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DECAYS of the TAU LEPTON* Fraction SLAC—292 DE86 009666 Abstract SLAC -292 Previous measurements of the branching fractions of the tau lepton result in a UC -34D discrepancy between the inclusive branching fraction and the sum of the exclusive (E) branching fractions to final states containing one charged particle. The sum of the exclusive branching fractions is significantly smaller than the inclusive branching DECAYS OF THE TAU LEPTON* fraction. In this analysis, the branching fractions for all the major decay modes are measured simr Uaneously with the sum of the branching fractions constrained Patricia R. Burchat to be one. The branching fractions are measured using an unbiased sample of tau decays, with little background, selected from 207 pA"1 of data accumulated with Stanford Linear Accelerator Center the Mark II detector at the PEP «+e~ storage ring. The sample is selected using Stanford University the decay products of one member of the T+T~ pair produced in e+e- annihilation Stanford, California 94305 to identify the event and then including the opposite member of the pair in the sample. The sample u divided into subgroups according to charged and neutral particle multiplicity, and charged particle ident; 'cation. The branching fractions are simultaneously measured using an unfold .echnique and a maximum likelihood February 1986 fit. The results of this analysis indicate that the discrepancy found in previous experiments is possibly due to two sources. First, the leptonic branching fractions measured in this analysis are about one standard deviation higher than the world average, resulting in a total exr< (2.0 ± 1.8)% over the world average in these Prepared fof the Department of Energy two modes. The mc od leptonic branching fractions correspond to a tau lifetime under contract number DE-AC03-76SF00515 l3 -*" ['\ u x 0.2) x 10~ s which is slightly longer than the average measured tau lifetime of (2.8 ± 0.2) x 10~13a. Secondly, the total branching fraction to one charged hadron plus at least one neutral particle is measured to be (7 ± 3)% higher Printed in the United States of America. Available from the National Techni­ than the branching fraction expected from a combination of previous measurements cal Information Service, U.S. Department of Commerce 5285 Port Royal Road, t and theoretical predictions. It is shown that decay modes involving the n are not Springfield, Virginia 22161. Price: Printed Copy A07, Microfiche A01. expected to contribute more than 3% to this excess. r Ph.D. Dissertation. 0ISTRIBUTI8K Of THIS 3t}CUM£HT IS UUUIMTGI Acknowledgments Table of Contents I wish to thank my advisor, Gary Feldman, for suggesting an interesting and Abstract \\ challenging thesis topic; John Yclton, for providing patient guidance in learning Acknowledgements \\[ analysis techniques; Mark Nelson, for invaluable advise, well documented programs, Table of Contents iv and a very useful thesis; and Ken Hayes, for friendly discussions and advise. List of Tables viii I am indebted to Gail Hanson, HaTtmut Sadrozinski, Dave Hutchinson and Abe List of Figures , x Seidell for the enjoyable experience of working with them and learning from them 1. Introduction 1 during my early days at SI-AC. 1.1 The Tau Lepton 1 With pleasure, I extend a warm thanks to those friends who made my graduate 1.2 Inclusive Branching Fractions 4 student days especially memorable: Barb and Thco Schaad, for their company and 1.3 Exclusive Branching Fractions 4 appetite for something different; all my friends at Campo Rello, especially Mary 1.3.1 Loptonic Decay Modes 6 James and Alain Schwartzman, for sharing a warm and friendly home: Bill Rowe, for many laughs; and Alan Weinstcin, for providing an example of how to combine 1.3.2 Somi-Leptonic Decay Modes 9 hard work and hard play. 1.3.3 Modes with an 17 in the Final State 22 1.3.4 Other Decay Modes 23 I wish to thank my family tn Canada for providing such a warm welcome each 1.4 The Discrepancy 25 time I go home. Finally, I wish to thank Tony Norcia, who has added to the pleasure 1.5 Goal of This Analysis 27 of my life both at home and at work, for his encouragement and support in seeing 3 me through this thesis. 2. The Mark II Detector ° J n . « 30 2.1 Overview . 32 2.2 Vertex Chamber • 36 2.:i Main Drift Chamber • 36 2.4 Timc-of-Flight System * iil iv 2.5 Solenoidal Magnet 36 4.4 The 3 Prong Tag 67 2.6 Lead - Liquid Argon Electromagnetic Calorimeter .... 38 4.4.1 Selection Criteria 67 2.7 Muon Identification System 39 4.4.2 Efficiency 69 2.8 Endcap Calorimeters 43 5. Unfolding the Branching Fractions 71 2.9 Small Angle Calorimeter 43 5.1 Observed Topologies 71 2.10 Event Trigger 43 5.2 The Unfold Technique 74 3. Particle Identification 46 5.3 The Efficiency Matrix 76 3.1 Muon Identification 47 5.4 Monte Carlo Simulation 80 3.1.1 /I/JT Discrimination 48 5.5 Particle Identification Efficiency 82 3.1.2 Muon Rejection 49 5.5.1 Muon Identification 82 3.2 Electron Identification 50 5.5.2 Electron Identification 85 3.2.1 cfir Discrimination 51 5.5.3 Pion Identification 87 3.2.2 Electron Rejection 53 5.6 Angular Distribution 90 3.3 Photon Identification 57 5.7 Momentum Distribution . ' 93 + 3.4 e e~~ Pair-Finding Algorithm 58 6. Backgrounds 96 4. Event Selection 62 6.1 Background from Two Photon Production of Tau Pairs ... 96 4.1 General Philosophy 62 6.2 Background from Two Photon Production of q — q . 98 4.2 Overall Cuts 63 6.3 Backgrounds to the 1 Prong Tag 98 4.3 The 1 Prong Tag 64 6.3.1 Leptonic Backgrounds 98 4.3.1 Selection Criteria 64 6.3.2 Muon Rejection Efficiency 99 4.3.2 Efficiency 66 6.3.3 Electron Rejection Efficiency 100 v vi 6.3.4 Background from Single Photon Production of Lepton Pairs 101 List of Tables 1.1 The known lepton families 2 6.3.5 Background from Two Photon Production of Lepton Pairs 101 1.2 Inclusive branching fractions . 5 6.3.6 Background from Single Photon Production of q - q . 103 1.3 Tau lifetime measurements 7 6.4 Background to the 3 Prong Tag 104 1.4 Published measurements of B{r~ -» v,K~f) 8 6.4.1 Background from Single or Two Photon Production of Lepton Pairs 104 t 1.5 Published measurement!! of B{T~ ~* v \iTV^) 9 6.4.2 Background from Single Photon Production of q - q 105 T 6.5 Summary of Backgrounds 106 1.6 Quantum numbers of the weak charged current .... 10 1.7 Published measureTients of B{T~ -» v -n~) 13 7. Results Ill T 1.8 Published measurements of B[T~ -* u p~) 14 7.1 Measured Branching Fractions Ill r 1.9 Published measurements of c?(r~ —* i/ 7r~7r+jT") .... 15 7.2 Systematic Errors 113 T 1.10 Published measurements of B[T~ —* I/ TT~7T+7T~7[°) ... 17 7.2.1 Particle Identification Efficiency 114 T 1.11 Published measurements of B{T~ —• v K~) 21 7.2.2 Relative Overall Efficiency U6 T 1.12 Published measurements of B{r~ -+ v K*~) 21 7.2.3 Background Estimates 117 T 7.2.4 Summary of Systematic Errors .... n7 1-13 Decay modes involving an n 23 7.3 Results .117 1.14 Branching fractions for all decay modes 26 + + 7.3.1 Limits on Decay Modes Involving the n 121 1.15 Published measurements of ff(c e~ —» r r" (t)) .... 27 7.4 Conclusions 122 2.1 Radii of sense wire layers in vertex chamber 34 Appendix A. Selection of Known Leptons in the Data .... 123 2.2 Hadron absorber thickness in muon system 41 REFERENCES 126 3.1 Extrapolation error for muon system 48 3.2 Longitudinal shower development 52 3.3 Shower width 53 5,1 Criteria for classification of a single charged particle ... 72 vii viii List of Figures 5.2 Topological distribution of sample 74 1.1 Feynman diagrams for the decay of the p. and r leptons ... 3 5.3 Efficiency matrix for the 1 prong tag 78 1.2 Feynman diagrams for r~ —• f ?r~ and 7r~ —» pTV^, ... 12 5.4 Efficiency matrix for the 3 prong tag 79 T + + + 1.3 Feynman diagrams for T~ —» u K~ and K~ —» >~P . ... 20 6.1 Background from e e~ -> c c~ T T" 97 T M 2.1 '''he Mark II detector at PEP 31 6.2 Background from single photon production of lcpton pairs . 102 2.2 The Mark II vertex drift chamber 33 6.3 Background from two photon production of lepton pairs . 102 2.3 Wire array for vertex detector 35 6.4 Backgrounds in 1 prong tag 107 6.5 Backgrounds in 3 prong tag 108 2.4 Wire array for main drift <.hamber 37 6.3 Background subtracted sample for 1 prong tag .... 109 2.5 Ganging scheme in the LA calorimeter modules .... 40 6.7 Background subtracted sample for 3 prong tag . .110 2.6 Cross sectional view of a muon proportional tube module . 42 7.1 Topological distribution for best fit 112 3.1 TEST1 distribution for electrons and pions 54 7.2 Measured branching fractions 113 3.2 LA calorimeter efficiency ......... 56 7.3 Correlation coefficients for 1 prong tag 113 3.3 Energy spectrum of photon candidates in Monte Carlo ... 59 7.4 Correlation coefficients for the 3 prong tag 114 3.4 Parameters used in pair-finding algorithm .....
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