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Study of Exclusive Charmless Semileptonic Decays of the B Meson

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Study of Exclusive Charmless Semileptonic Decays of the B Meson SLAC-R-743 Study of Exclusive Charmless Semileptonic Decays of the B Meson Amanda Jacqueline Weinstein SLAC-Report-743 December 2004 Prepared for the Department of Energy under contract number DE-AC02-76SF00515 This document, and the material and data contained therein, was developed under sponsorship of the United States Government. Neither the United States nor the Department of Energy, nor the Leland Stanford Junior University, nor their employees, nor their respective contractors, subcontractors, or their employees, makes an warranty, express or implied, or assumes any liability of responsibility for accuracy, completeness or useful- ness of any information, apparatus, product or process disclosed, or represents that its use will not infringe privately owned rights. Mention of any product, its manufacturer, or suppliers shall not, nor is it intended to, imply approval, disapproval, or fitness of any particular use. A royalty-free, nonexclusive right to use and dis- seminate same for any purpose whatsoever, is expressly reserved to the United States and the University. STUDY OF EXCLUSIVE CHARMLESS SEMILEPTONIC DECAYS OF THE B MESON a dissertation submitted to the department of physics and the committee on graduate studies of stanford university in partial fulfillment of the requirements for the degree of doctor of philosophy Amanda Jacqueline Weinstein December 2004 c Copyright by Amanda Jacqueline Weinstein 2005 All Rights Reserved ii I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Jonathan Dorfan (Principal Adviser) I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Vera Luth I certify that I have read this dissertation and that, in my opinion, it is fully adequate in scope and quality as a dissertation for the degree of Doctor of Philosophy. Patricia Burchat Approved for the University Committee on Graduate Studies. iii iv Foreword The couplings of the quarks to the weak charged current are represented by the el- ements of the Cabibbo-Kobayashi-Maskawa (CKM) matrix. Precision tests of the relations between these elements provide a cross check of the Standard Model predic- tion for charged current interactions, and a potential probe for physics beyond the Standard Model. Among the smallest and least known of the CKM matrix elements is V , which gives the probability of a weak-current transition from a bottom quark j ubj (b) to an up quark (u). Charmless semileptonic decays of the B meson involve a b u quark transition, ! along with the emission of a lepton and neutrino, and have decay rates directly proportional to V 2. A measurement of V may be derived from the branching j ubj j ubj fraction for any semileptonic b u transition to an exclusively reconstructed final ! state, such as π0=−`+ν or ρ0=−`+ν. In this case, the dominant theoretical uncertainty in V comes from our limited knowledge of the process by which the b and u quarks j ubj are bound into the initial and final state mesons, described by a set of hadronic form factors for each decay mode. In a sample of 83 106 BB events collected by the BABAR detector, we reconstruct × 310 31 B0 π−`+ν decays and 117 16 B+ π0`+ν decays. We combine the ! ! information from both channels, using isospin symmetry and the measured ratio of B lifetimes, to obtain the branching fraction (B0 π−`+ν) = 1:30 0:11(stat:)+0:26(syst:) 0:07(F F ) 10−4: (1) B ! −0:16 π`ν × With the large sample of BB events available, the measurement is currently limited v by systematic uncertainties in the description of background. The data are consistent with both quark model and light-cone sum rule predictions for the B π`ν form-factor. As a consequence, we consider both form-factor models ! in extracting V from the measured (B0 π−`+ν) branching fraction. Based on j ubj B ! the ISGW2 quark model predictions, we obtain V = (2:97+0:12(stat:)+0:28(syst:) 0:08(F F )) 10−3; (2) j ubjISGW 2 −0:13 −0:19 π`ν × and based on light-cone sum rule predictions, we obtain V = (3:07+0:12(stat:)+0:29(syst:) 0:08(F F )+0:47(Γ )) 10−3: (3) j ubjBall01 −0:13 −0:19 π`ν −0:43 thy × These results are consistent with, and of comparable precision to, existing measure- ments made using exclusively reconstructed decays, but have a substantially smaller statistical error. It is believed that both the experimental systematic and theoretical errors may be reduced with further study, and that this approach promises to provide one of the best measurements of V . j ubj vi Acknowledgments There is no way I could possibly acknowledge all those who have contributed to my graduate experience. I wish only to highlight those to whom I owe a particular debt of gratitude. First and foremost, I thank my advisors, Jonathan Dorfan and Vera Luth, for their advice, their patience, and their support throughout my graduate career. Second, I would like to thank those who have worked with me on this analysis over the years, in particular Jochen Dingfelder, Eckhard Elsen, and Mike Kelsey. Mike and Jochen especially have been invaluable sounding boards and sources of moral support (not to mention the occasional lecture on C++ coding technique), and I am deeply grateful for their help. I would like to thank the members of Group C, including my fellow students, for their friendship, their willingness to listen, and a great many useful physics discussions and tips, not to mention their tolerance of the fact that this measurement has occupied a rather appalling amount of their disk space. I would also like to thank Burt Richter for his very welcome support and equally welcome sage advice. I would like to thank the members of my Reading and Examination Committees, Patricia Burchat, Aaron Roodman, and Helmut Wiedemann, for their time. I also owe a debt to the BABAR Collaboration as a whole, and to the PEP-II team, for making this research possible. To Eleanor Mitchell, Mika Stratton, Anna Pacheco, and Maria Frank, all of whom have helped me navigate the treacherous waters of bureaucracy more than once in this past year, thank you. Thank you, also, to all my friends, who saw me through the worst of things with frequent offers of food, friendship, and a wireless connection (not necessarily in that vii order or always in that combination), and who have stuck by me despite my obsessive focus, my frequent lapses in memory, and my completely crazy schedule. And last but not least, to my family, especially my father, Marvin Weinstein, for everything. viii Contents Foreword v Acknowledgments vii 1 Theory of Semileptonic B Decays 1 1.1 Standard Model . 1 1.2 Electroweak Interactions . 2 1.3 Strong Interactions . 3 1.4 CKM Matrix and Current Experimental Constraints . 4 1.4.1 The Wolfenstein Parametrization . 5 1.4.2 CKM Tests and the Unitarity Triangle . 5 1.5 Semileptonic B Decays . 9 1.5.1 Form-factors and Helicity Amplitudes . 10 1.5.2 Form-Factors in Semileptonic Decays . 13 1.5.2.1 Quark Models (ISGW2) . 13 1.5.2.2 Lattice Calculations . 14 2 1.5.2.3 Parametrization of lattice results for f+(q ) . 15 1.5.2.4 Light Cone Sum Rules: LCSR . 16 1.5.3 B π`ν . 19 ! 1.5.4 B ρ`ν . 20 ! 1.5.5 Properties of q2 and Lepton Spectra . 20 1.6 Extracting V from B π`ν . 21 j ubj ! ix 2 The BABAR Detector 23 2.1 The PEP-II Collider . 23 2.2 The BABAR Detector . 24 2.3 The Silicon Vertex Tracker . 26 2.3.1 Layout and Electronics . 26 2.3.2 Reconstruction and Performance . 28 2.4 The Drift Chamber . 29 2.4.1 Design and Geometry . 29 2.4.2 Electronics and Readout . 31 2.4.3 Calibration and Single-Cell Performance . 32 2.4.4 Track Finding and Performance . 33 2.5 The Ring-Imaging Cerenkov Detector (Drc) . 34 2.5.1 Design and Geometry . 34 2.5.2 Electronics and Reconstruction . 36 2.6 The Electromagnetic Calorimeter . 37 2.6.1 Geometry and Electronics . 37 2.6.2 Reconstruction and Performance . 38 2.7 The Instrumented Flux Return . 40 2.7.1 The Resistive Plate Chambers . 40 2.7.2 Reconstruction . 41 2.8 Trigger . 42 2.8.1 The Level 1 Trigger . 42 2.8.1.1 The Drift Chamber Trigger . 42 2.8.1.2 The Electromagnetic Calorimeter Trigger . 43 2.8.2 The Level 3 Trigger . 43 2.8.2.1 Performance . 43 2.9 Particle Identification . 44 2.9.1 Electron Identification . 45 2.9.1.1 Criteria . 45 2.9.1.2 Performance . 46 2.9.2 Muon Identification . 46 x 2.9.2.1 Criteria . 46 2.9.2.2 Performance . 49 2.10 Kaon Identification . 49 2.10.1 Criterion and Performance . 49 3 Data Sample and Selection 51 3.1 Preliminary Event Selection . 52 4 Analysis: Candidate Selection 56 4.1 Signal Selection . 57 4.1.1 Neutrino Reconstruction . 57 4.1.1.1 Track and Photon Selection . 59 4.1.1.2 Quality Cuts . 62 4.1.2 Y System Selection . 64 4.1.2.1 Kinematic Consistency Requirements . 66 4.2 Background Classification . 69 4.3 Simulation . 70 4.3.1 Continuum Background . 73 4.3.2 Sample Inventory .
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