CERN-THESIS-2012-153 26/07/2012 - CERN Document Server

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SEARCH FOR FLAVOR CHANGING NEUTRAL CURRENTS IN TOP QUARK DECAYS USING A TWO-LEPTONS+TRACK SELECTION Doctoral Committee: BY ARELY CORTES GONZALEZ DISSERTATION Submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Physics in the Graduate College of the University of Illinois at Urbana-Champaign, <strong>2012</strong> Urbana, Illinois Professor Kevin Pitts, Chair Professor Tony Liss, Director of Research Professor James Eckstein Professor Scott Willenbrock
Abstract A search for flavor-changing neutral currents in top quark decays is described. Data collected at a center- of-mass energy of √ s = 7 TeV during 2011 corresponding to an integrated luminosity of 2.1 fb −1 are used. A cut-based analysis is used on top quark pair production events, where one top quark is assumed to follow the dominant Standard Model decay t → Wb, and the other the flavor changing neutral current decay t → Zq. Only the decays of the Z-boson to charged leptons and leptonic W-boson decays are considered signal. The search uses an event selection with two identified leptons (electrons and/or muons) and one lepton selected via inner-detector track requirements only (track lepton). Backgrounds to the search are evaluated using almost entirely data-driven techniques. An orthogonal channel, with three identified leptons is also described, and used in the final result. No significant excess is observed in the signal region. An upper limit on the t → qZ branching ratio of BR(t → Zq)
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Page 5 and 6: Acknowledgments I first want to tha
Page 7 and 8: Table of Contents List of Tables .
Page 9 and 10: List of Tables 2.1 The fundamental
Page 11 and 12: List of Figures 2.1 The potential V
Page 13 and 14: 5.15 Distributions of the 3ID analy
Page 15 and 16: This thesis is organized as follows
Page 17 and 18: the question whether each neutrino
Page 19 and 20: gauge invariant field strength tens
Page 21 and 22: for strong interactions. Although W
Page 23 and 24: particles. Because of the introduct
Page 25 and 26: which is introduced for mass genera
Page 27 and 28: 2.3 Top Quark The large mass of the
Page 29 and 30: ATLAS Preliminary Data 2011 Channel
Page 31 and 32: as the masses of the quarks involve
Page 33 and 34: LEP HERA Tevatron LHC BR(t → qγ)
Page 35 and 36: to Wtb couplings, where limits on a
Page 37 and 38: tion at the collision point, and F
Page 39 and 40: The positive x-direction is defined
Page 41 and 42: large air-core toroids (one barrel
Page 43 and 44: 3.3.3 Calorimetry The calorimetry i
Page 45 and 46: of ∼ 1% for the H → γγ and H
Page 47 and 48: esolution of 1 cm×1 ns with digita
Page 49 and 50: Luminosity measurement Accurate det
Page 51 and 52: least one hit on the A-side, at lea
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caps. The calorimeter selections ar
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ut instead fragment to color neutra
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Decays A large fraction of the part
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The ALGPEN program with HERWIG show
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Normalized to unit / 1.96429 GeV No
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Chapter 5 Event Selection 5.1 Intro
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Variable Cut pT >20 GeV Type MuID t
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Identification efficiency eff id 1
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(a) (b) Figure 5.4: Electron identi
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ε (P ) µ TL T 1 TL µ ε TL µ ε
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Jets in this analysis are reconstru
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impact parameter significance of we
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event is also discarded. Events wit
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event kinematics, however no b-jet
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Events / 10 GeV trackleptons / 10
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analysis, is 22%. Figure 5.13(a) sh
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transverse momentum of the leading
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Events / 10 GeV Events / 10 GeV 14
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two background sources, diboson and
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ID leptons provides three different
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3ID Final Selection ZZ and WZ 9.5
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data, has systematic uncertainties
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Jets The jet energy scale (JES) and
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the fake prediction was scaled down
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two or more, and events with E miss
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Source MC Background Luminosity ±
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Chapter 8 Limit Evaluation 8.1 Intr
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means: Q = L( X,s + b) L( , (8.5)
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computed from Equation 8.12 replaci
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Chapter 9 Conclusions A search for
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There are a total of 666236 γ+jets
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Appendix B Backgrounds to the 3ID a
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Appendix C Systematic Uncertainties
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References [1] J. L. Borges, Tres v
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[40] G. Lu, F. Yin, X. Wang and L.
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[82] ATLAS Collaboration, Expected
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