SEARCH FOR RARE MULTI-PION DECAYS OF THE TAU LEPTON USING THE BABAR DETECTOR DISSERTATION Presented in Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy in the Graduate School of The Ohio State University By Ruben Ter-Antonyan, M.S. * * * * * The Ohio State University 2006 Dissertation Committee: Approved by Richard D. Kass, Adviser Klaus Honscheid Adviser Michael A. Lisa Physics Graduate Program Junko Shigemitsu Ralph von Frese ABSTRACT A search for the decay of the τ lepton to rare multi-pion final states is performed + using the BABAR detector at the PEP-II asymmetric-energy e e− collider. The anal- 1 ysis uses 232 fb− of data at center-of-mass energies on or near the Υ(4S) resonance. + 0 In the search for the τ − 3π−2π 2π ν decay, we observe 10 events with an ex- ! τ +2:0 pected background of 6:5 1:4 events. In the absence of a signal, we calculate the − + 0 6 decay branching ratio upper limit (τ − 3π−2π 2π ν ) < 3:4 10− at the 90 % B ! τ × confidence level. This is more than a factor of 30 improvement over the previously established limit. In addition, we search for the exclusive decay mode τ − 2!π−ν ! τ + 0 +1:0 with the further decay of ! π−π π . We observe 1 event, expecting 0.4 0:4 back- ! − 7 ground events, and calculate the upper limit (τ − 2!π−ν ) < 5:4 10− at the B ! τ × 90 % confidence level. This is the first upper limit for this mode. + 0 In the search for the τ − 4π−3π (π )ν decay, we observe 24 events with an ! τ expected background of 21:6 1:3 events. In the absence of a signal, we calculate the + 0 7 upper limit (τ − 4π−3π (π )ν ) < 3:0 10− at the 90 % confidence level. This B ! τ × is an improvement by nearly an order of magnitude over the previously established limit. In addition, we set the first upper limits for the exclusive seven-pion decays + 7 + 0 7 (τ − 4π−3π ν ) < 4:3 10− and (τ − 4π−3π π ν ) < 2:5 10− . B ! τ × B ! τ × ii + 0 We also study the τ − 3π−2π π ν mode and measure its branching ratio ! τ + 0 4 (τ − 3π−2π π ν ) = (1:79 0:04 0:14) 10− , which is consistent with and B ! τ × more precise than the measurements from other experiments. iii Dedicated to my lovely wife Katya, our newborn daughter Natalie, and my parents Susanna and Samvel. iv ACKNOWLEDGMENTS I wish to express my gratitude to a group of people without whom this work would have never been possible. Foremost, I thank my family: my wife Katya, who was always there for me and shared my joy and frustration; my newborn daughter Natalie, who, even as an unborn child, motivated me to work hard and defend in time, right before her birthday; my parents and my brother, who were always incredibly encouraging and supportive. I wish to express my deepest appreciation to my advisor, wise and all-knowing Professor Richard Kass, whose help and guidance in my research were invaluable. He passed me his knowledge and experience for which I am very grateful. I have learned a lot from him in and outside the physics world. I could not have done this work without his support. I would like to thank my collaborators at SLAC, Dr. Carsten Hast and Dr. Thomas Allmendinger for their colossal help and support in this research project. They have had a tremendous impact on this analysis. I am grateful to Professor K.K. Gan, the τ guru, for his thoughtful advice on τ physics. He also supervised my hardware project for the ATLAS experiment at the LHC. Not only did he teach me everything about the pixel detector, he also taught me to appreciate fine French and Asian cuisine! v My gratitude also goes to Professors Klaus Honscheid and Harris Kagan for fruitful discussions and comments on my research. Klaus was also a member of my dissertation committee, who I also thank for their advice and comments on my dissertation. I thank the Tau Analysis Working Group at SLAC for helpful discussions, reviews and comments on my analysis. I would also like to acknowledge all graduate students and postdocs who I worked with at the Ohio State University. Among them, my officemate Dirk Hufnagel - a computer wiz in demand by all the high energy physics folks at the OSU, James Morris who replaced me on the BABAR's lettered release administrator job, Shane Smith - an engineer also in high demand at the OSU, Amir Rahimi, Kregg Arms, and the postdocs at SLAC: Paul (Jack) Jackson, Gabrielle Benelli and Teela Pulliam. Last, but not the least, I thank my friends and close relatives for their support. Everybody who showed interest and belief in this analysis, thank you! vi VITA April 20, 1977 . .Born - Yerevan, Armenia June 1998 . B.S. Physics, Moscow Institute of Physics and Technology, Moscow, Rus- sia September 1999 - Present . Graduate Student Associate, The Ohio State University, Columbus, OH; Particle Physics Experiments: ATLAS (2002-2003), BABAR (2003-Present) March 2003 . M.S. Physics, The Ohio State Univer- sity, Columbus, OH PUBLICATIONS + 0 B. Aubert et al. (BABAR Collaboration), \Search for the Decay τ − 4π−3π (π )ν ", ! τ Phys. Rev. D 72 012003 (2005). K. Arms et al. \ATLAS Pixel Opto-Electronics", Nucl. Instrum. Meth. A 554 458 (2005). + 0 R. Ter-Antonyan et. al., "Search for τ − 4π−3π (π )ν Decays", Nuclear Physics ! τ B (Proc. Suppl.) 144 21-26, (2004) . FIELDS OF STUDY Major Field: Physics Studies in Experimental Particle Physics: Professor Richard D. Kass vii TABLE OF CONTENTS Page Abstract . ii Dedication . iv Acknowledgments . v Vita . vii List of Tables . xi List of Figures . xv Chapters: 1. Introduction . 1 1.1 The Standard Model . 2 1.1.1 The Fundamental Particles . 2 1.1.2 The Fundamental Forces . 4 1.2 The τ Lepton . 9 1.2.1 Discovery of the τ Lepton and its Properties . 10 1.2.2 Multi-pion Decays of the τ Lepton . 12 2. The BABAR Experiment . 20 2.1 The PEP-II Collider . 21 2.2 The BABAR Detector . 23 2.2.1 Silicon Vertex Tracker . 27 2.2.2 Drift Chamber . 28 2.2.3 Detector of Internally Reflected Cherenkov Light . 31 2.2.4 Electromagnetic Calorimeter . 33 viii 2.2.5 Instrumented Flux Return . 34 2.2.6 Trigger . 35 3. Event Selection . 37 3.1 Signal and Background . 37 3.2 Data and Monte Carlo Samples . 38 3.3 Pre-Selection . 39 3.3.1 Tau1N Skim . 39 3.3.2 Photon Conversion Rejection . 41 3.3.3 Looper Rejection . 42 3.3.4 Charged Track Candidates . 42 3.3.5 Photon Candidates . 43 3.3.6 Event Thrust Magnitude . 45 3.3.7 Pre-selection Efficiency . 46 + 0 3.4 Additional Pre-selection Criteria for the τ − 3π−2π 2π ν Analysis 48 ! τ 3.4.1 Reconstruction of π0 . 48 3.4.2 Visible Energy . 49 3.4.3 Residual Neutral Energy on the Signal Side . 51 3.4.4 Efficiency of Additional Pre-Selection . 51 3.5 Invariant and Pseudo Mass of the τ . 52 3.6 Background Suppression . 54 3.6.1 Particle ID on the Signal Side . 55 3.6.2 Photon Conversion Veto on the Signal Side . 55 3.6.3 Tagging the 1-prong Side . 57 3.6.4 Background Suppression Summary . 59 4. Data and Monte Carlo Comparison . 62 5. Background Estimate in the Signal Region . 67 5.1 τ Background Estimate . 69 5.2 qq Background Estimate . 70 + 0 5.2.1 τ − 4π−3π (π )ντ . 71 ! + 0 5.2.2 τ − 3π−2π 2π ν . 75 ! τ 5.2.3 Summary of the background estimate . 76 6. Systematics Uncertainties and Monte Carlo Corrections . 79 6.1 Signal Efficiency Uncertainty . 79 6.1.1 Track Reconstruction Systematic Uncertainty . 79 6.1.2 Neutrals Systematic Uncertainty . 80 ix 6.1.3 Particle Identification Uncertainty . 81 6.1.4 Limited MC statistics . 82 6.1.5 Selection Stability . 82 6.1.6 Summary . 82 6.2 Background Estimate Uncertainty . 83 6.2.1 τ Background . 83 6.2.2 qq Background . 85 6.2.3 Summary . 87 6.3 Uncertainty on the Number of τ Pairs . 87 7. Results . 88 + 0 7.1 τ − 4π−3π (π )ντ . 88 ! + 0 7.2 τ − 3π−2π 2π ν . 90 ! τ 7.3 Summary . 93 8. Exclusive Decay Channels . 94 + 8.1 τ − 4π−3π ντ . 94 ! + 0 8.2 τ − 4π−3π π ν . 95 ! τ 8.3 τ − 2!π−ν . 97 ! τ 8.4 Summary . 103 + 0 9. Measurement of the τ − 3π−2π π ν Decay Branching Ratio . 105 ! τ 10. Summary and Conclusion . 111 Appendices: A. Glossary of Terms . 113 B. Pseudo Mass . 117 Bibliography . 119 x LIST OF TABLES Table Page 1.1 Three generations of leptons and quarks and their properties [6]. 3 1.2 Fundamental forces and associated mediators. 4 1.3 Phase space integral for τ decays to n charged pions and a neutrino [13]. 16 1.4 Isospin coefficients for seven-pion states. 18 3.1 Generic τ Monte Carlo Branching Ratios implemented in the TAUOLA. 39 3.2 Data and Monte Carlo samples used in this analysis. 40 3.3 Efficiency (%) of the Monte Carlo samples after each pre-selection cut. The cuts are multiplicative. The track quality cuts include the looper and the conversion rejection criteria.