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Charged-current and neutral-current event fraction determination based on fit vertices, time residuals… Nally, Christian Winslow 1996

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C H A R G E D - C U R R E N T A N D N E U T R A L - C U R R E N T E V E N T F R A C T I O N D E T E R M I N A T I O N B A S E D O N F I T V E R T I C E S , T I M E R E S I D U A L S A N D P M T H I T A N G L E S F O R T H E S U D B U R Y N E U T R I N O O B S E R V A T O R Y B y Chris t ian Winslow Nal ly B . Sc. (Engineering Physics) Queen's University. Kingston, Ontario, 1993 A T H E S I S S U B M I T T E D I N P A R T I A L F U L F I L L M E N T O F T H E R E Q U I R E M E N T S F O R T H E D E G R E E O F M A S T E R O F S C I E N C E in T H E F A C U L T Y O F G R A D U A T E S T U D I E S D E P A R T M E N T O F P H Y S I C S We accept this thesis as conforming to the required standard T H E U N I V E R S I T Y O F B R I T I S H C O L U M B I A A p r 1996 © Chris t ian Winslow Nally, 1996 In presenting this thesis in partial fulfilment 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 2075 Wesbrook Place Vancouver, Canada V6T 1Z1 Date: Abstract The Sudbury Neutrino Observatory (SNO) aims to improve our understanding of neutrinos and energy-generating processes in the Sun. The central purpose of SNO is to compare the flux of charged-current (CC) and neutral-current (NC) events caused by solar neutrinos. The target in SNO will be 106 kg of D 2 0 . One plan for enhancing the detection of N C events is by doping the D 2 0 with 3 5 C1. This thesis describes maximum likelihood fitters created for the purpose of fitting C C events and for fitting N C events that result from neutron capture by a 3 5 C1 nucleus. For each fit event, likelihood ratios are extracted from these fitters to aid in determining the fraction of each type of event in a data set of unknown mixture. It is concluded that using both timing and angle information marginally improves the ability to discriminate between C C and NC events, compared with using angle informa-tion alone. This is seen in the reduced estimated error for a technique that determines the C C event fraction in a 50/50 mixed set of C C and N C events. For prototype sets with 2000 events each, and a mixed set with 950 of each event type, an event fraction determination based on only angle information produces a fraction estimate of 0.494 ± 0.037. With identically sized data sets, an event fraction determination that includes both timing and hit angle information produces and estimate of 0.493 ± 0.034. i i Table of Contents Abstract ii Table of Contents iii List of Tables „ vi List of Figures vii Acknowledgements ix 1 Introduction 1 1.1 Context 1 1.2 The Standard Solar Model 1 1.3 The Solar Neutrino Problem 2 1.4 Previous Experiments , 3 1.5 The SNO Detector 5 1.6 The Physics of the SNO 6 1.7 Comparison of CC and NC Events 9 1.8 Searching for a Characterizing Parameter 9 2 Mean Opening Angle Classification 14 2.1 Average Opening Angle 14 2.2 Results of a x 2 Fit 14 iii 2.3 Introduction to Neural Networks 16 2.4 Training a F N N to Classify SNO Events 17 2.5 Training Data 19 2.6 Input Parameters 21 2.7 Classification Performance 21 2.8 Verification Using PDFs 22 3 Event Fraction Analysis 24 3.1 Event Fraction Determination 24 3.2 Event Fraction Error Estimation 26 4 Likelihood Ratio Characterization 27 4.1 A Type of Classification Parameter 27 4.2 Prototype Generation 28 4.3 Time Residual and Angle Extraction 28 4.4 Probability Density Estimation 29 4.5 Choosing a Window Width 30 4.6 One Dimensional PDFs 31 4.7 Two Dimensional PDFs 32 4.8 Likelihood Ratio Results 34 5 Maximum Likelihood Fitters 41 5.1 Building a Maximum Likelihood Fitter 41 5.2 Likelihood Maximization 42 5.3 Simulated Annealing 43 5.4 Very Fast Simulated Annealing 44 5.5 Selection of V F S A Parameters 46 iv 5.6 Using a Previous Fit as an Initial Vertex 48 5.7 Fitter Performance 48 6 Maximum Likelihood Ratio Characterization 53 6.1 M L R Results 53 6.2 Comparison and Discussion 54 6.3 Possible P D F Improvements 54 7 Conclusion 57 7.1 Conclusion 57 7.2 Future Work 58 Bibliography 59 A Glossary 61 B NC Gamma Rays 63 C Simulated Annealing Algorithm Outline 65 v List of Tables 1.1 Solar Neutrino Flux Measurements 3 2.1 F N N single electron training dataset contents 20 2.2 Gammas in simulated neutron capture on 3 5 C7 F N N training dataset. . . 20 2.3 Parameters used for event discrimination with the F N N 21 2.4 Summary of F N N results 23 3.1 x2 Fit Quantities 25 4.1 Results for L R Event Characterization 35 5.1 V F S A Parameters 46 6.1 Results for M L R Event Characterization 54 vi List of Figures 1.1 Solar Neutrino Flux •'. 4 1.2 Detector Cross Section 5 1.3 Schematic of Cerenkov Production 6 1.4 Sample Event from SNO 8 1.5 Nhits Spectrum for Pure D 2 0 . . : 12 1.6 Nhits Spectrum for D 2 0 with NaCl added 13 2.1 Opening Angle (9 ) Histograms for C C and NC events 15 2.2 Feedforward Neural Net Diagram 16 2.3 The Neural Networks Activation Function 18 2.4 Histogram of F N N Output 22 4.1 Time Residual P D F 32 4.2 Hit Angle P D F 33 4.3 C C Time and Angle P D F Estimate 36 4.4 N C Time and Angle P D F Estimate . 36 4.5 Tresiduai P D F likelihood ratios for time fitter vertices 37 4.6 Hit angle P D F likelihood ratios for time fitter vertices . . . 38 4.7 Tresiduai and hit angle P D F likelihood ratios for time fitter vertices . . . . 39 4.8 LRtime+angie distributions for Monte Carlo vertices 40 5.1 Maximum Likelihood fits fewer events 'In Cleveland' 47 vii 5.2 Fit depth compared to Monte Carlo likelihood 49 5.3 C C Distance Residuals 51 5.4 C C Time Residuals 51 5.5 N C Distance Residuals 52 5.6 N C Time Residuals 52 6.1 C C and N C Tresiduai P D F fit, hit angle P D F likelihood M L R histogram. . 55 6.2 C C and NC 2D P D F M L R Histograms 56 B . l Gammas from neutron capture on 3 5 C1 64 viii Acknowledgements I am especially grateful for the support of my supervisor, Prof. Chris Waltham. I would like to thank Robert Komar for helping with the neural network calculations and very helpful discussions about the rest of the work. Thanks go to Werner Keil for providing a well run L A N , and help with various UNIX 'tidbits'. My apologies go to the users of the U B C nuclear group L A N for my long running, memory hogging process runs. Thanks go to Peter Skensved of Queen's University for the original idea for this project and Steve Brice of Oxford for Figures and helpful discussions about neural networks. Thanks also go to Tom Radcliff of Queen's University, and Joshua Klein of the University of Pennsylvania for valuable conversations. ix Chapter 1 Introduction 1.1 Context This thesis is concerned with an aspect of data analysis for the Sudbury Neutrino Obser-vatory (SNO), currently being built at the 6800 ft level of the Creighton mine in Sudbury, Ontario. The purpose of SNO is to point to the solution of a discrepancy called the 'Solar Neutrino Problem'. This thesis is a contribution to the growing set of studies on analysis techniques that will aid the understanding of the data SNO will produce. The goal of this thesis is to investigate the ability of several parameters to aid in determining the fraction of C C and N C events in a data set with an unknown mixture of the two. 1.2 The Standard Solar Model The standard solar model (SSM) [Bahcall95] is a model of the sun's physical properties. There are four assumptions that serve as building blocks of the SSM. • The sun is in hydrostatic equilibrium, with particle and radiation pressures balancing gravitational collapse. • Energy is transported between layers in the sun by photons and convection of stellar matter. 1 Chapter 1. Introduction 2 • Nuclear reactions are driving energy production in the sun. • The sun started as a homogeneous mixture of gases. The SSM is built upon these assumptions, and uses as input measurable parameters such as solar luminosity and nuclear cross sections. It includes physical theories for radiation transport and convection of stellar material. Composition changes in layers of the sun stable to convection are primarily due to nuclear reactions, but recent versions of the SSM also account for diffusion processes. 1.3 The Solar Neutrino Problem Neutrinos are neutral fundamental subatomic particles. They are produced in the sun in connection with the energy generating fusion processes, and in /3-decays of fusion products. There are thought to be three kinds of neutrino, one for each of the three generations of subatomic particles, but the current theory of particle physics only allows electron type neutrinos {ve) to be created in the sun. Of the four fundamental forces of nature, neutrinos are only known to interact through the weak force, and therefore can pass through large amounts of matter without interacting. The SSM predicts a value for the flux of neutrinos from the sun. A l l measurements of the solar neutrino flux have indicated that it is lower than the expected value by a factor of two or three. This discrepancy is called the 'Solar Neutrino Problem' (SNP) [Bahcall89]. There are two general possibilities for a solution to the SNP. On one hand, there might be something fundamentally wrong with the SSM. Otherwise, there might be some new neutrino physics to be discovered. The Sudbury Neutrino Observatory aims to solve the SNP. Chapter 1. Introduction 3 Experiment Flux SSM Prediction Reaction Threshold (SNU) (SNU) (MeV) Homestake 2.56 ± 0.21 ve+'S7Cl -> e"+ 3 7 Ar 0.8 Kamiokande 2.8 ± 0.45 * 6.62 * ve + e~ ->• v'e + e~' 7.5 S A G E 69 ± 12 137+87 ^ e + 7 1 G a -> e-+ 7 1 Ge 0.23 Gallex 77 ± 10 l37+87 z^ e+ 7 1Ga e-+ 7 1 Ge 0.23 Table 1.1: Measurements of the Solar Neutrino Flux. 1 SNU = 10~ 3 6 interactions per target atom per second. * —»• The Kamiokande flux measurement is given in units of 106 c m - 2 s - 1 rather than in SNU, which would be threshold dependent. Among the possibilities for new neutrino physics is the possibility of neutrino os-cillations. A neutrino would 'oscillate' if it switched from one type into another. A particularly attractive solution to the SNP is the possibility of matter enhanced neu-trino oscillations. Dubbed the M S W effect, in honour of its inventors Mikheyev, Smirnov [Mikheyev] and Wolfenstein [Wolfenstein], this effect depends critically on the local elec-tron density. Possible observable effects of the MSW solution include a ve energy spec-trum that is distorted from the expected spectrum, a significant flux of and uT, and day-night variations in the neutrino flux caused by oscillations in the earth. 1.4 Previous Experiments Four measurements of the solar neutrino flux have been made and all show a flux less than that expected from the SSM. A l l of these experiments are predominantly sensitive to ve and have not made any conclusions about the solar flux of and/or vT. Figure 1.1 shows the fluxes of neutrinos from various sources in the sun, according to the SSM. Notice that the flux of low energy neutrinos is very much higher than that of the higher energy ones. This is due to the fact that the higher energy neutrinos are produced in reactions with low branching ratios. The experimental results are tabulated in Table 1.1. The Homestake experiment [Wildenhain] has been in operation since the 1967, and Chapter 1. Introduction 4 2.5 7 . ! IBS 17.5 0.0 5.0 10.0 1E.0 20.0 NEUTRINO ENERGY IN MeV. Figure 1.1: The neutrino spectra resulting from the main neutrino generating reactions as predicted by the standard solar model. [Bahcall95] The SNO detector will be sensitive only to neutrinos above some as of yet undetermined threshold near 5 MeV. The flux unit at Earth for the continuous spectra is c m _ 2 s _ 1 M e V _ 1 , and for the line spectra cm~ 2 s _ 1 . has undergone almost three decades of careful scrutiny. The Kamiokande experiment [Takeuchi] is a real-time water Cerenkov detector, which allows the time of each neutrino event to be recorded. Information relevant to the direction of each event is gathered as well. The direction information demonstrates that the sun is the actual source of the neutrinos. The S A G E [Gavrin] and G A L L E X [Anselmann] experiments confirm that the flux is lower than expected, and are noteworthy because of their low threshold. According to the SSM, the majority of solar neutrinos have an energy lower than 0.42 MeV, and the fact that these two experiments show a low flux rules out the possibility that it is only the higher energy neutrinos that have a flux lower than expected. Chapter 1. Introduction 1.5 The SNO Detector 5 The SNO detector will measure the flux of neutrinos from the sun, and produce infor-mation designed to point toward a solution to the SNP. The detector, shown in Figure 1.2, will comprise of an acrylic vessel 12.0 m in diameter that will hold ultra pure heavy water (D 2 0) . There is a geodesic sphere of photomultiplier tubes (PMTs) looking inward on the acrylic vessel. These tubes will detect photons produced by neutrinos reacting in the D 2 0 . 9450 PMTs & Reflectors Sudbury Neutrino Observatory Figure 1.2: Detector Cross Section Radioactivity in detector materials mimic the neutrino interactions in SNO and pose a problem for the measurement of the solar neutrino flux. Ultra pure light water will fill the cavity outside the acrylic vessel. This acts as a shielding from radioactivity in the Chapter 1. Introduction 6 P M T materials. The detector materials themselves have been scrutinized for cleanliness. The SNO detector essentially detects light from fast electrons, where fast means faster than the local speed of light in water. This threshold corresponds to the electron having a kinetic energy above 262 keV. The resulting Cerenkov radiation is the lightwave analogy to a sonic boom. Its generation is shown schematically in Figure 1.3. A l l of the reactions in SNO that are detected by the PMTs end in the creation of electrons that emit Cerenkov radiation. 1.6 The Physics of the SNO Cerenkov Angle Cerenkov Photons Electron Path e-Figure 1.3: Schematic of Cerenkov Production Using D 2 0 as a target, instead of relying on the electrons in normal water, provides SNO with some distinct advantages. The cross section for u interaction is higher due to a Chapter 1. Introduction 7 higher centre of mass energy for a nucleon-neutrino pair. D 2 0 provides higher sensitivity to Up and vT through a reaction that involves the breakup of the deuteron. There are three reactions of primary interest in SNO. vx + e~ —t vx + e~ ES ve + d->p + p + e~ C C i/x + d^vx-r-n + p NC The first is elastic scattering (ES), where a neutrino collides with an electron in the water. A l l three types of neutrino scatter from electrons through a neutral current interaction, but ve also scatters from electrons through a charged current interaction. Thus the cross section is 6 to 7 times higher for ve than for the other two neutrino types. Measurements of the rate of this reaction have not produced direct evidence for neutrino oscillations. The second reaction, the charged current (CC) reaction, is unique to SNO. It involves the inverse (3 decay of the deuterium nucleus. A fast electron is ejected, emitting Cerenkov photons. The energy of this electron is correlated with the energy of the incoming neutrino, and this allows the measurement of the C C energy spectrum. , The third reaction, also unique to SNO, is called the neutral current (NC) reaction. It involves the dissociation of the deuteron into a proton and a neutron. This reaction is independent of neutrino type, and SNO will use this reaction as a measurement of the total flux of high energy solar neutrinos. There are two main schemes for the detection of the neutron ejected from this third reaction. In one scheme, 3 He neutron counters (NCDs) are placed in the detector. In the other scheme, CI ions are introduced into the heavy water; 3 5 C I , which comprises 76 % of natural CI, is a good neutron detector. When a neutron is captured by a 3 5 C1 nucleus, gamma rays are emitted, as shown in the decay scheme in Appendix B. These gamma rays will subsequently transfer their energy Chapter 1. Introduction 8 to electrons, which in turn emit Cerenkov radiation, thus making them detectable by the PMTs in SNO. In the remainder of this thesis, neutral-current events as detected by neutron capture on 3 5 C1 are referred to with the notation NC. A n event in SNO is a set of P M T hits close together in time. A 5 MeV electron will typically result in around 50 P M T hits. The time of each P M T hit is.recorded. Often, most of the hits in the event are associated with a single electron. Some of the photons that cause a hit might have scattered on the way to the P M T . Some of the hits might be associated with random P M T firings, called noise hits. A typical event hit pattern is shown in Figure 1.4. Figure 1.4: Sample P M T Hit Pattern for a single electron created by the S N O M A N Monte Carlo. The electron kinetic energy is 5.0 MeV Chapter 1. Introduction 9 This thesis is concerned with discriminating C C events from N C events. 1.7 Comparison of CC and NC Events For a single e~, two effects tend to prevent the P M T hits from forming a clean Cerenkov ring. First, the direction of the electron will wander as it slows due to multiple scattering. Second, the angle of Cerenkov emission is a function of e~ energy such that cos 9 = ^ where 9 is the emission angle from the direction of propagation, j3 is v/c for the e~ and n is the index of refraction. Thus as the e~ slows, the emission angle closes from the f3 = 1 limit of 41.4° to 0.0°. These effects obscure the direction resolution to the point where the error in the reconstructed direction as computed by the mean direction to the P M T hits is approximately 25°. Charged current events are single e~ events with a known direction distribution from the sun. The essential difference between C C and NC events from the PMTs point of view is the presence of one or more gammas emitted from a neutron capture on the 3 5 CI nucleus (see the decay scheme in Appendix B). A gamma ray will result in more than one source for Cerenkov radiation if it Compton scatters multiple times. The presence of more than one gamma will compound this effect. This means that for N C events, there will sometimes be multiple sources of Cerenkov radiation. C C events and N C events will also have different energy spectra. 1.8 Searching for a Characterizing Parameter The SNO detector aims to point to a solution of the SNP by measuring both the flux and spectrum of electron type neutrinos (through the C C reaction) and the total flux of Chapter 1. Introduction 10 neutrinos (through the NC reaction). If these fluxes are the same, then the solar neutrinos are electron neutrinos, and the solution of the SNP probably involves modification of the SSM. If the total flux of neutrinos is significantly higher than the electron neutrino flux, then there are a significant number of muon and/or tau type neutrinos and the solution of the SNP would be that neutrinos are mixing among generations after being generated. In the worst case, we will not know whether a given event is C C or N C at all. In this case, we will be forced to estimate the fluxes of these reactions by performing two separate data runs. One with just pure D 2 0 , the other with the N C detection enhancing 3 5 C1. The Nhits spectrum for various sources of event is shown in Figures 1.5 and 1.6. Events labeled P M T (3 — 7 come from radioactivity in the PMTs. Internal j3 — 7 events are due to radioactivity in the D 2 0 . Events labeled 'PSUP + Cavity' come from radioactivity in the P M T support structure or from the rest of the SNO cavity. The first is for the pure D 2 0 case, while the second is for the case where 2500.0 kg of NaCl has been added. In the ideal (and very unrealistic) case, the exact nature of each event would be known as it occurs. In this case, only a single data run with 3 5 C1 would be required, and independent energy spectra for each event class could be created. This thesis is concerned with finding event characterizing parameters based on the ratio of a N C likelihood and a C C likelihood. These likelihoods will be calculated using probability densities for P M T timing, and P M T hit angles. The characterization ability of the parameters will be tested by using each parameter to estimate the event fraction in a set with a known mixture of C C and NC events. Event fraction determination is done by producing distributions of an event char-acterizing parameter. In the ideal case described above, for C C events this parameter would have values very different from its values for NC events. To determine whether the event was C C or NC, one would only have to evaluate this parameter. In a more realis-tic case, this parameter will have overlapping distributions for the two event types and Chapter 1. Introduction 11 event fraction determination proceeds via statistical analysis with a fit to two prototype distributions. This thesis ignores the fact that there will be ES events produced by SNO, and also ignores any background events that mimic the C C and NC signals. The analysis presented here would have to be applied to data sets where these event types were already removed, or the analysis would need to be extended to the case where more than two classes of event are present. Chapter 1. Introduction 12 o •g <n T3 o o a. c > UJ 10 10 10"-a 1 0 N 1 0 4 ^ 1 0 N 102 10' i . 1 o -=t A PMT p-y (scaled) A Internal p--f . ES X CC 0 NC • NC background X PSUP + Cavity t • LZI o 20 40 n I 60 80 Hits 100 120 Figure 1.5: Nhits spectrum for various event sources for a pure D 2 0 data run. Fluxes are scaled to 1/3 of the SSM. Chapter 1. Introduction 13 1 0 ° ^ 10' -i 1 0 10" -i 10*^ 10' i o ' i io- t 10 _ . r • PMT p-Y (scaled) A Internal p-7 O ES * CC « NC (with salt) • NC background X PSUP + Cavity .:.'i£J~J.'..:.. " V - V , . • 4 — a s -^n>.-.»:.:«.::::..i:g:t>,;; ' "> "• '^jr»:"rii o[>•• -• • - - -- •* -...»c- - v. -t> ;m>» a •>« * • •* -K-aC4-M< tt..fij. U - •« C M - . *• em-* ua> v-u-+ o ivMtt-* « » :.:::^ .::::.™::.:::::H*JP?:*3.: 20 40 60 80 100 120 Hits Figure 1.6: Nhits spectrum for various event sources for a D 2 0 data run with NaCl added. Fluxes are scaled to 1/3 of the SSM. Chapter 2 Mean Opening Angle Classification This chapter describes a simple parameter that can be used for event type characteriza-tion. Results are given for ratio determination using this parameter so that it may be held up as a benchmark for parameters described in later chapters. 2.1 Average Opening Angle A very simple parameter that distinguishes C C events from N C events is the mean opening angle 9 of the P M T hits in an event from the average direction of the hits from a fit vertex position. It is relatively easy to see the physical reason why this parameter distinguishes between C C and NC events. NC events will more often have more than one source of light for P M T hits, and when this is the case, the P M T hits will be distributed in a more isotropic fashion. The 9 parameter is simply a measure of the degree to which the distribution of P M T hits is isotropic. Histograms of 9 for sets of C C and N C events are shown in Figure 2.1. 2.2 Results of a x2 Fit Given a parameter that provides some ability to distinguish between the two event classes, one is able to estimate the fraction of events of each type in a mixed set by performing a 14 Chapter 2. Mean Opening Angle Classification 15 fit of the distribution of the parameter for that mixed set to a sum of two distributions from prototype sets of the two event classes. This method, to be described further in section 3.1, was used in conjunction with 6 to determine the most probable number of CC events and NC events in a mixed set with 950 CC events and 950 NC events. Prototype distribution sets were created that included 2000 CC events and 2000 NC events. The 9 distribution of the mixed set was fit to a mixture of the two prototype sets with the CC fraction used as the free parameter. This method produced an estimated CC event fraction of 0.521±0.047. The given error corresponds to 1.0 a statistical error; systematic errors have not been investigated. The remaining chapters of this thesis set out to find a parameter that provides a better basis for determining the fractions of CC events in a Chapter 2. Mean Opening Angle Classification 16 dataset. The remainder of this chapter describes how the 9 parameter was discovered as being the important distinguishing parameter among several others. 2.3 Introduction to Neural Networks The importance of the 9 parameter in relation to several others was initially investigated using a Feedforward Neural Net (FNN) classifier. The neural net simulation code that comes with [Masters] was used for the investigations described here. A high quality, free alternative is the Stuttgart Neural Network Simulator (SNNS) [SNNS]. parameter x vector input network: output input nodes hidden weights hidden biases & hidden nodes output weights output biases output nodes Figure 2.2: Diagram of a Feedforward Neural Net. Reprinted with permission from [Brice95] The diagram of a F N N is a graphical way of understanding how the F N N algorithm computes its output. An example F N N with one hidden layer is shown in Figure 2.2. This diagram depicts input neurons, hidden neurons and output neurons. Each neuron in the input layer is connected to each hidden neuron, which is, in turn connected to each output neuron. A calculation of the FNN's output is carried out as follows. The Chapter 2. Meati Opening Angle Classification 17 input values are given to the input neurons. The value of the input neuron is multiplied by a value associated with the link between it and a hidden neuron, called the 'weight' of that link. For each hidden neuron, the products of the input neuron values and their associated weights are summed. This is effectively the scalar product of the input vector and the weight vector for a given hidden neuron. The hidden neuron feeds its input through a non-linear function called an activation function. The result of this function is considered to be the hidden neuron's output. A similar process of taking the scalar product of neuron values with weight vectors occurs between the hidden layer and the output layer. This process is described by the following set of equations. The output of a particular hidden neuron is calculated as n H = A(£XlWl + 6) i = l where the sum over i is over each input neuron, n is the number of input neurons, Xi is the value input from a neuron in the input layer, Wi is the weight vector associated with the link to that neuron, 0 is the bias value associated with that hidden neuron, and A is the activation function for the neuron. The activation function is a 'sigmoid', that is to say 's-shaped'. The performance of a neural net is often not sensitive to the exact functional form of the activation function. For the trials described here an interpolated logistic function A(x) = 1+),-x was used. The logistic function is plotted in Figure 2.3. 2.4 Training a FNN to Classify SNO Events A F N N can be viewed as a function approximator. The function being approximated is from from n to c dimensions, where n is the number of input neurons and c is the number of output neurons. The function is learned through a training procedure based Chapter 2. Mean Opening Angle Classification 18 i t J- , , , , H- , , , , , , , j , , , , J Figure 2.3: The Neural Networks Activation Function on sets of prototype data. Training occurs by varying the weights and biases until the net approximates the function represented by the training data. A F N N is trained to classify by creating a training set for each class that the network is intended to recognize. Each input vector in the training set is associated with one event such that its components are either parameters that describe an event, such as Nnits, or are calculated from the P M T Hit pattern, for example, an error estimate in the fit vertex. Each one of these input vectors is associated with a desired output vector. For the case of distinguishing between two event classes, only one output neuron is required. The F N N will be trained to have a high output value for one event class, and a low output value for the other class. The goal of training is to minimize the mean square error over a complete data set, Chapter 2. Mean Opening Angle Classification 19 defined as M = l / n ^ ( T i e - y i e f (2.1) e=l i= l where e runs over each event in the training set, n is the number of events, i runs over the output neurons, c is the number of output neurons, T,e is the target value for the ith output neuron for event e, and Y e^ is the observed activation for the ith output neuron for event e. The reason for training the net in this way is that the derivative of M with respect to the weights is easily computed, and this provides a way of updating the weights in such a way as to make them produce output closer to the desired value for that class. The ability of the network to approximate a given function is constrained by the number of neurons and the number of hidden layers it has. This is similar to fitting many data points to a polynomial. One should be careful to have many more data points in the training set than free parameters in the net. This was not a problem for the investigations presented here because there were several thousand events in the training sets used, and only one or two hidden layers with few a few hidden neurons in each. 2.5 Training Data Two data sets, produced with S N O M A N , were used for the results reported in this Chap-ter; different prototype sets are developed for later work. For all sets, event interactions were distributed uniformly throughout the D 2 0 . One data set was a combination of various single e~ Monte Carlo runs with various electron energies resulting in a spectrum of N^ts similar to that for NC. This was done to minimize the FNNs ability to distinguish between the two event types based on the Nhits parameter alone. It was hoped that by doing this, the F N N might develop sensitivity to differences between the two event classes in the form of an Nhits dependence of one of the Chapter 2. Mean Opening Angle Classification 20 Ee (MeV) number out of 10000 4.0 2375 5.0 2250 6.0 2500 7.0 2375 8.0 500 Table 2.1: Electron energies in the single e~ event set. The events from these discrete en-ergy electrons were added together to form an event set with an Nhits spectrum spectrum close to that of the N C events. E1 (MeV) number out of 10000 8.578 291 7.790 and 0.788 855 7.414 and 1.165 1042 6.978 and 1.601 226 6.628 and 1.951 302 6.628 and 1.164 and 0.787 152 6.620 and 1.959 801 6.111 and 1.951 and 0.517 1332 6.111 and 1.164 and 0.787 and 0.517 668 5.715 and 2.863 664 1.0 and 2.0 and 2.0 and 3.578 3667 Table 2.2: Various decays included as a simulation of the gammas resulting from neutron capture on 3 5 C1. The last entry is included to simulate the many decays that were not extracted from the decay scheme. other input parameters. Each single e~ was started in a random direction. The contents of the data set are shown in Table 2.1. The other data set was a 'hand made' simulation of N C events, in the sense that it was constructed by combining the results of gamma ray Monte Carlo runs. The contents of this combined set are shown in Table 2.2. This set was created before the physics for neutron capture on 3 5 C1 was built into the S N O M A N Monte Carlo. Angular correlations between the gamma rays in the 3 6 C I were not taken into account for the data set described here. Each gamma ray was released in a random direction. Chapter 2. Mean Opening Angle Classification 21 Nhits The number of P M T hits for a given event AX The uncertainty in the position of the fit vertex AT The uncertainty in the time of the fit vertex 9 The mean of the angle between the direction to each hit and the fit direction ve The standard deviation in 6 &time The standard deviation in the P M T hit times expected from the hit XR S N O M A N Fitter Reduced X 2 Table 2.3: Parameters used for event discrimination with the F N N . 2.6 Input Parameters The input parameters were extracted and/or calculated from the S N O M A N time fitter, a subroutine that returns a fit vertex and direction based on P M T times. The complete list of parameters considered is in Table 2.3. Each parameter was rescaled between 0.0 and 1.0 as a requirement for the use of the neural net. 2.7 Classification Performance To measure classification performance, a sample from the set of NC events was fed into the classifier. Each event was labeled as being 'NC-like', or 'single e~-like' depending on whether the output neuron value was above or below 0.5. Then a set of the single e~ events were classified in a similar manner. The performance of the classifier was measured by recording the percentage of events that were classified correctly. The results are summarized in Table 2.4. Histograms of the F N N output are shown in Figure 2.4. Chapter 2. Mean Opening Angle Classification 22 3 5 0 3 0 0 2 5 0 2 0 0 1 SO 100 5 0 0 0 0.1 0 .2 0 .3 0.4 0 .5 0 .6 0.7 0 .8 0 .9 1 H i s t o g r a m of F N N Output Figure 2.4: Histograms of one of the F N N output neurons. These histograms are for the F N N with all 7 input parameters from . The sum of the output neuron activations was 1.0, so the other output neuron contains no extra information. 2.8 Verification Using PDFs To verify the results of these investigations, a separate test based on the estimation of the probability density functions (PDFs) of the input parameters was done. Estimation of PDFs is described in Section 4.4. This classifier worked by evaluating its input parameters for each event, and comparing the values of the P D F for each class at that point. The event was deemed to be of the class with the higher P D F value. When a P D F comparison was done for both 9 and o-g, the performance matched that of the initial F N N . A P D F comparison using only the 9 parameter was performed and Chapter 2. Mean Opening Angle Classification 23 Results % NC correct % single e correct F N N all parameters 71% 74% F N N 9ae ' 67% 76% P D F doe 69% 75% P D F e 64% 75% Table 2.4: Summary of Results. The similar performance for the two F N N runs shows that the majority of the classification ability is inherent in the 9 and og parameters. Error estimates on these percentages are approximately 0.6% produced results that were only slightly worse than those from the initial F N N . This suggested that 9 is a parameter that provides much of the ability to distinguish between these event types. It was concluded that the F N N was not finding correlations in the other parameters. Histograms for the other parameters appeared, for the most part, to overlap for both classes, so none of those parameters would provide much in the way of classification ability on their own. Chapter 3 Event Fraction Analysis This chapter outlines the method used throughout this thesis to perform event ratio determination. It requires a single parameter that has lower values for one event type, and higher values for the other event type. The method is essentially a x2 fit °f the distribution of the parameter for a mixed set to a sum of the distributions of the parameter for C C and N C events. The one free parameter in the fit is taken to be the C C fraction. The method was adopted from standard statistics by Steve Brice [Brice96]. 3.1 Event Fraction Determination The method used for event fraction determination requires a parameter that can be evaluated for each event. One example of this sort of parameter is the mean opening angle 6 discussed previously in chapter 2. Further parameters will be developed in chapters 4 and 6. The technique implements a fit of the distribution of the parameter for the mixed set to a sum of the two distributions of the same parameter for C C and N C prototype sets. The quantities of concern in this method are listed in table 3.1. The index k = 1 •refers to C C events and k = 2 refers to NC events. The method is implemented under the constraint that a1 + a2 = 1, so that there is only one free parameter in the fit. There 24 Chapter 3. Event Fraction Analysis 25 Quantity Description N The Number of Events in the Mixed Set Ui Bin Contents for the Mixed Set (Unnormalized) Ui Bin Contents for the Mixed Set (Normalized) Nk Number of Prototype Events in Class k i Prototype Histogram for Class k (Unnormalized) ft Prototype Histogram for Class k (Normalized) Ak Number of Events in Prototype Set k ak Fraction of Events in Class k x2 The Goodness of Fit Criterion Table 3.1: Quantities Used in the x2 Fit. k = 1 refers to C C events, k = 2 refers to NC events. is a contribution to the error estimate in each bin from both the prototype sets and the mixed set. If the fit were perfect, the bin contents of the mixed set (Ui) would be exactly equal to the contents of the sum of the two prototype distributions (Y^k^ft)- There will be some expected error in this fit for each bin, related to how many events were accumulated in the mixed set and in each of the prototype sets. These independent errors are added in quadrature, where a2;, is the error estimate for the bin contents of the mixed set, and (AhOjk)2 is the error estimate in the scaled bin contents for the kth. prototype set. For the purposes of this thesis, prototype distributions always contain 2000 events each, and the mixed set is always made up of 950 C C events and 950 N C events. In reality, the prototype sets might be produced by a tuned Monte Carlo code and so may have many more events than this. The main thrust of this thesis is to compare the ability of each parameter in distinguishing C C from NC. This is done by comparing the estimated statistical error for an event fraction determination using each parameter. The parameter that produces the lowest statistical error is taken to be the best parameter. Chapter 3. Event Fraction Analysis 26 The x 2 function can then be written as x 2 = £ where (3.1) — Ui + Yk jfkAkfi = N L-ik \ Nk ak)akfk\ Or, written entirely in terms of normalized histograms [ui-Zk*krff X 2 = ^ E" Ui + Zk(wak)akfi\ 3.2 Event Fraction Error Estimation For the purposes of error estimation the x 2 was simply tabulated with a 0.001 interval from 0.0 to 1.0. The best fit was selected by looking in the resulting list for the smallest X 2 , and error estimates were produced by finding the values of the parameter that cor-responded to a 1.0 increase in the x 2 - This technique was used to arrive at the 0.521 ± 0.047 C C event fraction estimate presented in the previous chapter for 9 event fraction determination on a 50/50 mixture. Chapter 4 Likelihood Ratio Characterization This chapter describes a characterization parameter formed by taking the ratio of two likelihoods. Likelihoods are calculated using various PDFs, and results are given for the event fraction determination ability of three parameters based on vertices returned by the S N O M A N Time Fitter. 4.1 A Type of Classification Parameter A likelihood is a measure of how probable obtaining a given set of data is, assuming that it was caused by physics well represented by a certain model. A likelihood is calculated using a P D F in the following way: Nhita Levent(x,y,z,t) = Y[ Pi{x,y,z,t) (4.1) i=i The function P(x,y,z,t) is a P D F that represents the assumption about the origin of the data. A P D F is a representation of our knowledge of the character of a certain event class. One of the PDFs used in this thesis is the time residuals P D F for C C events. Another is the hit angles P D F for NC events. In this thesis, all PDFs are formed from data sets using the Parsen windowing technique, as described in Section 4.4. One can calculate the likelihood of obtaining a given set of event data, assuming that the 27 Chapter 4. Likelihood Ratio Characterization 28 event is an N C event. In this case, one would take the event information and calculate the likelihood based on a P D F formed with NC events. Thinking of likelihoods in this way suggests a possible classification parameter. For a given event and an associated fit vertex, one could calculate the likelihood of obtaining the data if the event were NC, and the likelihood of obtaining the data if the event was C C . The characterizing parameter is formed by taking the ratio of the two likelihoods produced in this process. This kind of parameter will be referred to as a Likelihood Ratio (LR), and is defined as jr^-4.2 Prototype Generation In order to produce the required PDFs, two event sets were produced with S N O M A N and used as class prototypes. These event files were written to disk in the format that the actual data from SNO will be written to tape. One set was used to create PDFs for the C C class, and the other was used to create the NC class PDFs. The events in these sets were distributed uniformly throughout the D 2 0 . The N C events were built using S N O M A N version 2.08 which takes into account angular correlations between the emitted gamma rays. This thesis does not investigate whether it is possible to create similar PDFs using calibration data. If this is not possible, then to achieve the results presented in this thesis, one would have to use the S N O M A N Monte Carlo to produce such sets, after having tuned the S N O M A N Monte Carlo to reproduce all calibration data with sufficient precision. 4.3 Time Residual and Angle Extraction Three different P D F definitions are used in this thesis. Starting from a vertex, one can calculate the TresidUai for any P M T hit. The time residual for a given P M T hit is defined Chapter 4. Likelihood Ratio Characterization 29 as: (4.2) where Texpected is the time that the P M T would have been hit if light had traveled from an assumed vertex (x,y,z,t) to that P M T . The P M T hit angle can be measured from the average direction to the P M T s from the vertex in question. S N O M A N , with additional user code, was used to extract P M T hit angles and time residuals from the Monte Carlo seed vertices. Density estimates for the Tresiduai P D F , the hit angle P D F and for the joint P D F in TresidUai and hit angle were created for both C C and N C events. 4.4 Probability Density Estimation A histogram is a representation of the distribution of a given parameter. One can observe the salient features rather easily, but one must choose both a bin width, and where to start the binning. When all is said and done, the distribution is 'blocky', and one must be satisfied that choosing a different bin width and starting point will not affect the results of the analysis. Only in the limit of high statistics and small bin widths does the histogram become smooth. A n alternative is to fit a curve to a histogram. This is reasonable when one has some idea what the distribution should look like. If there is reason to believe that the curve is exponential, then fitting an exponential to the histogram will provide a decent estimate. Very often, however, one does not know the form of the distribution, and using this method will impose restrictions on the shape of the data. This method may end up hiding unexpected but relevant features of the shape. Another method for estimating PDFs is 'Parsen windowing' [Silverman]. It has the / Chapter 4. Likelihood Ratio Characterization 30 advantage that it lets the data speak more for itself. The estimate is made by summing 'kernel functions', each one centred on a datapoint in the set. A typical kernel function is the normal distribution. The user of this method is left, however, with the problem of choosing the width of the kernel function, h. This parameter is often called the window width. The P D F estimate then takes the following form: 1 71 r - X nh^ v h ' where n is the number of data points used to estimate the density, x is the point at which the density is being estimated, Xt is the zth data point, and K is the kernel function. 4.5 Choosing a Window Width There is no prescribed method for choosing a window width in the Parsen windowing scheme. It is often best to just produce various density estimates using different window widths, and choose the best one based on previous knowledge of the density. One knows absolutely nothing about the parameter for which one is producing a density estimate then one has little to go on with regard to choosing a window width. This is reasonable because if one truly knows nothing about the distribution then the best estimate might be delta functions at each of the observed points, but this kind of estimate is rarely reasonable when the parameter is in a continuous space. The choice of the window width is mixed up with how many observations one has and how fast the density can fluctuate. For a given set of observations, a larger window width results in a smoother density estimate. One way of getting around this deadlock is to take the window width that would be optimal if the density was a Gaussian distribution with a sample standard deviation equal to that of the observed set and then adjust this value until the estimate fits with some Chapter 4. Likelihood Ratio Characterization 31 previous knowledge of how the density should behave. This is different from assuming that the distribution is Gaussian as one would be doing if one fit a Gaussian distribution to a histogram. Instead, this is choosing the window width for a Parsen estimate. If the density is in fact Gaussian with the measured width, this method will produce a very good estimate of it. If there are data points that suggest some other features to the shape, they will have an effect of the estimate. The following choice of h provides us with a method that would be ideal if the density being estimated was actually Gaussian. hopt = (4/3) V V n " 1 / 5 (4.3) where hopt is the window width that is optimal in the Gaussian limit, n is the number of points in the sample set, and a is the sample standard deviation. One problem with the Parsen windowing technique is that it can be computationally intensive. For the case of a Gaussian kernel, the estimate requires the calculation of an exponential once for every point in the prototype set each time the density estimate is computed. Overall speed can be increased by computing a 'lookup' table for the P D F , and then interpolating the P D F estimate from this table. 4.6 One Dimensional PDFs A time residual P D F was formed for both C C events and NC events using a normal kernel. These PDFs were sampled on a grid from -150.0 ns (i.e. a P M T that was 150 ns early) to 180.0 ns with 3301 samples. These PDFs are shown in Figure 4.1. One can see that the distribution is centred close to 0.0 as expected. The P D F is dominated by the 'in time' P M T hits. Late light, due to scattered and reflected photons, is shown as having a positive time residual. Early P M T hits are due to noise hits. PDFs for the angle of a P M T hit from the mean direction to the P M T hits are shown in Figure 4.2. Chapter 4. Likelihood Ratio Characterization 32 A total of 3141 samples were taken from 0. to TT radians. Time Residual PDFs CC and NC Events 0.0 | . , . , . .8.0 I . 1 . . . 1 . 1 -200.0 -100.0 0.0 100.0 200.0 Time Residual (ns) Figure 4.1: Normalized Parsen window estimate of the C C and N C time residual PDFs. 4.7 Two Dimensional PDFs Estimating a two dimensional P D F using Parsen windowing proceeds in the same fashion as outlined above, expect that the kernel function is now two dimensional. One obvious choice for a kernel function would be a bivariate normal distribution. It is preferable to have a kernel that has the same shape as the data in an overall sense. For instance, consider the effect of a data point on the various P D F sample points. If the data is highly correlated in the two dimensions being investigated, then the effect of a data point on the P D F sample points should not be radially symmetric around the data point. This Chapter 4. Likelihood Ratio Characterization 33 Angle PDFs o.o i . , . , . -1.0 -2.0 -3.0 0.0 1.0 2.0 3.0 4.0 radians Figure 4.2: Normalized Parsen window estimates of the C C and N C Hit Angle PDFs can be taken into account by computing the covariance matrix for the data and using it to first transform points into a coordinate set where the data has unit variance and the basis vectors for the new coordinate set lie along the principle axes of the data. This is achieved by using a P D F estimate of the following form: P = 1 ± K{h~\x - X%)TS-\x - Xz)) nhdsJ(detS) i=i where P is the P D F estimate, x is the P D F sample point vector, Xi is a data point, n is the number of points in the data set, h is the window width, d is the dimension of the space the P D F is in (2 in this case), and K is the kernel function. The sum is over all the points in the prototype set. Two dimensional PDFs, in Tresiduai and hit angle, were constructed for C C and N C Chapter 4. Likelihood Ratio Characterization 34 events using prototype sets sorted in angle and a truncated kernel of the following form: 1 — (x • x)3 for x • x < 1 K{x • x) (4.4) 0 for x • x > 1 where x is the vector from the data point to the P D F sample point. This allowed a faster computation of the P D F estimate by determining which P M T hits could possibly have any effect at all on P D F sample points with each angle sample coordinate. Also, the second derivative properties of this kernel allow the sampling interval to be larger without a significant increase in P D F estimate error. See [Silverman] for details. The sampled P D F estimates are shown in Figures 4.3 and 4.4. Initially, the window widths calculated with equation 4.3 were found to be too high for the angle components, as seen by over smoothed PDFs. Lower ones were put in by hand until the angle distribution along the time peak tended to zero at the limits of 0 and n, as it should. 4.8 Likelihood Ratio Results Two datasets with close to 1000 events each were analysed with the S N O M A N time fitter, resulting in a single vertex for each event. For each set of P M T hits and fit vertex, a C C likelihood and an NC likelihood was calculated with each P D F . These likelihoods were used to calculate the three likelihood ratios (LRs) for Tresiduai only, hit angle, or both Tresiduai and hit angle. Histograms for the likelihood ratios LRtime, L R a n g i e , and LRtime+angle are shown in Figures 4.5, 4.6, and 4.7. Event fraction analysis was done using each of LRtime, LRangle, and L R t i m e a n g i e . Pro-totype histograms for each of these parameters were created with 2000 events each. In Chapter 4. Likelihood Ratio Characterization 35 Parameter Extracted C C Fraction LRtime LRangle LRtimeangle 0.189 ±°0-\f2 0.489 ± 0.042 0.497 ± 0.039 9 • 0.521 ± 0.047 Table 4.1: Results for L R Event Characterization. The L R t i m e parameter clearly cannot be used to form an event characterizing parameter. This was because the histograms for C C and N C events overlap almost exactly for this parameter. These three results are compared with the performance of the 9 parameter. each case, an attempt was made to extract the C C event fraction from a set of data that was known to have 950 C C and 950 NC. Results are summarized in Table 4.1 A method was devised of finding the theoretical limit of how well any given parameter could determine event fractions. When analysing the prototype events, the Monte Carlo vertex was used to calculate the likelihood in each P D F . If any fitter provided vertices that resulted in a better performance than using the Monte Carlo vertices of the events, then it would be because the fitter was systematically doing poorly on one of the event types. This would be a dubious method of creating a characterization parameter and it would be better to understand the systematics of the fitter, which should suggest a better kind of characterization parameter. In this light, the histogram for the L R t i m e + a n g i e parameter calculated using the Monte Carlo vertices is shown in Figure 4.8. Here it can be seen that indeed, we have come close to the limit with the current P D F definitions. Chapter 4. Likelihood Ratio Characterization 36 log_dansity Or ime_index Figure 4.3: C C P D F Estimate in Both Time Residual and Hit Angle. The P D F was generated with a data set with close to 50000 events. The angle index that runs from 1 to 30 corresponds to the angles from 0.0 to t t radians. The time residual index from 1 to 330 corresponds to times from -150.0 to 180.0 ns. Figure 4.4: N C P D F Estimate in Both Time Residual and Hit Angle. The P D F was generated with a data set with close to 50000 events. The axis values are identical to those in Figure 4.3 Chapter 4. Likelihood Ratio Characterization 37 1.04 T i m e F i t t e r V e r t e x T i m e R e s i d u a ! P D F L i k e l i h o o d R a t i o H i s t o g r a m Figure 4.5: Histograms for the likelihood ratio formed using the Tresidual PDFs (LRtime ) Chapter 4. Likelihood Ratio Characterization 38 Figure 4.6: Histograms for the likelihood ratio using the hit angle PDFs (LRangie ) Chapter 4. Likelihood Ratio Characterization 39 Figure 4.7: Histograms for the likelihood ratio using the joint Tresidual and hit angle PDFs Chapter 4. Likelihood Ratio Characterization 40 Figure 4.8: Likelihood ratio produced using the Monte Carlo vertices and the Tresiduai , hit angle PDFs. Of course, when fitting events, the actual event vertex is not available. This histogram shows the distributions that would result if one could use perfect fitters. Chapter 5 Maximum Likelihood Fitters One possible problem with the results presented in Section 4.8 is that the assumptions built into the S N O M A N time fitter might be minimizing the differences between C C and N C events. The S N O M A N time fitter assumes at the outset that there is a single source of Cerenkov light; it is possible that the returned fit vertices for NC events are biased in such a way as to mask their inherent nature as NC events. This possibility motivates the idea of creating a fitter that is designed to fit N C events. It is possible to create such a fitter, and a fitter tailored to C C events using the PDFs already developed in the previous chapter. Maximum likelihood fitting involves searching for a vertex with the highest likelihood as calculated with a given PDF . These fitters were created, and are described in this chapter. 5.1 Building a Maximum Likelihood Fitter The purpose of an event fitter is to extract information from an event about the physical interaction that caused that set of P M T hits. Any given fitter makes assumptions about the nature of the event. Most SNO fitters constructed to date assume that there is one vertex for the creation of Cerenkov light. This restriction will be dropped here. In this thesis, all information extracted from an event is based on its vertex. The hit angles 41 Chapter 5. Maximum Likelihood Fitters 42 are computed by first finding the average direction to the P M T hits from the returned vertex. The hit angles are subsequently measured from this average direction. A n event is assumed to have a clearly defined vertex. For C C events, it is the position and time of interaction of a neutrino with a deuteron. For NC events, it is the position and time of a neutron capture on a 3 5 C1 nucleus. This NC vertex will be separated from the Cerenkov generation points by the distance that the emitted gamma rays travel before transferring its energy into electrons through Compton scattering. Consider the case where a fitter is trying to extract the most likely vertex for a given event. Given an event from SNO, different vertices will have a different likelihood of having occurred for each of the various hypotheses about the nature of the event. If one assumes that the event is C C then one might expect the hit pattern to have certain features. One expects light from C C events to be less isotropic than light from N C events. A method characterizing these features numerically is with a P D F of parameters that are calculated from the set of P M T hits. One of the important features of the Maximum Likelihood method is that it does not throw away any of the data. The time fitter built into S N O M A N tries to limit itself to P M T hits that are associated with direct hits, as opposed to scatter or noise. This results in 'pull ' , an effect which results in the fit time being dragged late in order to accommodate scattered light that cannot be tagged as such. When producing PDFs for any parameters, all of the physics of the situation is included. One is not required to throw away any information contained in noise tubes or hits due to scattered photons. 5.2 Likelihood Maximization A likelihood, even when maximized, can be a very small number because it is a product of many numbers less than one. Instead of trying to find a vertex that maximizes a Chapter 5. Maximum Likelihood Fitters 43 likelihood, it is computationally better to find a vertex that minimizes the negative sum of density logarithms. This is what has been done for the fitters described here. Minimization of a function can be difficult, depending on the 'landscape' of the function to be minimized. It can be difficult to find the deepest minimum if there are many different minima. The more valleys there are the more searching and computing one must do to find them. Two kinds of minimization were used in this thesis. One method of finding the minimum of a function is to use gradient descent. This is a very common technique, and a description and implementation is described in Section 10.5 of [Press]. This technique was used whenever it was believed that a vertex was known that was reasonably close to the global minimum. The other technique is called simulated annealing, and is used whenever there is reason to believe that there are local minima between the initial vertex and the global minimum. 5.3 Simulated Annealing The name for simulated annealing derives from an analogy with solidification. If a molten metal is cooled rapidly it will often solidify into a phase that is not the lowest energy state for the final temperature. If it is cooled slowly, however, it stands a far better chance of finding the lowest energy phase, because thermal motion over longer periods of time allows the system to explore many configurations. The analog to temperature while searching for a global minimum of a function is the magnitude of the typical steps that a probing point takes, and the degree to which the algorithm lets the current accepted point jump out of local minima. Annealing occurs in the following way. First, a starting point for the minimization is chosen, and labeled as the current point. Next, a new point is picked by adding a random Chapter 5. Maximum Likelihood Fitters 44 vector to the point, with the components of this vector picked from a distribution. Next, the function is evaluated at the new point, and compared with the function value at the old point. If the new point has a lower function value, it is automatically adopted as the 'current' point for the next step. If the new point has a higher function value, then a random number is drawn. That random number determines whether the new point will be adopted as the current point. This gives the annealer the ability to jump out of a local minimum. The theory and implementation of various kinds of simulated annealing is described in [Ingber]. Any simulated annealing method involves prescribing three functions: the distribution from which new points are selected, the probability that a new point will be accepted (based on the value of the function being optimized at both the new and old location), and a function that describes the lowering of the temperature with each 'time' step. 5.4 Very Fast Simulated Annealing A variant of simulated annealing called Very Fast Simulated Annealing (VFSA) is used in this thesis. This algorithm was developed to address some problems with simpler versions of simulated annealing. V F S A allows each model parameter (x,y,z,t) to be-have independently, in that there is a separate temperature and a separate temperature schedule for each variable. There are a number of free parameters to be set when using V F S A . It is through these free parameters that the algorithm can be tuned to fit a specific problem. For the fitters discussed in this thesis, the models are event vertices requiring four parameters (x,y,z,t). For each model description parameter, there is an initial temperature, and a cooling schedule. There is also an initial acceptance temperature, and corresponding Chapter 5. Maximum Likelihood Fitters 45 cooling schedule. One must also choose the number of temperatures, and the number of probes to attempt per temperature. For V F S A in four dimensions, there are 12 free parameters; there are four initial temperatures and four temperature schedule parameters (a pair for each of x,y,z and t), an initial acceptance temperature and corresponding temperature schedule, and finally a number of new points per temperature, and a number of temperatures. For each free parameter that was specific to a model parameter, x, y and z were always given the same value, and t was given a different one, reducing the overall number of free parameters to 8. This is motivated by previous knowledge that since the events were distributed uniformly throughout the D 2 0 , and directions were chosen isotropically, there is no distinction between the spatial directions. Temperatures are calculated, one after the other, using i T = T0e~ah* where a is the cooling schedule parameter and T 0 is the specific initial temperature and k is the 'time step'. A pseudo code implementation of simulated annealing is shown in Appendix C. The new model is selected using the following set of equations. Ui ~ [0,1] (5.1) V i = sgn(Ul - 1)^1(1 + I ) « M ^ - D _ 1 } ( 5 2 ) ynew = vold + y^vrnax _ ^min) (53) vmin < ynew < ^max where Ui is drawn at random, Ti is the current temperature for function variable i (i=l corresponding to x etc.), vfd is the current vertex, w" e w is the vertex about to be probed vertex, u™m and are limits set by the user of the code. The perturbing distribution is a function of the annealing temperature, such that as the temperature lowers, the new point will be closer and closer to the current point. Chapter 5. Maximum Likelihood Fitters 46 Parameter Initial Tempurature Cooling Schedule (x,y,z) 0.01 2.0 (t) 0.01 1.6378 Probe Acceptance 100.0 1.0 Table 5.1: V F S A parameter values. 'Probe acceptance' refers to the temperature asso-ciated with whether or not a newly probed vertex will be accepted if the new function value is worse than the one for the current vertex. 5.5 Selection of VFSA Parameters The following strategy was used to set the annealing parameters. At first, the probing vertex should sample places all over the detector. Gradually, the probing should be restricted to a finer and finer area, stopping when the resolution limit of the detector has been reached. In general a large number of probes are required to sufficiently sample the likelihood space. For the results presented here, the temperature was lowered 1000 times, and for each temperature, the likelihood space was sampled 30 times. The annealing parameters were chosen using some knowledge of the detector, some knowledge from experience with the likelihoods, and some experience with the perfor-mance of the annealing as a function of the number of anneal probes per temperature. Values are shown in Table 5.1. During annealing different initial vertices did produce different results. This is seen as a failure in the choice of annealing parameters. Consider starting from the centre of the detector. When the initial vertex time was chosen to be 25.0 ns before the first P M T hit, as is done in the S N O M A N Time Fitter, then when fitting to a Tresidual P D F the final vertex time was more than 20 ns too early about half the time. A more intelligent choice of initial vertex time is choosing the time that makes the average time residual to each of the P M T hits zero. This time will often be within 20.0 ns of the actual event Chapter 5. Maximum Likelihood Fitters 47 time. When the vertex was annealed with this choice of initial vertex, annealing followed by gradient descent proved a good choice for minimization, and improved fit residuals in the tails. Annealing did improve the number of fits that went very badly. Following SNO lore, a fitter is said to 'Fit the Event in Cleveland' when the fit residual is greater than 104 cm. For the smart initial vertex, annealing, and gradient descent combination there were about half as many events in Cleveland, as is seen on the log-log plot in Figure 5.1. t-o • • • • 'Co * " 10 3 L-V <iJio2 f • • • • • • • • • * • '• • I-• a cr io t o •z. ^ 1 Is;-***-i . I M M . I 1 10 102 io 3 io 4 105 C.r*A Figure 5.1: Maximum Likelihood fits fewer events 'In Cleveland'. Chapter 5. Maximum Likelihood Fitters 48 5.6 Using a Previous Fit as an Initial Vertex The behaviour of the likelihoods near the global minimum is of great interest. A way of fast tracking to the interesting part of the function is to use a vertex from a previous fit as the seed for a new minimization. Very often, this tends to put the vertex in the global minimum for a new fitter. This method suffers from the problem that the next minimization is biased by the previous fit, but in the face of a very rough likelihood, this was seen as a necessary evil in order to be able to analyse the behaviour of the new likelihoods. This tended to leave in the poor performance of the S N O M A N Time Fitter with regard to fitting in Cleveland, but gains were made is the speed of the algorithm, because annealing was no longer required. The two dimensional PDFs produced likelihoods that had very many local minima. This is deduced because whenever annealing was used with any initial vertex other than one provided by a previous fit, the subsequent gradient descent minimizer did not move the vertex by more than 1.0 cm or 0.1 ns. This difficulty was bypassed by feeding the result of the Tresiduai fit to the Tresidual 9 fit. 5.7 Fitter Performance It is impossible to tell whether any given minimization search is finding the global min-imum, for indeed if one knew where the global minimum was one wouldn't need to do the search in the first place. In this case however, one does know where the Monte Carlo vertex is, and it is possible to check whether the fitter is finding a likelihood that is better than the likelihood for the Monte Carlo vertex. If this showed that the likelihood for the Monte Carlo vertex was better than the one found, then one would certainly know that the minimization process was not working. Histograms of the difference between Chapter 5. Maximum Likelihood Fitters 49 the negative log likelihoods for the Monte Carlo vertex and the fit vertex for the Tresidual P D F and the TreSiduai and hit angle P D F are shown in Figure 5.2. This shows that the Tresidual only fit is finding a vertex with a better fit almost all of the time, whereas the Tresidual and hit angle P D F likelihood search is finding a higher likelihood than the Monte Carlo vertex around 85 % of the time. Figure 5.2: These histograms are a measure of the ability of the fitters to find the global minimum. These histograms show the difference between the negative log likelihood at the Monte Carlo vertex and at the returned fit vertex, for each of the Time P D F fitter and the 2D P D F . These histograms were formed using 1000 C C events. After analysing a set of data, one is able to compare the vertices returned by the fitter with the Monte Carlo vertices that produces the hit patterns. This provides a measure of the ability of the fitter to reproduce the event vertex. Two event sets, one of 1000 C C events and another with 1000 NC events were run through each fitter. Figures 5.3 and 5.5 show the distance residuals for C C and N C events respectively. These Figures show that there is little difference in the various fitters ability to find the ?00 - 2 HI Likelihood Depths Chapter 5. Maximum Likelihood Fitters 50 event vertex location. A cut is made so that the distance residual is less than 100.0 cm. Figures 5.4 and 5.6 show the time residuals for C C events and N C events respectively. Note that the fit time is centred around the Monte Carlo time for the Tresiduai and angle fits, but is late for the S N O M A N time fit. Chapter 5. Maximum Likelihood Fitters 51 9 0 \ -0 2 0 -10 6 0 6 0 1 0 0 D i s tance R e s i d u a l s ( c m ) Figure 5.3: C C Distance Residuals 140 \ -C C T ime Res iduo l H i s t o g r a m s (ns ) Figure 5.4: C C Time Residuals Chapter 5. Maximum Likelihood Fitters 52 8 0 F Dis tance R e s i d u a l s ( c m ) Figure 5.5: NC Distance Residuals NC T ime Res iduo l H i s t o g r a m s (ns ) Figure 5.6: NC Time Residuals Chapter 6 Maximum Likelihood Ratio Characterization This chapter deals with event fraction determination, and the creation of event char-acterizing parameters. Given the fitters described in the previous chapter, the M L R characterization parameter is investigated. It is calculated by taking the ratio of the likelihood returned by the N C fitter and the likelihood returned by the C C fitter. 6.1 M L R Results Event fits returned by the T r e s i a - u a i fit and the T r e s i c i u a i and angle fit were used to calcu-late likelihoods and form ratios of the maximum likelihoods (MLRs) . The parameters produced were used as fraction determination parameters. In each case, prototype sets with 2000 events each were used. The distribution of a mixture set, with 950 C C and 950 N C events was then fit against a normalized sum of the two prototype distributions. Results are shown in Table 6.1. For the case of fitting to the Tresiduai PDFs, the likelihoods used were from the angle only PDFs. A histogram of the characterization parameter is shown in Figure 6.1. For the case of using the two dimensional PDFs for the fit, the likelihoods were calculated 53 Chapter 6. Maximum Likelihood Ratio Characterization 54 Fit Type Likelihood P D F Extracted C C Fraction 2D P D F 2D 0.493 ± 0.034 Tresiduai P D F hit angle 0.494 ± 0.037 Fit Type Parameter Extracted C C Fraction S N O M A N Time TRtime 0.189 S N O M A N Time TRangle 0.489 ± 0.042 S N O M A N Time LRtimeangle 0.497 ± 0.039 Tresiduai P D F 9 0.521 ± 0.047 Table 6.1: M L R Event Fraction Determination Results. For fits performed using PDFs, each event was fit as a C C event and then as a NC event. The M L R was produced by taking the ratio of the returned negative log likelihoods for each event type. A l l previous results are included for comparison. from the two dimensional PDF . A histogram of this characterization parameter is shown in Figure 6.2. 6.2 Comparison and Discussion In view of the fact that the 2D PDFs have provided only a factor of two reduction in event fraction error estimate compared with the 9 event fraction determination, it is concluded that much of the ability to characterize NC and C C events in the manner described here is contained in P M T hit angle information. 6.3 Possible PDF Improvements It is clear from the results above that much of the ability of these classification parameters lies in the hit angle information. This highlights the importance of the angle PDFs, and any improvements in the definitions of the angle PDFs may provide direct improvement in the ability of these parameters. More sophisticated definitions of the angle information may be fruitful. This might involve applying Hough transforms [Radcliff] to look for Chapter 6. Maximum Likelihood Ratio Characterization 55 70 5 0 3 0 NC Events 0 .9 1 1.1 1.2 MLR fo r T .Res P D F fit a n d Ang le P D F l ike l ihoods Figure 6.1: Histograms of Ratios of the Maximum Likelihoods for C C and NC events. These ratios were calculated using the vertex returned by a maximum likelihood fit using the Tresi(iuai P D F . The likelihoods were calculated using the hit angle P D F . more robust estimates of the direction of the event. It might also involve having separate P D F definitions for each event class. For instance, one could use the initial Monte Carlo direction as a direction definition for single e~ events, but use some other definition for direction for the N C events. This would mean fitting for both the event vertex and direction, as in [Klein] because alternate definitions for direction might not be directly calculable as they are for the mean P M T hit direction definition. Moving to different P D F definitions for each class opens up a variety of new fitters for development. Also, restricting the analysis to C C and N C event types, each pair of definitions would have to be checked against each other for event fraction determination Chapter 6. Maximum Likelihood Ratio Characterization 56 NC Even ts 0.99 1 1.01 1.02 MLR fo r 2D P D F F i ts a n d L ike l ihoods 1.04 Figure 6.2: Histograms of Ratios of the Maximum Likelihoods for C C and N C events. These ratios were calculated using likelihoods returned by the maximum likelihood anal-ysis fitters. Both the fits and the likelihoods used the two dimensional Tresiduai , hit angle PDFs. ability. Chapter 7 Conclusion 7.1 Conclusion For event sets containing only C C and NC events, it has been shown that, compared with using only angle information, there is some additional ability to distinguish between C C and N C events in a joint P D F for Tresiduai and P M T hit angle. The sets used were assumed to contain only C C and NC events. This means that if the analysis presented here were to be used, data would first have to have ES and other background events removed. Out of all of the parameters investigated in this thesis, the best performance was found for the case where events were fit first with the N C and then with the C C fitter, and the resulting likelihoods, calculated with the two dimensional Tresiduai , hit angle PDFs, were used to form a ratio (MLR). This parameter distinguished between C C and N C events such that for prototype sets with 2000 events each, a mixed set with 950 N C and 950 C C events was estimated to have a C C fraction of 0.493 ± 0.034. 57 Chapter 7. Conclusion 58 7.2 Future Work A next step in the work presented in this thesis, would be to include more event classes in the analysis. This would allow the analysis of datasets comprised of more than two event types, including such things as ES events, and backgrounds of various kinds. Extensions of this work should include some estimates of the systematic errors in the event fraction estimates. This would be done by analysing several separate Monte Carlo data sets, and looking at the distribution of results with respect to the exact result. This work stands to be improved most by investigating improvements to the P D F definitions. This could be done by first looking for a P D F definition that provided as much differentiation in hit angle alone, and then forming the two dimensional PDFs after settling on a definition for the angle PDFs. This is recommended on the basis the vast majority of the discrimination information is in the angle information. One possibility is to use the Hough transform to look for a direction that maximizes the number of P M T hits in a Cerenkov cone, as in [Radcliff]. Given that fitters are naturally produced as a side effect of the creation of these PDFs, it is worthwhile spending some more effort on finding the global minimum of the likelihoods. In particular, it is preferable that these fitters not rely on being fed an initial vertex from another fitter. One possibility is to use a smart initial vertex, and then use simulated annealing over a smaller spatial and temporal neighbourhood, or possibly a grid search as in [Moorehead], followed by gradient descent. Bibliography [Mikheyev] S.P. Mikeyev and A. Yu. Smirnov, Nuovo Cimento C9, 17 (1986); Yad. Fiz. 42, 1441 (1985) [Sov. J. Nucl. Phys. 42, 913 (1985)]. [Wolfenstein] L . Wolfenstein, Phys. Rev. D17, 2369 (1978) and Phys. Rev. D20, 2634 (1979). [Wildenhain] P. Wildenhain, Ph.D. thesis, University of Pennsylvania, 1995. [Takeuchi] Y . Takeuchi, Ph.D. thesis, Tokyo Institute of Technology, 1995. [Gavrin] V . N . Gavrin, talk given at T A U P 95 (Conference on Theoretical and phe-nomenological Aspects of Underground Physics), Sep. 19-23, 1995, Toledo, Spain. [Anselmann] G A L L E X Collaboration, P. Anselmann et a l , Phys. Lett. B 357, 237 (1995). [Silverman] B .W. Silverman Density Estimation for Statistics and Data Analysis. Chapman and Hall. 1986. [Klein] J.R. Klein Maximum Likelihood Fitting to Both Time and Angle. SNO-STR-94-055 (Restricted SNO Internal Document) [Bahcall95] J .N. Bahcall, M . H . Pinsonneault. Rev. Mod. Phys. Oct 95. or J. N . Bahcall and M . H. Pinsonneault, Rev. Mod. Phys. 64, 885 (1992). [Bahcall89] J .N. Bahcall Neutrino Astrophysics. Cambridge University Press. 1989. [Press] W . H . Press, S.A. Teukolsky, W.T. Vetterling, B.P. Flannery. Numerical Recipes in Fortran: The Art of Scientific Computing Second Edition. Cam-bridge University Press. 1992. [Brice95] S. Brice. An Overview of the Feedforward Neural Network Technique and its Application to SNO Event Classification SNO-STR-95-037 (SNO internal document) [Brice96] S. Brice. The Results of a Neural Network Statistical Event Class Analysis. SNO-STR-96-001 (SNO internal document) 59 Bibliography 60 [SNNS] Stuttgart Neural Network Simulator. See http://vasarely.informatik.uni-stuttgart.de/snns/snns.html or search the W W W for SNNS. [Masters] Masters, T. Practical Neural Network Recipes in C+ + Academic Press, 1993. [SNO] SNO Collaboration, G.T. Ewan et. al, Sudbury Neutrino Observatory Pro-posal. Report No. SNO-87-12, 1987 (unpublished) [Ingber] Ingber, L. Simulated Annealing: Practice versus theory Mathl. Comput. Modelling, V 18, N 11, 1993, p 29 - 57. [Radcliff] Radcliff, T .J . Three methods for event-type identification in the SNO De-tector SNO-STR-95-002 (SNO Internal Document) [Moorehead] Moorehead, M . E . Grid Fitter for Reducing Tails in Spatial Distributions SNO-STR-95-042 (SNO Internal Document) [Bartholomew] Bartholomew, Groshev, et al. Nuclear Data A3, 1967. Appendix A Glossary C C Charged Current event (CC). In this thesis, this refers to events as simulated by S N O M A N . These are single electron with an energy that has been selected from the expected charged current spectrum. F N N A Feedforward Neural Network (FNN) is an algorithm loosely based on ideas about how neurons in the human brain compute. M L A Maximum Likelihood Analysis (MLA) is a technique for finding the most probable value of a parameter associated with a given model for the source of a certain set of data. N C Neutral Current event (NC). In SNO terms, this is a neutrino event that dissociates the deutron in heavy water, into a proton and a neutron. This thesis concentrates on the scenario where the neutron ejected in this reaction is detected by neu tron capture on 3 5 C1. P D F Probability Density Function. P M T Photomultiplier Tube. An evacuated tube about the size of a human head, that is designed to detect single photons. 61 Appendix A. Glossary 62 S N O M A N The Sudbury Neutrino Observatory's Monte carlo and data ANalysis package. SNP Solar Neutrino Problem SSM Standard Solar Model V F S A Very Fast Simulated Annealing. An algorithm for finding the global minimum of a function. Appendix B NC Gamma Rays This diagram [Bartholomew] shows the gamma rays that result from neutron capture on 3 5 CI. This reaction results in a 3 6 C1 nucleus in an excited state. This nucleus reaches the ground state via the emission of gamma rays. There are many energy levels that the nucleus might go through in this transition, however, and the number of gamma rays that result from any given neutron capture depend on the route that the 3 6 C1 nucleus takes to the ground state. 63 Appendix B. NC Gamma Rays 64 S 5 R S s R 5 r s § 2 l S § M = I S i I S i S M 5 1 •.M2010 I.S7MM 0 J • • 711 2 1* 2 2-la " Figure B . l : Gammas from neutron capture on 3 5 C1 Appendix C Simulated Annealing Algorithm Outline The following is an algorithm for the Simulated Annealing. This becomes Very Fast Simulated Annealing (VFSA) when the distributions used to select the new model, and make acceptance decisions are those shown in Section 5.4. loop for each temperature(T) loop over number of random moves/temperature select new model move to new model, or to model limits if new model has lower function value, adopt it else draw random number and decide whether to adopt it end random moves loop end temperature loop 65 


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