated until the particle either leaves the detector or disappears in an interaction. The interaction length, or mean free path, A, for a particle to interact via a specific process in a medium is the inverse of its macroscopic cross-section for that process, E , in that medium. The macroscopic cross-section is given by:  X = N £ ^<T(E,Z ,A )  (3.43)  J  AvP  i  i  i  where NAV is Avogadro's number, Z{ is the atomic number of the i  th  element, A ; is  atomic weight, p is density, a is the total microscopic cross-section for the reaction, p,- is the proportion by weight of the i  ih  element in the material and E is the energy of the  particle [16]. The method for defining detector volumes and for simulating specific physical processes is discussed in the following sections.  Detector Volumes  In G E A N T , component volumes are defined by a shape identifier, shape parameters and physical properties. Volumes can be placed within other volumes and positioned by the definition of a reference frame. In the B B S I M simulation, the z-axis is defined as along the direction of the high energy (electron) beam.  Chapter 3.  Detector and Background  Radiation  Simulation  54  Since G E A N T provides a wide variety of shape identifiers, the B B S I M package is able to provide a detailed simulation of the beam line components in the IR. Specifically, B B S I M includes an approximation to the complex geometry of the synchrotron radiation masks, which have elliptical apertures. Also included in the IR simulation are the B l and Q l beam line magnets with their dipole and quadrupole fields as well as the Q2 septum masks that shield the Q2 coil. Figure 3.19 shows the IR region of the simulation and a sample event. A n incoming positron ( L E B particle) scatters off of a synchrotron radiation mask tip producing a shower of electrons, positrons and photons. For the purpose of studying the background distribution in the drift chamber, all interior volumes are simulated. In addition to the IR, the silicon vertex detector and the support tube are included in the simulation, as they are between the IP and the drift chamber. The drift chamber is defined as a cylinder with an inner radius 22.5 cm and an outer radius 82 cm with 22.7° angled aluminum endplates, and carbon fiber inner and outer cylindrical tubes. It is offset from the interaction point such that its length is 1.1 m in the backward direction and 1.7 m in the forward direction. In B B S I M the material in the drift chamber is modeled as a Helium:isobutane mixture (80:20) with the aluminum field wires and tungsten sense wires treated as vapors and added to the mix. The result is a homogeneous mixture of gas and wires. This approximation is adequate for the purposes of background simulations since no detector readout is modeled. The only exterior factor pertaining to the study of backgrounds in the drift chamber is the 1.5 T external magnetic field. A field map of the magnetic field is included in the B B S I M simulation.  Chapter 3.  Detector and Background  Radiation  Simulation  55  Figure 3.19: G E A N T simulation of beam line (IR) components. The interaction of a L E B lost beam particle is also shown. The solid line is the incoming positron and the dotted lines are produced photons.  Chapter 3.  Detector and Background  Radiation  Simulation  56  M o n t e Carlo Technique  Monte Carlo techniques are used to simulate random behavior on a computer. A common method of Monte Carlo simulation, and the one used in G E A N T , is the composition and rejection method [3, 16, 17]. This method requires that the probability density function, f(x), for a process is well enough known that it can be computed for any x and its shape enclosed entirely within a shape which is C times a simple distribution h(x). Often h(x) is a uniform distribution or a sum of uniform distributions. Both f(x) and h(x) are normalized, meaning that C  > 1. A candidate x is generated by a random number generator according to h(x). The  values of f(x) and Ch(x) are calculated. Next, another random number, u, is generated on the interval (0,1). If uCh(x) < f(x),  x  is accepted. Otherwise, the process is repeated  until a suitable x is found. If x and uCh(x) are considered to combine as points on a two-dimensional plot, these points fill the area of Ch(x) in a smooth manner and points under f(x) are accepted. The efficiency of this method is simply the ratio of areas, or 1/C. It is therefore desirable that C be kept close to 1.0. Accordingly, Ch(x) is chosen to be as close to f(x) as possible using sums of uniform distributions. This is known as importance sampling, since more trial values of x are generated in the peak regions of f(x) [3].  Synchrotron R a d i a t i o n  A study was done prior to the research presented in this thesis optimizing the placement of the water-cooled synchrotron radiation masks inside the B l magnet. Synchrotron radiation will be effectively minimized in the  BABAR  detector. Detailed treatment of  the simulation, design and optimization of the masks can be found in [18, 20]. These  Chapter 3.  Detector and Background  studies show that in the  BABAR  Radiation  Simulation  57  detector, synchrotron radiation is a very small effect  when compared to lost beam particle backgrounds. Consequently, synchrotron radiation has not been included in the studies discussed in this thesis. Residual Gas Collisions With the minimization of synchrotron radiation, the largest contributor to backgrounds in the  drift chamber becomes the interaction of the beam particles with residual  BABAR  molecules in the beampipe. These interactions can result in high-energy beam particles and photons striking masks and the beampipe near the IP and producing electromagnetic showers. In a bremsstrahlung interaction, the primary particle is not stopped but continues at a different energy and a photon is produced. Hence, both the scattered particle and a photon must be tracked in a simulation after the interaction. The total cross-section, O~B for bremsstrahlung is given by [3]: a Z 3  <J  B  ~  •  2  -2-4  . 3.44  where a = e /hc, Z is the atomic number of the nucleus and m is the mass of the 2  e  electron/positron. Energy loss for a beam particle due to bremsstrahlung is given by [3]:  dE  i  E  =-%  ,  < > 345  where E is the initial particle energy, x is the distance traveled in the medium and XQ is the radiation length of the particle in the medium. electron/positron bremsstrahlung is defined as [3]:  The radiation length for  Chapter 3.  Detector and Background  1 X  0  _ 4Z(Z "  Radiation  Simulation  + iy N e  J  A  137A  58  183  M"'"KZ^ ^TTI  3  ( 3  4 6  )  where r is the classical electron radius and A is the mass number of the nucleus. e  The bremsstrahlung photon is then propagated through the medium as is the electron/positron at its lower energy. Either of these can then interact again.  Electromagnetic Showers In the case of the photoelectric effect, the incident photon is stopped and the energy of the "knock off" electron is calculated and the electron propagated. The macroscopic cross-section, E , for the photoelectric effect is calculated as:  Yi = per  (3.47)  where p is the medium density and a is the microscopic cross-section [16].  For this  simulation, fits were performed in different intervals j of the photon energy and for such an interval, the microscopic cross section is given by [16]: L  v^-^  + ^  + ^T  + ^icrng  )  (3.48)  with j changing from 1 to m;, where m; is the number of fitting intervals used for the element i. For composites or mixtures of N elements, the cross-section for the j  ih  element  is calculated as:  N  °-j =  where f  is the fraction by mass of the k  th  k  E fk°~j,k  (3.49)  element in the material. Therefore: N  Ci,total = ^ k=l  fkQiJk  (3.50)  Chapter 3.  Detector and Background  Radiation  Simulation  59  for i = 1 to 4 [16]. For Compton scattering, the quantum mechanical Klein-Nishima differential crosssection is randomly sampled to determine the energy of the scattered photon. The differential cross-section is given by [16]:  $(E,E')  X mrrlm 0  =  esin#  2  e  + e  E  2  1  1 +e  (3.5i;  2  where E is the energy of the incident photon, E' is the energy of the scattered photon, e is E'IE, n is the electron number density of the material, r is the classical electron e  radius, m is the mass of the electron and XQ is the radiation length of a photon in the e  material. Assuming an elastic collision, the scattering angle, 0, of the scattered photon is given by the Compton formula [16]:  m,  cos - l  8 =  EE  (3.52)  l(E-E')  For pair production, the incident photon is again absorbed and the e /e~pair is +  tracked and can interact. The total cross-section of e /e~ pair production by a photon +  is parameterized as [16]:  a{Z,  = A(Z  + l)(iMX) + F {X)Z 2  + F (X)/Z) 3  (3.53)  in barn/atom, where X = / n ( E / m ) , m = electron mass, E~, = photon energy, and 7  e  e  5  Fi(X)  = ~^2c X . The parameters c are taken from a least squares fit to actual data. n  n  n  This parameterization is applicable for a range of atomic numbers from 1 to 100 and to incident photon energies of 1.5 MeV to 100 GeV [16]. The energy of one of the particles in the e e~ pair is given by [16]: +  Chapter 3.  Detector and Background  Radiation  E = e£  Simulation  60  (3.54)  7  where the variable e is restricted kinematically to the range m / E < e < 1 — m /E. The e  e  cross-section is symmetric with respect to the interchange of e with 1 — e. Therefore, we can restrict e to lie within the range eo < e < 1/2 where CQ = m /E. e  The differential  cross-section is then given by [16]:  ^ =1 : ^ / ^ ( 0  (3-55)  where the functions / (e) are given by [16]: 2  « > = £  and represent conservation of energy and momentum. The functions a,- and  <- > 3  57  are com-  plicated functions which account for the shielding of the nucleus by atomic electrons. A more detailed derivation of these functions is found in [16]. Output  Results from the background simulation using the B B S I M G E A N T interface are output in H B O O K [21] files. H B O O K allows for the storage of one and two dimensional histograms and ntuples. In this application the x, y, z, r, and <j> of a drift chamber hit, the radius of curvature of the tracked particle at each step, the energy loss of the particle, the layer number of the hit, and the track number of the particle producing the hit are stored in a one dimensional H B O O K ntuple.  Chapter 3.  Detector and Background  Radiation  Simulation  61  The H B O O K file is then read into P A W [23], an interactive utility for visualizing experimental data for analysis. P A W was developed and is maintained at C E R N and is a very powerful tool for high energy physics analysis. 3.2.4  OBJEGS  The first Monte Carlo simulation package used to simulate the  BABAR  drift chamber was  O B J E G S . This package is an interface to EGS4 (Electron Gamma Simulation 4) [22], a widely used Monte Carlo. O B J E G S was designed to facilitate the input of simple detector geometries. Beam line and detector components are input in terms of the inner and outer radius, length, and material type. Rays generated by Decay T U R T L E are then input one particle at a time and propagated through detector components. Finally, events are summed over time. The O B J E G S package was not satisfactory for simulating the  BABAR  detector due  to its restriction to cylindrical geometries. This restriction precluded realistic modeling of some of the more complicated components in the interaction region and the detectors. For example, the synchrotron radiation mask is not cylindrically symmetric but rather is elliptical in shape.  A n approximation to this shape was attempted by using two  different versions of O B J E G S geometries, one for the H E B and one for the L E B . A particular geometry is then used depending on which side of the detector a particle struck. For example, a particle striking the forward end of the IR would use the L E B geometry to simulate the shower produced by that particle. For this reason, the O B J E G S simulation was used only for preliminary results on background distributions and will not be discussed here in detail. It is important to note, however, that G E A N T provides a detector/geometry interface to EGS4 as well as a number of other simulation packages (GHEISHA, H A D I N T , E G S , to name a few).  Accordingly, the previous sections on  particle propagation are applicable to the O B J E G S method.  Chapter 4  Discussion of Results  This thesis discusses the results of two distinct Monte Carlo studies of the distribution of background radiation in the  BABAR  drift chamber. The first was an O B J E G S study to lo-  cate "hot spots" in the chamber volume to determine whether major design changes were necessary to exclude these high background regions. The second was a B B S I M / G E A N T study of occupancies and aging carried out with a more realistic geometry simulation. The drift chamber is still in the design stage and consequently many of the detector design parameters changed from the time of the first study to the second. In fact, the design parameters at the time of writing are different than those used in the second study. For this reason, the two simulations will be discussed separately. Also, a brief summary of the design and Monte Carlo input parameters is included in Table 4.2. It is important to note that the changes to the design have a very minimal impact from the point of view of backgrounds in the drift chamber. The same cannot be said for other detector components. For example, the changing endplate thickness has a large effect on background rates in the forward endcap of the electromagnetic calorimeter.  4.1  Endplate Design Studies with the OBJEGS Simulation Package  The first study of background distribution in the drift chamber was carried out to determine whether radical design changes would be necessary to exclude "hot spots" from the drift chamber volume. Due to the location of the support tube immediately inside the  62  Chapter 4.  Discussion of Results  63  Table 4.2: Design and Monte Carlo input parameters for the two studies discussed as well as the current values.  Gas mixture  Wires  OBJEGS  BBSIM  Current  He:C02:isobutane  He:isobutane  Same as B B S I M  (78:15:7)  (80:20)  with A l wires  with A l and W wires  50  Al  55 //m gold plated A l  80, 120 urn gold plated A l  20 / i m gold plated W  20//m gold plated W  Endplates  None included  22.7° angled A l , 1.05 cm  flat A l , 2.4 cm  Inner wall  .5 mm Be  1.99 mm carbon fiber  1 mm Be  Outer wall  5 mm Be  3.61 mm carbon fiber  3.2 mm carbon fiber  inner radius and the offset of since the chamber in the forward direction, high occupancies were anticipated in the forward end at the inner layers (due to photon conversions in the support tube). Several endplate designs were considered to address concerns about high backgrounds. Proposed options included highly conical endplates following the 300 mr acceptance line of the detector, as well as stepped endplates. These designs were only to be seriously considered if their benefit in terms of excluding hot spots were significant since either option would be difficult to machine and would make the wiring of the inner layers of the drift chamber problematic. These preliminary studies were undertaken using the O B J E G S interface to EGS4 discussed in Section 3.2.4. The defined drift chamber volume was divided into 32 azimuthal segments and 28 radial layers for the purpose of background calculation. Additionally, 40 z segments were defined with a total length of 2.74 m, offset in z with 1.64 m in the -\-z and 1.1 m in the — z  Chapter 4.  Discussion of Results  direction. This is the center offset of the  64  BABAR  drift chamber, chosen to optimize drift  chamber performance with the boosted center of mass in the lab frame. The inner region was segmented into very thin (0.2 cm) radial "scoring layers" filled with the nominal gas mixture for better counting resolution of low momentum looping electrons in the inner layers. "Scoring layers" are small volumes defined in the larger volume of the drift chamber used purely for binning purposes. Hits in these inner layers were later averaged to reflect the actual geometry of the innermost layer of the chamber. The outer "scoring layers" were defined to approximate superlayers (see Section 2.3.2). The gas used in the simulation was He(78%):(702(15%):Isobutaiie(7%). A multiplication factor of 0.312 was uniformly applied to the density of the helium gas mixture to approximate the presence of aluminum wires. The large number of wires in the drift chamber (~ 28000) makes including individual wires in the Monte Carlo complicated and costly. Instead, the equivalent mass and density of the wires is included evenly distributed throughout the drift chamber volume. This approximation is adequate for the purpose of studying background distribution, but is would not be satisfactory for other applications (ie. modeling chamber tracking). Particles are tracked until their energy falls below user defined minimum energy thresholds. These energies were defined to be 0.1 MeV/c  for photons and 0.611  MeV/c  for electrons. Particles with energies lower than these thresholds do not contribute significantly to background rates [11]. Background rates were examined for an integration time of 4 //sec, the nominal resolving time of the O B J E G S simulated drift chamber design (PEP-II running at nominal luminosity) [11]. The O B J E G S input file specifying the geometry and cuts is included in Appendix A . A total of 1502 events were simulated with an initial input of 10000 rays generated by Decay T U R T L E , representing a total of 789 ps of beam. While background events  Chapter 4.  Discussion of Results  65  are relatively rare, the Decay T U R T L E rays used as input were saved specifically for background analysis purposes, ie. they involved the beam particle or photon striking the beam pipe, a synchrotron mask or other aperture near the IP. Since all input rays are preselected to be likely background producing candidates, the input of 10000 rays resulted in events approximately 10% of the time, much higher than normal rates. This was done purely to save computing time. Results are all calculated using the full beam time simulated (789 fis) and are therefore absolute background rates in real time. In other words, 789 fis of simulated beam time resulted in 10000 potentially interesting events that were then put in to the Monte Carlo. The bunch crossing rate is 238 M H z (4.3 ns bunch separation) so that the 789 (is of simulation represents ~ 1.8 x 10 bunch 5  crossings. Backgrounds from both the H E B and L E B interacting with synchrotron radiation masks both upstream and downstream were studied.  Since the goal was to look at  background distributions along the chamber length (z) in order to address the endplate design issues, no <>/ distinction was made in the preliminary analysis. Figures 4.20 and 4.21 show the results of this study. Here, H E B  -  refers to the high  energy beam incident on the upstream (low energy beam entrance) mask, H E B  +  the  H E B incident on the downstream mask, and so on. Photon rates are not insignificant, peaking as high as 45 per 4 //sec in the +0.125 to +0.375 m region of the detector for the H E B . Note that while the peaks are indeed shifted in the z direction, these shifts +  are all contained within ±0.5 m and therefore the use of the proposed highly conical or stepped endplates would not eliminate the peak regions. The relative intensity of background electron radiation is 3 orders of magnitude smaller than that of the photons. The ^-distributions, shown in Figure 4.21, were found to be more uniform in all but the innermost layer of the detector. However, rates remain  Chapter 4.  Discussion of Results  66  the most intense within the region of cos 6 < 300 mrad. The relatively low occupancies, which peak at less than 1 electron per 4 psec, suggest that the slight improvements afforded by the proposed endplates would not be worth the design difficulties. The conclusion of the endplate design study is that there is no advantage to radically altering the flat endplate design for the purpose of excluding areas of high occupancy in the inner layers of the drift chamber.  4.2  Occupancy Studies  Chamber occupancy is defined as the fraction of cells that contain random hits during the event resolving time (nominally Ips for the B B S I M design). If the occupancy exceeds a few percent per wire per resolving time, pattern recognition for tracking becomes impossible. Simulations to determine occupancy were carried out using the G E A N T interface B B S I M discussed in Section 3.2.3. The chamber geometry defined for the B B S I M simulation is very similar to that in the previous section. The chamber length is 2.8 m with an inner radius of 22.5 cm and an outer radius of 80 cm. The volume is filled with a He(80%):Isobutane(20%) mixture with aluminum and gold plated tungsten wires modeled as vapors. The resolving time is defined as the maximal drift time of the chamber and is set to 1 psec as determined in studies of cell geometries and gas mixtures [12]. A l l components between the drift chamber and the IP are included in the model including the silicon vertex detector, the beryllium support tube, and the beam pipe. Additionally, the IR components within the fiducial volume of the drift chamber are modeled, including the synchrotron radiation masks. A very good approximation to the actual shape of the masks is possible due to the greater flexibility of the B B S I M interface to G E A N T . The input geometry used for this simulation is included in Appendix B .  Chapter 4.  Discussion of Results  67  Figure 4.20: z distribution intensity of beam-gas photon radiation in photons/4 yusec. The label HEB~  means the high-energy beam incident on upstream mask, LEB  low-energy beam on the downstream mask, and so on.  +  the  Chapter 4.  Discussion of Results  20  41  62  68  83  104  125  146  167  188  219  230  Intensity of beam-gas background electron radiation in 10.-' electrons per 4 microseconds.  Figure 4.21: z distribution intensity of beam-gas electron radiation in 1 0  - 3  electrons/4  //sec. The label HEB~ means the high-energy beam incident on upstream mask, the low-energy beam on the downstream mask, and so on.  LEB  +  Chapter 4.  Discussion of Results  69  X '  4-  LAYER  Figure 4.22: Simulated background occupancies for each drift chamber layer due to converted photons from showers produced by lost particle interactions. A total of 10000 Decay T U R T L E rays were input representing 789.9 ps. As mentioned in the previous section, the results are calculated using the full beam time of 789.9 /J,S. The results for the occupancy simulations are shown in Figure 4.22. The occupancy distribution is peaked at the innermost layers with expected occupancies of 0.55% w i r e  -1  /is . This peak is expected due to photon to e / e -1  +  -  conversions  in the support tube immediately outside of the drift chamber inner wall. The average occupancy in the outer layers is 0.16% w i r e  -1  / J S . The smearing of the occupancies in - 1  the outer layers is due to the modeling of the wires as vapors, the actual occupancies are expected to be significantly lower in the outer layers. Pattern recognition becomes very difficult when occupancies reach the level of ~10%. These occupancies are then well within acceptable levels according to this simulation. It should be noted, however, that no attempt has been made to address the impact of cross-talk (pulses produced in  Chapter 4.  Discussion of Results  70  Figure 4.23: The background distribution in <j>. neighboring wires due to a large pulse on a given wire). Since background events will tend to produce low momentum particles that will loop tightly in the inner layers, depositing large amounts of charge on a few wires, the effect of cross-talk might be large. Figure 4.23 shows the <f> distribution of background events integrated over 789.9 ps. Once again higher rates are observed at the innermost layers. Figure 4.24 shows the energy deposited per layer for all events. This distribution is noticeably peaked below 5  MeV/c.  Chapter 4.  Discussion  of Results  71  Figure 4.24: Energy distribution per layer in MeV for integration time of 789.9 ps. A l l energy deposits greater than 50 keV are shown as entries in the histogram.  Chapter 4.  4.3  Discussion of Results  72  Aging Studies  Aging in the drift chamber is defined as changes in gain or increases in noise due to material deposited on the wires from the gas-avalanche process. Using the B B S I M simulation described above, the charge deposition on a wire corresponds to an integrated charge of 0.001 C / c m / 1 0 sec for the innermost drift chamber layer. This value is calculated as 7  follows:  accumulated charge = (E/I)e/£  (4.58)  where E is the energy deposited in some time interval in a given layer, I is the effective ionization energy of the gas mixture, e is the charge of an electron and £ is the length of the wire in the layer. Values are quoted for uniform distribution along the wires. In fact, as shown in Figure 4.21, there is a factor of ~ 2 or more variation in the distribution in z. Taking this variations into account, the expected integrated charge for some regions in the inner layers may be as high as 0.005 C / c m / 1 0 sec. This significant amount of 7  charge will lead eventually to deposits of hydrocarbons on the wires. These deposits will result in sparking and current draw. A value of 0.1 C / c m is generally considered to be a conservative limit on the tolerable integrated charge on a wire. This would give the drift chamber an expected lifetime of at least 6 years, which is acceptable.  Chapter 5  Conclusions  The simulations of the distribution of background radiation in the  BABAR  drift chamber  presented in this thesis sought to address endplate shape/design issues, occupancies and drift chamber aging. The studies showed that the background distribution was peaked at the inner layers and in the forward direction, as anticipated due to the location of the support tube immediately outside the drift chamber inner wall and due to the offset in the forward direction. However, rates are not expected to be especially problematic and radical design considerations like highly conical or stepped endplates are not considered necessary. Indeed, the difficulty in machining such endplates and in mounting electronics on the inner layers far outstrips the benefits they would afford in terms of excluding hot spots from the drift chamber volume. The final design will include a flat backward endplate and a flat forward endplate with one step. Occupancies were shown to be well within acceptable limits with a peak in the inner layers of 0.55% w i r e  -1  /xs" and an average in the outer layers of 0.16% w i r e 1  -1  xzs . If -1  occupancies reach the level of approximately 10% w i r e fis" , pattern recognition would -1  1  become extremely difficult. However, cross-talk has not been accounted for in this study and might have a significant effect on the performance of the inner layers. Aging estimates based on occupancy studies show an expected drift chamber lifetime of approximately 6 years. This is an acceptable value with a goal for major chamber refurbishing or upgrade of approximately 5 years from turn-on.  73  Appendix A  O B J E G S Input File  The following is an actual input file for O B J E G S Monte Carlo simulation. All geometry, materials and magnetic fields are .specified. AP75HI D A T A A l This O B J E G S runcard file defines masking and detector for the Apiary 6.3 flat-beam B-Factory lattice. See Hearty B-ABARNote  [15].  Mod 8/14/91 by Hobey DeStaebler to be very appro Apiary 7.3 1. H E B sees Q l only; L E B both Q l , Q 2 2. L E B off axis in Q l . 3. No change in detector, just beamline. 4. Magnets, offsets from Mike Sullivan 5. Change B l 7.5 k G , 45cm long, start 25 cm. 6. Q l becomes G=1.03 K g / c m , z=90-210 cm. 7. Estimate radii from Ap 6.3 O B J E G S 8. Slice magnets-at least 10 slices per kind of field. 9. Distinguish D E B (w=+l) and L E B (w=-l) directions. In table below, a = medium 1 = Ta, 2 = Fe, 3 = A l , 4 = Be, 5 = Cu, 6 = Pb, 7 = C02(92)/Ethane(8) 2 atm, 8 = W , 9 = He(78)/C02(15)/Isobutane(7), 10 = Sm2Col7, 11 = Ar(50)/ethane(50), 12 = Ar(80)/Methane(20) 8.6 atm, 13 = C226, 14 = t i , 15 = Nal, 16 = Si, 17 = C s l , 18 = L A r , 19 = G10  74  Appendix  A.  OBJEGS  Input File  b and c control information recorded in IDA or handypak: b = 0 photons or electrons, b = 1 electrons only c = 0 neither ida nor handypak, c = 1 handypak only, c = 2 ida and handypak d and bfield are used to define magnetic regions name  z left  z right  r inner  r outer  a  b  c  d  bfield  Support 2  60.  900.  16.8  18.0  3  0  0  0  0.0  Quad 1A  90.  120.  7.0  12.  10  0  0  0  0.0  Quad IB  120.  150.  7.0  12.0  !Q  0  0  0  0.0  Quad 1C  150.  180.  7.0  12.  10  0  0  0  0.0  Quad ID  180.  210.  7.0  12.  10  0  0  0  0.0  Q l Shield  90.  210.  12.  15.  8  0  0  0  0.0  Bl  20.  25.  3.75  5.08  10  0  0  0  0.0  Bl  25.  30.  4.00  6.80  10  0  0  0  0.0  Bl  30.  35.  4.25  8.05  10  0  0  0  0.0  Bl  35.  40.  4.5  9.40  10  0  0  0  0.0  Bl  40.  45.  4.75  9.92  10  0  0  0  0.0  Bl  45.  50.  5.00  10.44  10  0  0  0  0.0  Bl  50.  55.  5.25  10.96  1.0  0  0  0  0.0  Bl  55.  60.  5.5  11.48  10  0  0  0  0.0  Bl  60.  65.  5.75  12.01  10  0  0  0  0.0  Bl  65.  70.  6.00  12.53  10  0  0  0  0.0  BEAMPIPE 4  210.  430.  6.0  6.4  5  0  0  0  0.0  E N D C A P OA  150.  154.  45.  50.  17  0  0  0  0.0  E N D C A P OB  154.  160.  45.0  50.0  17  0  0  0  0.0  E N D C A P OC  160.  187.  45.0  50.0  17  0  0  0  0.0  % BEGOBJ  Appendix  A.  OBJEGS  . name  Input File  z left  z right  r inner  r outer  a  b  c  d  bfield  ENDCAP 1  150.  154.  50.0  100.0  17  0  0  0  0.0  ENDCAP 2  154.  160.  50.0  100.0  17  0  0  0  0.0  ENDCAP 3  160.  187.  50.0  100.0  17  0  0  0  0.0  E C Shieldl  187.  197.  45.0  100.0  6  0  0  0  0.0  E C Shield2  180.  154.  18.0  45.0  6  0  0  0  0.0  E C Shield3  154.  197.  40.0  45.0  6  0  0  0  0.0  MASK G  70.  90.  7.0  11.0  8  0  0  0  0.0  MASK H  210.  280.  6.7  10.7  8  0  0  0  0.0  MASK I  330.  430.  6.7  10.7  8  0  0  0  0.0  Catcher  430.  450.  0.  12.  8  0  0  0  0.0  -330.  -280.  6.7  13.  10  0  0  0  0.0  280.  330.  6.7  8.7  8  0  0  0  0.0  MASK  -86.4120  -70.0  5.0  5.2  1  0  0  0  0.0  MASK  -84.4120  -65.5  4.8  5.0  1  0  0  0  0.0  MASK  -82.4120  -61.0  4.6  4.8  1  0  0  0  0.0  MASK  -80.4120  -56.5  4.4  4.6  1  0  0  0  0.0  MASK  -78.4120  -52.6  4.2  4.4  1  0  0  0  0.0  MASK  -76.4120  -47.5  4.0  4.2  1  0  0  0  0.0  MASK  -74.4120  -43.0  3.8  4.0  1  0  0  0  0.0  MASK  -72.4120  -38.5  3.6  3.8  1  0  0  0  0.0  MASK  -70.4120  -34.0  3.4  3.6  1  0  0  0  0.0  MASK  -68.4120  -29.5  3.2  3.4  1  0  0  0  0.0  MASK  -66.4120  -25.0  3.0  3.2  1  0  0  0  0.0  % Duplicate Quad 2M A S K J+  Appendix  A.  OBJEGS  name  Input File  z left  z right  r inner  r outer  a  b  c  d  bfield  MASK  -64.4200  -19.5  2.8  3.0  1  0  0  0  0.0  MASK  -62.2619  -14.5  2.6  2.8  1  0  0  0  0.0  MASK  -60.2699  -8.5  2.5  2.6  1  0  0  0  0.0  MASK  -60.2699  -8.6  2.4  2.5  1  0  0  0  0.0  MASK  -58.2779  -13.2918  2.2  2.4  1  0  0  0  0.0  MASK  -56.1199  -18.1059  1.0  2.2  1  0  0  0  0.0  MASK  -54.1279  -23.0859  1.8  2.0  1  0  0  0  0.0  MASK  -52.1359  -27.8999  1.6  1.8  1  0  0  0  0.0  MASK  -49.9779  -32.7139  1.4  1.6  1  0  0  0  0.0  MASK  -47.9859  -34.6939  1.2  1.4  1  0  0  0  0.0  MASK  -45.9939  -42.5079  1.0  1.2  4  0  0  0  0.0  BEAMPIPE 1  -8.5  +8.5  2.5  2.6  4  0  0  0  0.0  BEAMPIPE 0  -8.5  +8.5  2.4975  2.5  5  0  0  0  0.0  BEAMPIPE-3  -210.  -86.412  5.0  5.2  5  0  0  0  0.0  MASK 1  -210.  -205.  5.2  6.4  1  0  0  0  0.0  BEAMPIPE+2  8.5  11.8  2.59  2.79  5  0  0  0  0.0  BEAMPIPE+2  11.8  15.1  2.68  2.88  5  0  0  0  0.0  BEAMPIPE+2  15.1  18.4  2.77  2.97  5  0  0  0  0.0  BEAMPIPE+2  18.4  21.7  2.86  3.06  5  0  0  0  0.0  BEAMPIPE+2  21.7  25.0  2.95  3.15  5  0  0  0  0.0  BEAMPIPE+3  25.  28.  3.06  3.26  5  0  0  0  0.0  BEAMPIPE+3  28.  31.  3.17  3.37  5  0  0  0  0.0  BEAMPIPE+3  31.  34.  3.28  3.48  5  0  0  0  0.0  BEAMPIPE+3  34.  37.  3.39  3.59  5  0  0  0  0.0  BEAMPIPE+3  37.  40.  3.50  3.70  5  0  0  0  0.0  Appendix  A.  OBJEGS  name  Input File  z left  z right  rinner  r outer  a  b  c  d  bfield  BEAMPIPE+4  40.  43.  3.65  4.05  5  0  0  0  0.0  BEAMPIPE+4  43.  46.  3.80  4.20  5  0  0  0  0.0  BEAMPIPE+4  46.  49.  3.95  4.35  5  0  0  0  0.0  BEAMPIPE+4  49.  52.  4.10  4.50  5  0  0  0  0.0  BEAMPIPE+4  52.  55.  4.25  4.65  5  0  0  0  0.0  BEAMPIPE+4  55.  58.  4.40  4.80  5  0  0  0  0.0  BEAMPIPE+4  58.  61.  4.55  4.95  5  0  0  0  0.0  BEAMPIPE+4  61.  64.  4.70  5.10  5  0  0  0  0.0  BEAMPIPE+4  64.  67.  4.85  5.25  5  0  0  0  0.0  BEAMPIPE+4  67.  70.  5.00  5.40  5  0  0  0  0.0  BEAMPIPE+4  70.  73.  5.15  5.55  5  0  0  0  0.0  BEAMPIPE+4  73.  76.  5.30  .5.70  5  0  0  0  0.0  BEAMPIPE+4  76.  79.  5.45  5.85 '  5  0  0  0  0.0  BEAMPIPE+4  79.  82.  5.60  6.00  5  0  0  0  0.0  BEAMPIPE+4  82.  210.  6.  6.4  5  0  0  0  0.0  SUPPORT 1  -60.  60.  19.7  21.0  -13  0  0  0  0.078  SI L A Y 1  -4.5  4.50  2.8  2.83  +16  0  0  0  0.0  SI L A Y 2  -9.0  9.00  5.1  5.13  +16  0  0  0  0.0  SI L A Y 3  -9.0  9.00  7.4  7.43  +16  0  0  0  0.0  SI L A Y 4  11.0  11.03  4.0  8.5  +16  0  0  0  0.0  SI L A Y 5  14.0  14.03  4.0  8.5  +16  0  0  0  0.0  DC W A L L  -110.  164.  22.5  22.55  4  0  3  0  0.0  DC O T H E R 1  -110.  164.  22.55  23.  -9  0  0  0  .312  Support 3  -110.  164.  23.  23.4  -13  0  0  0  0.078  Appendix  A.  OBJEGS  Input File  name  z left  z right  r inner  r outer  a  b  c  d  bfield  DC O T H E R 2  -110.  164.  23.40  24.08  -9  0  1  0  .312  DC S C O R E 1  -110.  164.  24.08  24.28  -9  0  1  0  .312  DC S C O R E 2  -110.  164.  23.28  24.48  -9  0  1  0  .312  DC S C O R E 3  -110.  164.  23.48  24.68  -9  0  1  0  .312  DC S C O R E 4  -110.  164.  23.68  24.88  -9  0  1  0  .312  DC S C O R E 5  -110.  164.  23.88  25.08  -9  0  1  0  .312  DC SCORE 6  -110.  164.  25.08  25.28  -9  0  1  0  .312  DC S C O R E 7  -110.  164.  25.28  25.48  -9  0  1  0  .312  DC S C O R E 8  -110.  164.  25.48  25.68  -9  0  1  0  .312  DC S C O R E 9  -110.  164.  25.68  26.08  -9  0  1  0  .312  DC S C O R E 10  -110.  164.  26.08  26.48  -9  0  1  0  .312  DC S C O R E 11  -110.  164.  26.48  26.88  -9  0  1  0  .312  DC S C O R E 12  -110.  164.  26.88  27.28  -9  0  1  0  .312  DC S C O R E 13  -110.  164.  27.28  27.68  -9  0  1  0  .312  DC S C O R E 14  -110.  164.  27.68  28.08  -9  0  1  0  .312  DC S C O R E 15  -110.  164.  28.08  28.48  -9  0  1  0  .312  D C S C O R E 16  -110.  164.  28.48  28.88  -9  0  1  0  .312  DC S C O R E 17  -110.  164.  28.88  29.28  -9  0  1  0  .312  DC S C O R E 18  -110.  164.  29.28  29.68  -9  0  1  0  .312  DC S C O R E 19  -110.  164.  29.68  29.91  -9  0  1  0  .312  DC O T H E R 3  -110.  164.  29.91  32.61  -9  0  1  0  .312  DC O T H E R 4  -110.  164.  32.61  35.61  -9  0 ,1  0  .312  DC O T H E R 5  -110.  164.  35.31  40.71  -9  0  1  0  .312  DC O T H E R 6  -110.  164.  40.71  46.53  -9  0  1  0  .312  Appendix  A.  OBJEGS  Input File  name  z left  z right  r inner  r outer  a  b  c  d  bfield  DC OTHER 7  -110.  164.  46.53  57.33  -9  0  1  0  .312  DC O T H E R 8  -110.  164.  57.33  68.45  -9  0  1  0  .312  DC O T H E R 9  -110.  164.  68.45  78.37  -9  0  1  0  .312  DC O U T W A L L  -100.  164.  79.37  79.87  3  0  0  0  0.0  Grid  -150.  150.  80.  94.  0  0  0  0  0.0  CALOR1  -150.  150.  94.0  98.0  17  0  0  0  0.0  CALOR 2  -150.  150.  98.0  104.  17  0  0  0  0.0  CALOR 3  -150.  150.  104.  131.  17  0  0  0  0.0  -150.  150.  0.0  94.  0  0  0  3  10.  B1 +  25.  70.  0.  7.  0  0  0  2  -7.5  Bl-  -70  -25  0.0  7.  0  0  0  2  7.5  Q1+  90.  210.  0.0  12.0  0  0  0  4  -1.03  Qi-  -210.  -90.  0.0  12.  0  0  0  4  -1.03  Q2-  -330.  -280.  0.0  12.  0  0  0  4  1.15  % ENDOBJ Det Sol  % END M A G % B E G I N I T 0.600 .001 ecut and pcut 47183756402 R A N D O M N U M B E R S E E D 100001 1 1. N C A S E S IQI W T H I S T % 190 E0 = 9000.0 X 0 = 0.0 Y0 = 0.0 Z0 = 0.0 U0 = 0.0 V0 = 0.0 W0 = 1.  Appendix B  B B S I M Input File  The following is the actual geometry input file for the G E A N T / B B S I M runs upon which this thesis is based.  Appendix  B.  BBSIM  Input File  82  ! History: ! 05-Nov-1995 Lockman: added origin of dhca (odcha) ! 25-Nov-1995 Rankin: modified definitions of delay hits to use px etc. ! 10-Dec-1995 Krai: Add layer(+cell) number(s) in D C L A Y + D C W I R hits for .ds. ! Remove fitter structure dcflags. incname dcha ! include file name l structure dcha ! drift chamber  ! BaBar Drift Chamber - E P A C Technical Design Report superlayer design ! From E P A C draft and the file Feb 21 17:30 /u/ec/adam/dcsim/auvw.90.18 ! Updated sre/dcha code to agree with dcha.db on March 16, 1995.  real rext 80. ! External radius of the drift chamber real rref 80. ! Reference radius real rint 22.5 ! Internal radius of the drift chamber real dsw 0.0020 ! Sense Wire Diameter real dfw 0.0055 ! Field Wire Diameter  Appendix  B.  BBSIM  Input File  83  ! no guard wires real dgw 0.0070 ! Guard Wire Diameter  ! Support tube: carbon fiber/rohacell foam sandwich - 0.5 % rl ! D C inner wall: 0.400 mm carbon fiber tube - 0.15 % rl ! rf shield foil: 0.130 mm aluminum - 0.15 % rl ! Equivalent to 0.8 % of X 0 (carbon fiber r.l.=24.90cm) ! real ecagi 0.199 ! Cage Thickness (internal) real ecagi 0.0747 ! Cage thickness w.out support tube (0.3%*24.9cm)  ! Outer wall: 3.0 mm solid carbon fiber - 1.3 % rl ! rf shield foil rl 0.130 mm aluminum - 0.15 % rl ! Equivalent to 1.45% of X 0 (carbon fiber) real ecage 0.361 ! Cage Thickness (external)  ! 6.5 mm carbon fiber end plate  2.8 % rl  ! Sacrificial epoxy layer machined for  0.5 % rl  ! feedthrough registration ! Feedthrough plastic longer than endplate,.... ! but has longer rl - a wash to first order ! Brass wire locating feedthrough inserts ! aluminum crimp pins ! brass crimp pins  0.3 % rl 0.3 % rl  0.3 % rl  ! Equivalent to 4.2% of X 0 (carbon fiber) real ecagp 1.05 ! Cage Thickness (End-Plates)  Appendix  B.  BBSIM  Input File  lElectronics: !(1) Backward: Preamp boards, H V , cables, etc  10.0 % rl  !(2) Forward: G10 boards with copper ground plane ! Some sort of brass connectors soldered  0.8 % rl  0.5 % rl  ! onto the GlO/copper boards. ! Forward and Backward i f shield: ! 0.5 mm aluminum sheet  0.6 % rl  ! Carbon fiber support spider  0.4 % rl  ! (arbitrarily chosen to make 1% total for rf shield) ! Equivalent to 11% rl backward, 2.3% rl forward real eelec(2) 1.03 0.22 ! Equivalent silicon thicknesses (r.l.=9.36cm) I  ! int nsl 1 ! Number of Super-Layers (10=sl, l=no si) int nsl 10 ! Number of Super-Layers (10=sl, l=no si) ! int nlay 40 ! Number of Layers per Super-Layer(4=sl,40=no si) int nlay 4 ! Number of Layers per Super-Layer (4=sl, 39=no si) real psi(2) 22.7 22.7 ! End-Plates Psi Angles; T D R : psi(l) used for both real zgas(2) -111.3 165.7 ! Extremal Z values (gas volume) real z(2) -127.3 171.7 ! Extrem Z (chamb) char dummy "string" ! needed to avoid absurd compilation problem ! with g++ compiler real full Jength 276. ! full chamber length along z, used in auvlay.f real cell-size 0.838666 ! (2/3)*Rectangular cell size (radially) real max_sag 1.5 ! Max Sag for No-Super-Layer chamber (for nsl=l only) ! 1.5 = Fixed maximum sag (cm), independent of cell height ! -1.5 = Automatically calculated = cell height ! 0.0 = All-axial no-superlayer chamber  Appendix  B.  BBSIM  Input File  real sigz 9999.0 ! Resolution in z (z-strips) huge! real sigrp 0.0140 ! resolution in the drift coordinate real zcat 0.0 ! Z-Pad Layer Thickness (0=no z-layer, 0.5=default) real rzcat(2) 23.0 79.5 ! wire phi half-angle in degrees real dphi(10) / 0. 11.058 -10.222 0. 8.844 -8.397 0. 7.583 -7.297 0. ! Radius of first sense wire per layer, at fullJength/2 "end of chamber" real r(10) / 24.18 30.26 35.29 40.82 46.90 51.93 57.46 63.54 68.57 74.10 ! Number of phi cells / superlayer int nphi(10) 90 108 126 144 162 180 198 216 234 252 ! Superlayer type (redundant with 'ster') char tsl(10) " A " " U " " V " " A " " U " " V " " A " " U " " V " " A " ! Sense wire Layer radii at z = Z0 = waist of chamber (recalculated in coc real rlayl(4) 99.999 99.999 99.999 99.999 real rlay2(4 ) 99.999 99.999 99.999 99.999 real rlay3(4 ) 99.999 99.999 99.999 99.999 real rlay4(4 ) 99.999 99.999 99.999 99.999 real rlay5(4) 99.999 99.999 99.999 99.999 real rlay6(4 ) 99.998 99.999 99.999 99.999 real rlay7(4 ) 99.999 99.999 99.999 99.999 real rlay8(4 ) 99.999 99.999 99.999 99.999 real rlay9(4 ) 99.999 99.999 99.999 99.999 real rlayl0(4) 99.999 99.999 99.999 99.999  Appendix  B.  BBSIM  Input File  ! stereo angles for each layer (recalculated in code) real sterl(4)) 0.00 0.00 0.00 0.00 real ster2(4)) 0.00 0.00 0.00 0.00 real ster3(4)) 0.00 0.00 0.00 0.00 real ster4(4)) 0.00 0.00 0.00 0.00 real ster5(4)) 0.00 0.00 0.00 0.00 real ster6(4)) 0.00 0.00 0.00 0.00 real ster7(4)) 0.00 0.00 0.00 0.00 real ster8(4)) 0.00 0.00 0.00 0.00 real ster9(4)) 0.00 0.00 0.00 0.00 real sterl0(4) 0.00 0.00 0.00 0.00 wire pattern in a superlayer char wpat(23) / )>P » ))  11 H p  g  11 11 11  J  "p " " p P" " p " " " "g p " " "  J  " p " " p p" "p " " " " p g " " " j "P " " p p"  ii  p )) ))  5)  »g p  n  ii  n  I  "P " hit info real odcha(3) 0.00 0.00 0.00 ! origin of dcha in lab end dcha  dimension nheLdclay 12 ! number of hit elements index delay / xO yO zO pxO pyO pzO path dEdx typ time flag layer  Appendix  B.  BBSIM  Input File  structure delay ! Hit structure for the layers char set " D C H A " char hnam(12) / "xO" "yO" "zO" "pxO" "pyO" "pzO" "path" "dEdx" "typ" "time" "flag" "layer" iut hbit(12) / 32 32 32 32 32 32 32 32 16 32 32 16 real hoff(12) / 500. 500. 500. 100. 100. 100. 0. 0. 0. l.e-5 0. 0. real hfac(12) / l.e4 l.e4 l.e4 l.e6 l.e6 l.e6 l.e4 l.e6 1. I.el4 1. 1. end delay l I  dimension nheLdcwir 9 ! number of hit elements index dewir x y z rcell phcell dedx drift idclay idcell structure dewir ! Hit structure for the sense wires char set " D C H A " char hnam(9) "x" "y" "z" "rcell" "phcell" "dedx" "drift" "idclay" "idcell" int hbit(9) 32 32 32 32 32 32 32 16 16 real hoff(9) 500. 500. 500. 0. 0. 0. 500. 0. 0. real hfac(9) l.e4 l.e4 l.e4 l.e4 l.e5 l.e<3 l.e4 1. 1. end dewir !  structure dcopt ! generation/reconstruction options int hwirtyp 1 ! 0:none, l:one bank (default), 3:three banks int lhwir 1 ! 0:don't store, ^,=l:store hits (default), £=2:hit dump int prtdeb 1 ! debug level 0=off, l=status (default), 2=print hits end dcopt  Appendix  B.  BBSIM  Input File  88  template dcmat ! dcha materials template (material number assigned in dc_tmed) int number ! material number - reassigned dynamically in dc_tmed char name ! name real a ! atomic weight real z ! atomic number real dens ! density real radl ! radiation length real absl ! absorption length end dcmat I  ! He:isobutane 80:20 gas averaged with material contribution from wires make dcmat gaz 0 "He-ibu-wire" 31. 15. .84E-03 .34E+05 .1E+06 ! the following materials are not specific to the dcha. They should be defined once make dcmat car 0 "Carbon-Fiber" 13.208 6.600 1.590 24.9 63.6 make dcmat sil 0 "Silicon" 28.090 14.000 2.330 9.36 30.3 make dcmat tug 0 "Tungsten" 183.85 74.000 19.3 0.35 10.3 l template dcmed int number ! medium number - reassigned dynamically in dc_tmed char natmed ! medium name char name ! corresponding material name int isvol ! sensitivity int ifield ! field flag real fieldm ! max. field in kgauss real tmaxfd ! max. deflection (degrees) real stemax ! max step permitted (cm) real deemax ! max. energy loss real epsil ! boundary crossing precision real stmin ! minimum step  Appendix  B.  BBSIM  Input File  89  make dcmed mot 0 "DCHA-mother" " A I R " 0 2 15. 10.0 0.5 0.02 0.01 1.0 make dcmed cag 0 "DCHA-cage" "Carbon-Fiber" 0 2 15. 10.0 0.5 0.02 0.01 1.0 make dcmed gaz 0 "DCHA-gaz" "He-ibu-wire" 1 2 15. 10.0 0.5 0.02 0.01 1.0 make dcmed fwi 0 "DCHA-fwi" " A L U M I N I U M " 0 2 15. 10.0 0.5 0.02 0.01 1.0 make dcmed swi 0 "DCHA-swi" "Tungsten" 0 2 15. 10.0 0.5 0.02 0.01 1.0 !! make dcmed gwi 0 "DCHA-gwi" " A L U M I N I U M " 0 2 15. 10.0 0.5 0.02 0.01 1.0 make dcmed ele 0 "DCHA-ele" "Silicon" 0 2 15. 10.0 0.5 0.02 0.01 1.0  Appendix C Author List  BABAR  L A P P Annecy,  Annecy-Ie-Vieux,  France  D. Boutigny, Y . Karyotakis, S. Lees-Rosier, P. Petitpas  I N F N , Sezione di Bari and Universita di B a r i , Bari,  Italy  C. Evangelista, A . Palano  Beijing Glass Research Institute,  Beijing,  China  G. Chen, Y . T . Wang, 0 . Wen  Institute of High Energy Physics,  Beijing,  China  Y . N . Guo, H . B . Lan, H.S. Mao, N . D . Q i , W . G . Yan, C . C . Zhang, W . R . Zhao, Y.S. Zhu  University of Bristol,  Bristol,  UK  N . Dyce, B . Foster, R.S. Gilmore, C.J.S. Morgado  University of Bergen,  Bergen,  Norway  G. Eigen  University of British Columbia, Vancouver,  British Columbia,  Canada  C. Goodenough, C. Hearty, J . Heise, J . A . McKenna  Brunei University,  London,  UK  T. Champion, A . Hasan, A . K . McKemey  Budker Institute of Nuclear Physics,  Novosibirsk,  Russia  A . R . Buzykaev, V . N . Golubev, V . N . Ivanchenko, S.G. Klimenko, E . A . Kravchenko, 90  Appendix  C.  BABAR  Author List  G . M . Kolachev, A.P. Onuchin, V.S. Panin, S.I. Serednyakov, A . G . Shamov, Ya.I. Skovpen, V.I. Telnov California Institute of Technology, Pasadena, California,  USA  D.G. Hitlin, J . Oyang, F . C . Porter, M . Weaver, A . J . Weinstein, R. Zhu University of California, Davis, Davis, California,  USA  F. Rouse University of California, I I R P A , L a Jolla, California,  USA  A . M . Eisner, M . Sullivan, W . Vernon, Y . - X . Wang University of California, Irvine, Irvine, California,  USA  K . Gollwitzer, A . Lankford, M . Mandelkern, G . McGrath, J . Schultz, D. Stoker, G. Zioulas University of California, Los Angeles, Los Angeles, California,  USA  K . Arisaka, C. Buchanan, J . Kubic, W . Slater University of California, San Diego L a Jolla, California,  USA  V . Sharma University of California, Santa Barbara, Santa Barbara, California,  USA  D. Bauer, D. Caldwell, A . L u , H . Nelson, J . Richman, D. Roberts, M . Witherell, S. Yellin University of California, Santa Cruz, Santa Cruz, California,  USA  J. DeWitt, D. Dorfan, A . A . Grillo, C. Heusch, R.P. Johnson, E . Kashigin, S. Kashigi W. Kroeger, W . Lockman, K . O'Shaughnessy, H . Sadrozinski, A . Seiden, E . Spencer Carleton University and C R P P l , Ottawa, Ontario, K. Edwards, D . Karlen, M . O'Neill^  Canada  Appendix  C. BABAR  Author List  92  University of Cincinnati, Cincinnati, Ohio, USA S. Devmal, B . T . Meadows, A . K . S . Santha, M . D . Sokoloff University of Colorado, Boulder, Colorado, USA A . Barker, B . Broomer, E. Erdos, W . Ford, U . Nauenberg, H . Park, P. Rankin, J . Roy, J . G . Smith Colorado State University, Fort Collins, Colorado, USA J. Harton, R. Malchow, M . Smy, H . Staengle, W . Toki, D. Warner, R. Wilson Technische Universitat Dresden, Institut fur K e r n - und Teilchenphysik, Dresden,  Germany  J. Brose, G. Dahlinger, P. Eckstein, K . R . Schubert, R. Schwierz, R. Seitz, R. Waldi Joint Institute for Nuclear Research, Dubna,  Russia  A. Bannikov, S. Baranov, I. Boyko, G . Chelkov, V . Dodonov, Y u . Gornushkin, M . Ignatenko, N . Khovansky, Z. Krumstein, V . Malyshev, M . Nikolenko, A . Nozdrin, Yu. Sedykh, A . Sissakian, Z. Silagadze, V . Tokmenin, Yu. Yatsunenko University of Edinburgh, Edinburgh, UK K . Peach, A . Walker I N F N , Sezione di Ferrara, Ferrara, Italy L. Piemontese Laboratori Nazionali di Frascati dell' I N F N , Frascati, Italy R. Baldini, A . Calcaterra, R. De Sangro, I. Peruzzi (also Univ. Perugia), M . Piccolo, A. Zallo I N F N , Sezione di Genova and Universita d i Genova, Genova, Italy A. Buzzo, R. Contri, G. Crosetti, P. Fabbricatore, S. Farinon, R. Monge, M . Olcese, R. Parodi, S. Passaggio, C. Patrignani, M . G . Pia, C. Salvo, A . Santroni  Appendix  C. BABAR  Author  List  93  University of Iowa, Iowa City, Iowa., USA U. Mallik, E . McCliment, M.-.Z. Wang Iowa State University, Ames, Iowa, USA H.B. Crawley, A . Firestone, J.W. Lamsa, R. McKay, W . T . Meyer, E.I. Rosenberg Northern Kentucky University, Highland Heights, Kentucky,  USA  M . Falbo-Kenkel University of Lancaster, Lancaster, UK C. K. Bowdery, A . J . Finch, F . Foster Lawrence Berkeley Laboratory, Berkeley, California,  USA  G.S. Abrams, D. Brown, T. Collins, C . T . Day, S.F. Dow, F . Goozen, R. Jacobsen, R.C. Jared, J . Kadyk, L . T . Kerth, I. Kipnis, J.F. Krai, R. Lafever, R. Lee, M . Levi, L. Luo, G.R. Lynch, M . Momayezi, M . Nyman, P.J. Oddone, W . L . Pope, M . Pripstein, D. R. Quarrie, J . Rasson, N . A . Roe, M . T . Ronan, W . A . Wenzel, S. Wunduke Lawrence Livermore National Laboratory, Livermore, California,  USA  0. Alford, J . Berg, R . M . Bionta, A . Brooks, F.S. Dietrich, O.D. Fackler, M . N . Kreisler, M . A . Libkind, M . J . Mugge, T. O'Connor, L . Pedrotti, X . Shi, W . Stoeffl, K . van Bibber, T . J . Wenaus, D . M . Wright, C.R. Wuest, R . M . Yamamoto University of Liverpool, Liverpool, UK J.R. Fry, E . Gabathuler, R. Garnet, A . Muir, P. Sanders University of London, Imperial College of Science, Technology and Medicine, London, UK P. Dornan, A . Duane, L . Moneta, J . Nash, D. Price University of London, Queen Mary &i Westfield College, London, UK D.V, Bugg, P.F. Harrison, I. Scott, B . Zou  Appendix  C.  BABAR  Author  List  University of London, Royal Holloway &i Bedford New College, Egham, Surrey, UK  Y . Gao, M . G . Green, D.L. Johnson, E . Tetteh-Lartey University of Louisville, Louisville, Kentucky,  USA  C. L. Davis M c G i l l University, Montreal, Quebec, Canada D. Britton, R. Fernholz, D. MacFarlane, P. Patel, C. Smith, B . Spaan, J . Trischuk University of Manchester, Manchester, UK J. Allison, R. Barlow, G. Lafferty, K . Stephens University of Maryland, College Park, Maryland,  USA  C. Dallapiccola, M . Foucher, H . Jawahery, A . Skuja University of Massachusetts, Amherst, Massachusetts,  USA  J. Button-Shafer, J.-J. Gomez-Cadenas, S.S. Hertzbach, R . R . Kofler, M . G . Strauss Massachusetts Institute of Technology, Cambridge, Massachusetts,  USA  R . F . Cowan, M . J . Fero, R . K . Yamamoto I N F N , Sezione di Milano and Universita di M i l a n o , Milano, Italy M . Calvi, C . Cattadori, R. Diaferia, F . Lanni, C . Matteuzzi, F . Palombo, A . Sala, T. Tabarelli University of Mississippi, Oxford, Mississippi,  USA  M . Booke, S. Bracker, L. Cremaldi, K . Gounder, R. Kroeger, J . Reidy, D. Summers Universite de Montreal, Montreal, Quebec, Canada G. Beaudoin, M . Beaulieu, B . Lorazo, J.P. Martin, P. Taras, V . Zacek Mount Holyoke College, South Hadley, Massachusetts, H . Nicholson, C.S. Sutton  USA  Appendix  C. BABAR  Author  List  95  I N F N , Sezione d i Napoli and Universita d i Napoli, Napoli, Italy N . Cavallo, L . Lista, S. Mele, P. Parascandolo, C. Sciacca University of Notre Dame, Notre Dame, Indiana, USA J . M . Bishop, N . N . Biswas, N . M . Cason, J . M . LoSecco, A . H . Sanjari, W . D . Shephard Oak Ridge National Laboratory/Y-12, Oak Ridge, Tennessee, USA F.S. AlsHiiller, R . G . Alsmiller, Jr., T . A . Gabriel, J.L. Heck L A L Orsay, Orsay, France D. Breton, R. Cizeron, S. Du, A . - M . Lutz, J . M . Noppe, S. Plaszczynski, M . - H . Schune, E. Torassa, K . Truong, G. Wormser I N F N , Sezione d i Padova and Universita di Padova, Padova, Italy F. Dal Corso, M . Morandin, M . Posocco, R. Stroili, C. Voci Ecole Polytechnique Palaiseau, L P N H E , PaJaiseau, France L. Behr, G . Bonneaud, P. Matricon, G. Vasileiadis, M . Verderi L P N H E des Universites Paris 6 et Paris 7, Paris, France M . Benayoun, H . Briand, J . Chauveau, P. David, C. De L a Vaissiere, L . Del Buono, J.F. Genat, 0 . Hamon, P. Leruste, J . Lory, J.-L. Narjoux, B . Zhang I N F N , Sezione d i Milano and Universita di Pavia, Pa via, Italy P.F. Manfredi, V . Re, V . Speziali, F. Svelto University of Pennsylvania, Philadelphia, Pennsylvania, USA L. Gladney I N F N , Sezione di Pisa, Universita di Pisa* and Scuola Normale Superiore*, Pisa, Italy G. Batignani ", S. Bettarini, F . Bosi, U . Bottigli*, M . Carpinelli, F . C o s t a n t W , 1  Appendix  C.  BABAR  Author List  96  F. Forti, D. Gambino, M . Giorgi^ A . Lusiani*, P.S. Marrocchesi, M . Morgantit, G. Rizzo, G. Triggiani*, J . Walsh Prairie View A & M University, Prairie View, Texas,  USA  M . Gui, D . J . Judd, K . Paick, D . E . Wagoner Princeton University, Princeton, New Jersey, USA C. Bula, C. L u , K . T . McDonald I N F N , Instituto Superiore di Sanita, Roma, Italy C. Bosio I N F N , Sezione di R o m a and Universita " L a Sapienza," Roma, Italy F. Ferroni, E. Lamanna, M . A . Mazzoni, S. Morganti, G . Piredda, R. Santacesaria Rutgers University, Rutgers; New Jersey, USA P. Jacques, M . Kalelkar, R. Piano, P. Stamer Rutherford Appleton Laboratory, Chilton, Didcot,  UK  P.D. Dauncey, J . Dowdell, B . Franek, N.I. Geddes, G.P. Gopal, R. Halsall, J.A. Lidbury, V . J . Perera C E A , D A P N I A , CE-Saclay,  1  Gif-sur-Yvette,  France  R. Aleksan, P. Besson, T. Bolognese, P. Bourgeois, A . de Lesquen, A . Gaidot, L . Gosset, G. Hamel de Monchenault, P. Jarry, G. London, M . Turluer, G. Vasseur, C. Yeche, M . Zito Shanghai Institute of Ceramics ( S I C C A S ) , Shanghai, China J.R. Jing, P.J. L i , D.S. Yan, Z.W. Y i n University of South Carolina, Columbia, South Carolina, M . V . Purohit, J . Wilson 1  Subject to approval of funding agency.  USA  Appendix  C. BABAR  Author  List  97  Stanford Linear Accelerator Center, Stanford, California, USA D. Aston, R. Becker-Szendy, R. Bell, E. Bloom, C. Boeheim, A . Boyarski, R . F . Boyce, D. Briggs, F . Bulos, W . Burgess, R . L . A . Cottrell, D . H . Coward, D.P. Coupal, W. Craddock, H . DeStaebler, J . M . Dorian, W . Dunwoodie, T. Fieguth, D. Freytag, R. Gearhart, T. Glanzman, G. Godfrey, G. Haller, J . Hewett, T. Himel, J. Hoeflich, W. Innes, C P . Jessop, W . B . Johnson, H . Kawahara, L. Keller, M . E . King, J . Krebs, 2  P. Kunz, W . Langeveld, E . Lee, D.W.G.S. Leith, V . G . Luth, H . Lynch, H . Marsiske, T. Mattison, R. Melen, K . Moffeit, L. Moss, D. Muller, M . Perl, G. Oxoby, M . Pertsova, H. Quinn, B . N . Ratcliff, S.F. Schaffner, R . H . Schindler, S. Shapiro, C. Simopolous, A . E . Snyder, E . J . Soderstrom, J . Vav'ra, S. Wagner, D. Walz, R. Wang, J . L . White, W. Wisniewski, N . Y u Stanford University, Stanford, California, USA P. Burchat, R. Zaliznyak Academia Sinica, Taipei, Taiwan H.-Y. Chau, M . - L . Chu, S.-C. Lee University of Texas at Dallas, Richardson, Texas, USA J . M . Izen, X . Lou I N F N , Sezione di Torino and Universita di Torino, Torino, Italy F. Bianchi, D. Gamba, G. Giraudo, A . Romero I N F N , Sezione di Trieste and Universita di Trieste, Trieste, Italy L. Bosisio, R. Delia Marina, G. Delia Ricca, B . Gobbo, L. Lanceri, P. Poropat T R I U M F , Vancouver, British Columbia, R. Henderson, A . Trudel '• Retired  '  Canada  Appendix  C. BABAR  Author  Tsinghua University,  List  Beijing,  China  Y . P . Kuang, R . C . Shang, B . B . Shao, J.J. Wang  Vanderbilt University,  Nashville,  Tennessee, USA  R.S. Panvini, T . W . Reeves, P.D. Sheldon, M . S . Webster  University of Victoria, Victoria,  British Columbia,  Canada  M . McDougald, D . Pitman  University of Wisconsin,  Madison,  Wisconsin,  USA  H.R. Band, J.R. Johnson, G.S. Mitchell, R. Prepost, G . H . Zapal  York University, W . Frisken  Toronto, Ontario,  Canada  Bibliography  [1] D. Griffiths, Introduction to Elementary Particles, (1987), Wiley. [2] F. Halzen and A . Martin, Quarks and Leptons, (1984) Wiley.  [3] L. Montanet et al, Particle Data Group, Particle Physics Book, (July 1994) American Institute of Physics, Phys. Rev. D 5 0 , 1173. [4] P.R. Burchat, Physics at an Asymmetric-Energy B Meson Factory, Lake Louise Winter Institute, (1991) Conference Proceedings. [5] A . D . Sakharov, The Baryon Asymmetry  of the Universe, (May 1991) Usp. F i z .  Nauk. 161, 110. [6] W.S.C. Williams, Nuclear and Particle Physics, (1991) Oxford University Press. [7] F . Wilczek, The Cosmic Asymmetry A m . 243, No. 6, 60.  Between Matter and Antimatter,  (1980) Sci.  [8] C S . Wu, E . Ambler, R . W . Hayward, D.D. Hoppes, R.P. Hudson, (1957) Phys. Rev. D105, 1413. , [9] L. Wolfenstein, CP Noninvariance Part. Phys. 14, 135.  in K° Decay,  (June 1985) Comments Nucl.  [10] Y . Nir and H.R. 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