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Measurement of Top Quark-Antiquark Pair Production in Association with a Z Boson with a Trilepton Final State in Pp Collisions at 8 Tev Center of Mass Energy

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Measurement of Top Quark-Antiquark Pair Production in Association with a Z Boson with a Trilepton Final State in Pp Collisions at 8 Tev Center of Mass Energy UC San Diego UC San Diego Electronic Theses and Dissertations Title Measurement of top quark-antiquark pair production in association with a Z boson with a trilepton final state in pp collisions at 8 TeV center of mass energy Permalink https://escholarship.org/uc/item/0gt3c4cr Author MacNeill, Ian Christopher Publication Date 2015 Peer reviewed|Thesis/dissertation eScholarship.org Powered by the California Digital Library University of California UNIVERSITY OF CALIFORNIA, SAN DIEGO Measurement of top quark-antiquark pair production in association with a Z boson with a trilepton final state in pp collisions at 8 TeV center of mass energy A dissertation submitted in partial satisfaction of the requirements for the degree Doctor of Philosophy in Physics by Ian Christopher MacNeill Committee in charge: Professor Avraham Yagil, Chair Professor Claudio Campagnari Professor Aneesh Manohar Professor George Tynan Professor Frank Würthwein 2015 Copyright Ian Christopher MacNeill, 2015 All rights reserved. The dissertation of Ian Christopher MacNeill is approved, and it is acceptable in quality and form for publication on microfilm and electronically: Chair University of California, San Diego 2015 iii DEDICATION To my parents, who started me asking questions and taught me to value knowledge. iv EPIGRAPH Numquam aliud natura, aliud sapientia dicit. Never does nature say one thing and wisdom say another. —Juvenal v TABLE OF CONTENTS Signature Page . iii Dedication . iv Epigraph . .v Table of Contents . vi List of Figures . ix List of Tables . xii Acknowledgements . xvii Vita ......................................... xix Abstract of the Dissertation . xx Chapter 1 Introduction . .1 1.1 Standard Model of Particle Physics . .2 1.1.1 Fermions . .3 1.1.2 Bosons . .5 1.1.3 Beyond the Standard Model . .8 1.2 Proton-Proton Collisions . 10 Chapter 2 The Large Hadron Collider and the Compact Muon Solenoid Ex- periment . 13 2.1 The LHC and other Particle Accelerators . 13 2.2 Description of the CMS Detector . 16 2.2.1 Solenoidal Magnet . 18 2.2.2 Silicon Tracker . 19 2.2.3 Electromagnetic Calorimeter . 21 2.2.4 Hadronic Calorimeter . 23 2.2.5 Muon Detector . 25 2.3 Luminosity Measurement . 27 2.4 Triggering . 29 Chapter 3 Particle Reconstruction . 33 3.1 Charged Particle Reconstruction . 33 3.2 Vertex Reconstruction . 36 3.3 Electron Reconstruction . 37 3.4 Muon Reconstruction . 39 vi 3.5 Particle Flow Candidate Reconstruction . 41 3.6 Jets Reconstruction . 42 3.7 Reconstruction of Jets from b-quarks . 44 Chapter 4 Signal and Backgrounds . 48 4.1 ttZ Production Details . 48 4.2 ttZ Decay Details . 49 4.3 Motivation for Measuring the 3 Lepton Final State . 51 4.4 Backgrounds . 52 4.4.1 Backgrounds From Irreducible Sources . 52 4.4.2 Non-prompt Leptons . 53 4.4.3 Non-top Originating b-Jets . 54 Chapter 5 Real and Simulated Collision Sources . 56 5.1 Collision Data Samples . 56 5.2 Monte Carlo Simulation Samples . 57 Chapter 6 Event Selections . 63 6.1 Triggers for Data Acquisition . 64 6.2 General Event Cleanup and Vertex Selection . 65 6.3 High Level Event Identification . 65 6.4 Selections for Identifying Electrons . 66 6.5 Selections for Identifying Muons . 67 6.6 Selecting the Z-boson Leptons and the 3rd Lepton . 69 6.7 Vetoing Events with a 4th Lepton . 69 6.8 Selecting Jets . 69 6.9 Tagging Jets As Originating from a b-quark . 70 6.10 Optimizing Selections . 71 Chapter 7 Background Estimation Methods . 74 7.1 Monte Carlo Based Estimation . 74 7.1.1 Irreducible Sources . 76 7.2 Data Derived Estimation . 77 7.2.1 Estimation of Events Passing Selections Due to Fake Leptons . 78 7.2.2 Estimation of Events Passing Selections with Non- top b-quarks . 89 Chapter 8 Efficiencies and Uncertainty . 93 8.1 Lepton MC to Data Scale Factors and Uncertainties . 94 8.2 Systematic Uncertainty Due to Triggers . 99 8.3 b-tagging Efficiency and Associated Errors . 99 8.4 Uncertainties Involved in Event Generation . 100 8.4.1 Uncertainties Due to Parton Distribution Functions . 100 vii 8.4.2 Uncertainties Measured in Top Samples . 103 8.4.3 Uncertainties Due to Generators . 104 8.5 Jet Energy Scale and Resolution . 106 8.6 Pile Up . 108 Chapter 9 ttZ Cross Section Measurement . 110 9.1 Data and Prediction Agreement in Preselection . 110 9.2 Yields from Data After Full Selections . 112 9.3 Measured Signal and Background Subtraction . 116 9.3.1 Method for Reconstructing a Top Mass . 116 9.3.2 Distributions with Full Selections . 117 9.4 Cross Section Calculation and Significance . 118 9.5 Combined Cross Section and Significance . 123 Chapter 10 Summary and Conclusions . 126 Bibliography . 127 Appendix A Complete Listing of Data Samples and Tables of Triggers . 132 A.1 Triggers . 132 A.2 Datasets . 134 Appendix B Complete Listing of Monte Carlo Simulations . 136 B.1 Details of Simulated Samples . 136 Appendix C Additional Figures . 144 C.1 ttZ Preselection . 144 viii LIST OF FIGURES Figure 1.1: Quarks (purple), leptons (green), and bosons (red and yellow) are shown grouped together by their kind. Anti-particles are not shown. Quarks and leptons stacked above each other have a special relation- ship via the weak interactions [1]. .3 Figure 1.2: The above diagrams represent the various interactions of bosons in the standard model. As noted before a boson is considered to medi- ate a force between two other particles [2]. .6 Figure 1.3: This diagram traces the interactions between various elementary particles in the Standard Model, including self interactions. Lines connecting a black oval show interactions between the particles within only the two connected ovals while lines connecting a square show interactions with all of the particles within the square [3]. .8 Figure 2.1: Plot of experimental accelerator energies and their first year of oper- ation [4]. Accelerators were built with a rough factor of 10 increase in energy over the last. The LHC is the first to start to fall off this increase. 14 Figure 2.2: Picture of the interior of the LHC showing the beam pipe and some equipment. Particles circulate around the ring in the very center of this pipe (which is mostly filled with electronics) [5]. 15 Figure 2.3: Map of the LHC including the beam ring [6] with locations of the various detectors. 16 Figure 2.4: The CMS detector shown with axial slices. This view demonstrates the cylindrical construction of the CMS detector where each cylin- drical layer measures a different aspect of the particles that traverse the detector [7]. 17 Figure 2.5: A cross sectional slice of the CMS detector [8]. The inner radius is on the left and the outer radius is on the right. Sections shown (left to right) are the silicon detector, the ECAL, the HCAL, the solenoid, and the muon detector. 18 Figure 2.6: Projection of the tracker in the r-z plane. Modules are shown as lines. Angles are measured in h [9]. 21 Figure 2.7: The crystal layout of ECAL by pseudorapidity range [10]. Crystals are oriented to be as close to parallel to the the path of particles that traverse them. 23 Figure 2.8: Map of the HCAL layout flattened into r-z space [11]. The magnetic coil provides extra material to help contain the hadronic particles. For particles that happen to reach the coil and then shower, there are additional components outside the coil to measure their energy. 24 ix Figure 2.9: Layout of the CMS muon system with sub-detectors projected in the r-z plane. The Drift Tubes (green), the Cathode Strip Chambers (blue), and the Resistive Plate Chambers (red) are shown [12]. 27 Figure 2.10: Plot of 8 TeV data collection over time. The delivered data (blue) is slightly above the recorded data (yellow) as the detector is not able to record every collision due to various transient issues. 29 Figure 2.11: Schematic of triggering for two level and three level systems. Left: CMS method with a single High-Level Trigger (HLT) providing all filtering after the Level-1 decision. Right: the HLT performed in two stages (L2 and L3) [13]. 31 Figure 3.1: Graphical representation of the stages of an electron in the detector and the components that must be combined in the reconstruction. The electron bremsstrahlungs a photon which leaves a region of en- ergy in the ECAL. The electron itself leaves energy in the ECAL near the photon’s energy along a somewhat predictable curved tra- jectory [14]. 39 Figure 3.2: Plot of data and MC overlaid. The MC contains samples of jets originating from different quarks including b and light flavor [15]. Agreement is generally very good in the bulk of the sample. 47 Figure 4.1: Feynman diagrams of the Leading Order (LO) diagrams for ttZ and multiplicities of the diagrams. In the gluon produced diagrams (Left), it is best to think of the Z as a radiative process off of the top. In the quark produced diagrams (Right), the Z production is best thought of as quark annihilation. 49 Figure 4.2: Fraction of time ttZ decays to various final states. States that con- tain tau decays have been separated from the states containing only electrons and muons because the measurement in this document only targets states with electrons and muons from boson decays. Decay fractions are from the Particle Data Group [16]. 51 Figure 4.3: Diagram of gluon splitting where the b-quarks are from not top events. The gluon can radiate from any hadronic source. 55 Figure 9.1: Yields with break down by leptonic channel of data and predictions after tri-lepton plus two b-tag selections.
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