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ZZ Production in Proton-Proton Collisions at √S = 13 Tev in Four

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ZZ Production in Proton-Proton Collisions at √S = 13 Tev in Four ZZ Production in Proton-Proton Collisions at ps = 13 TeV in Four-Lepton Events Using the CMS Detector at the LHC by Nathaniel Woods A dissertation submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy (Physics) at the University of Wisconsin { Madison 2017 Defended on 25 October 2017 Dissertation approved by the following members of the Final Oral Committee: Wesley Smith Bjorn Wiik Professor of Physics · Sridhara Dasu Professor of Physics · Matthew Herndon Professor of Physics · Lisa Everett Professor of Physics · Pupa Gilbert Professor of Physics · i Abstract Several studies of four-lepton production in proton-proton collisions are presented. The dataset used corresponds to an integrated luminosity 35.9 fb−1 at a center-of- mass energy of 13 TeV, collected by the CMS detector at the LHC. All reported measurements use the 2`2`0 final states, where `; `0 = e or µ. The total ZZ cross section for all events with two Z bosons in the mass range 60{120 GeV is measured and found to be σ(pp ZZ) = 17:2 0:5 (stat) 0:7 (syst) 0:4 (theo) 0:4 (lumi) pb. ! ± ± ± ± The Z branching fraction to four leptons is found to be (Z 4`) = 4:8 0:2 (stat) B ! ± ± −6 0:2 (syst) 0:1 (theo) 0:1 (lumi) 10 for events with 80 < m4 < 100 GeV and m > ± ± × ` `` 4 GeV for all opposite-sign, same-flavor lepton pairs. Differential cross sections are measured as functions of a number of kinematic and jet-related observables. All these results agree with standard model predictions. A search for fully electroweak ZZ + 2jets production is performed, and an excess consistent with standard model vector boson scattering is found at the 2:7σ level. Searches for anomalous triple and quartic gauge couplings are performed, and the four-lepton invariant mass distributions are used to set the most stringent limits to date on a number of parameters affecting neutral gauge boson interactions. ii Acknowledgements They say it takes a village to raise a child, and a grad student is just a toddler whose grape juice has fermented, so I have many people to thank. I am indebted to too many advisors, colleagues, friends, and family members to name them all here, so I will highlight just a few for particular appreciation; the many omissions are solely for the purpose of keeping this document under 200 pages. I must first thank the members of the Wisconsin CMS group, who have, under- standably, played a central role in my life over the past five and a half years. The professors, Wesley Smith, Sridhara Dasu, and Matt Herndon, are great physicists and master tacticians, and their clear-eyed advice has been extremely valuable through- out my time here. Sascha Savin has been a great guide in the day-to-day business of being a physicist, and he has taught me an enormous amount about physics, data analysis, work habits, intracollaboration politics, and just about everything else. The postdocs, Evan Friis, Maria Cepeda, Senka Duric, Isobel Ojalvo, Bhawna Gomber, and C´ecileCaillol, are uniformly brilliant and endlessly helpful. Everything would grind to a halt and nothing would ever get done if not for Dan Bradley, Ajit Mo- hapatra, Carl Vuosalo, and the rest of the computing team, to whom I should also apologize for all the times I completely broke everything. Pam Klabbers and Aleˇs Svetek are founts of knowledge about the CMS trigger, and are fully responsible for any competence I appeared to display while on shift. I would never have accomplished anything without the other UW CMS grad students. Tom Perry, Aaron Levine, Laura Dodd, Devin Taylor, Tyler Ruggles, Nick Smith, Kenneth Long, James Buchanan, and Usama Hussain, and others with whom I overlapped more briefly, have been great friends and comrades-in-arms. The intelligence, collective knowledge, and snark on continuous display in the group chat and in our office chats were invaluable to both iii my work and my mental health. Ivan Furic and Darin Acosta helped me get my foot in the door during my REU at the University of Florida and taught me a lot when I had essentially no knowledge base to work from. The friends I made in Madison and Geneva are far too numerous to mention individually, but they are wonderful people and I'm lucky to have them in my life. A special shoutout goes to my flatmates, Laser Kaplan, Svet Hristozkov, Heather Russell, and Tessa Carver, for not just putting up with my idiosyncrasies but actively making home a place I wanted to go. I never would have gone anywhere in life without my smart, funny, supportive family: my parents Anne and David, my siblings Julia and Peter, a small army of extended family, and a clowder of cats led by The Boss, Gadget. I could not ask for a stronger foundation on which to build my future. iv Contents Abstract..................................... i Acknowledgements............................... ii List of Figures ix List of Tables xiii 1 The Standard Model1 1.1 Introduction................................ 3 1.2 Matter and Forces ............................ 3 1.3 Electroweak Unification and Symmetry Breaking, and the Higgs Boson 10 1.4 Diboson and Multiboson Physics .................... 13 1.4.1 Vector Boson Scattering ..................... 16 1.5 Limitations and Possible Extensions................... 17 1.5.1 Anomalous Gauge Couplings................... 18 1.6 Proton-Proton Collisions......................... 21 1.7 ZZ Measurements in Context ...................... 23 2 ZZ Phenomenology and Previous Results 24 2.1 Nonresonant ZZ=Zγ∗ Production and Decay .............. 25 2.1.1 Vector Boson Scattering ..................... 27 v 2.1.2 Prior Measurements ....................... 28 2.2 Resonant ZZ∗/Zγ∗ Production...................... 34 2.2.1 Z Boson Decays to Four Leptons ................ 35 2.2.2 Higgs Boson Production..................... 35 2.2.2.1 Prior Measurements .................. 36 2.3 Anomalous Gauge Couplings....................... 38 2.3.1 Previous Limits.......................... 40 2.4 Background Processes .......................... 42 3 The CMS Experiment and the CERN LHC 44 3.1 The CERN Large Hadron Collider.................... 44 3.1.1 Accelerator Chain, Layout, and Detectors ........... 46 3.1.2 Operating Parameters ...................... 49 3.1.2.1 Design.......................... 50 3.1.2.2 Run I .......................... 51 3.1.2.3 Run II.......................... 52 3.2 The Compact Muon Solenoid Detector................. 53 3.2.1 Terminology and Geometry ................... 54 3.2.2 Magnet and Inner Tracking System............... 55 3.2.2.1 Pixel Detector ..................... 58 3.2.2.2 Strip Tracker...................... 59 3.2.3 Electromagnetic Calorimeter................... 59 3.2.4 Hadronic Calorimeter....................... 61 3.2.5 Muon Spectrometer........................ 63 3.2.5.1 Drift Tubes....................... 64 3.2.5.2 Cathode Strip Chambers................ 65 vi 3.2.5.3 Resistive Plate Chambers ............... 66 3.2.6 Data Acquisition and Trigger .................. 67 3.2.6.1 Level-1 Trigger..................... 68 3.2.6.2 High-Level Trigger................... 70 3.2.7 Luminosity Determination.................... 71 4 Simulation 74 4.1 Monte Carlo Event Generation ..................... 74 4.1.1 Matrix Element and Hard Process Generation......... 75 4.1.2 Parton Shower, Hadronization, and Underlying Event . 76 4.1.3 Pileup Simulation......................... 78 4.1.4 Samples Used in this Analysis.................. 78 4.2 Detector Simulation............................ 79 5 Object Reconstruction and Selection 82 5.1 Track Reconstruction and Vertex Identification............. 83 5.2 Particle Flow Reconstruction....................... 85 5.2.1 PF Candidates .......................... 86 5.2.1.1 Muons.......................... 87 5.2.1.2 Electrons and Prompt Photons............ 88 5.2.1.3 Charged and Neutral Hadrons............. 88 5.2.2 Jets ................................ 89 5.2.3 Missing Transverse Energy.................... 91 5.3 Object Identification and Selection ................... 91 5.3.1 Electrons ............................. 92 5.3.2 Muons............................... 95 5.3.3 Final State Photon Radiation.................. 96 vii 5.3.4 Jets ................................ 97 5.3.5 Misidentified Objects....................... 97 5.4 ZZ Candidate and Event Selection.................... 98 5.4.1 Z/γ∗ Candidate Selection .................... 99 5.4.2 ZZ Candidate Selection...................... 100 5.4.3 Dijet and VBS Signal Selection . 102 6 Analysis Strategy 103 6.1 Background Estimation.......................... 104 6.2 Systematic Uncertainties......................... 106 6.3 Fiducial and Total Cross Section Calculation . 109 6.3.1 Signal Strength Extraction.................... 111 6.3.2 Z 4` Branching Fraction.................... 112 ! 6.4 Differential Cross Sections........................ 112 6.4.1 Unfolding ............................. 113 6.4.2 Uncertainties ........................... 114 6.5 VBS Signal Extraction.......................... 115 6.6 Anomalous Gauge Coupling Searches.................. 117 7 Results 119 7.1 Four-Lepton Yields and Inclusive Cross Sections . 120 7.1.1 Full Spectrum........................... 120 7.1.2 Z 4` Resonance ........................ 120 ! 7.1.3 Higgs Resonance ......................... 125 7.1.4 ZZ Production .......................... 126 7.2 Differential Cross Sections........................ 136 7.3 Vector Boson Scattering ......................... 138 viii 7.4 Anomalous Coupling Limits....................... 138 8 Conclusions 156 8.1 Summary ................................
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