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Looking for Beyond the Standard Model Physics in Dijet-Plus-Lepton Final-State Events Collected with the ATLAS Detector

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Looking for Beyond the Standard Model Physics in Dijet-Plus-Lepton Final-State Events Collected with the ATLAS Detector Looking for Beyond the Standard Model Physics in Dijet-Plus-Lepton Final-State Events Collected with the ATLAS Detector by Vincent R. Pascuzzi A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto c Copyright 2019 by Vincent R. Pascuzzi Abstract Looking for Beyond the Standard Model Physics in Dijet-Plus-Lepton Final-State Events Collected with the ATLAS Detector Vincent R. Pascuzzi Doctor of Philosophy Graduate Department of Physics University of Toronto 2019 A search for a new resonant state decaying exclusively to two quarks in events with at least two jets and one isolated charged lepton using 139 fb−1 of proton-proton collision data collected by the ATLAS p detector at s = 13 TeV is presented. Requiring a charged lepton in dijet final states provides sensitivity to extensions of the Standard Model which predict the production of dijet resonances in association with leptonically-decaying vector bosons or top quarks. Whereas inclusive searches are limited to resonance masses greater than 1 TeV due to jet-based triggers which have a minimum jet transverse momentum of about 400 GeV, lepton triggers can be as low as 24 GeV, thereby permitting access to resonance masses of order 100 GeV. The search presented in this thesis considers a dijet invariant mass range between 0:22{6:3 TeV. The largest deviation between the number of observed events and the Standard Model background estimate was found at a dijet invariant mass mjj = 1:3 TeV with a global statistical significance of 0:5 standard deviations. In the absence of a statistically significant deviation of the data from the background expectation, exclusion limits are set at a 95% confidence level on the presence of generic Gaussian-shaped signals of different masses and widths. Such signals are excluded with cross sections ranging from 40{0:03 fb between 0:25{6:0 TeV. Limits are also set in the context of a Technicolor model and a Simplified Dark Matter model. A follow-up search using a machine learning approach is discussed last. ii To Jules, Lala and Dora (and Ada). iii Acknowledgements A special \thanks" first to my supervisor, Peter Krieger, for taking me on as a graduate student and getting me into this mess I'm currently in. (If it were not for PK, I may have have been stuck doing cosmology or astrophysics at another institution, or worse { have a \real" job by now.) One thing I especially appreciated were the many opportunities I was given as your student: spending nearly two years at CERN, and all of the travelling you allowed me to do for workshops, conferences, and \collaboration weeks". I'm a strong believer in networking, and these opportunities let me interact with some people I regard as being true experts in the field. Even more, the infinite patience he had while reading and re-reading (and re-re-reading) the dozens of drafts of this thesis that has ultimately resulted in what I consider to be a “fine” dissertation. To my PhD committee members, Pekka Sinervo and Michael Luke: thanks for your help getting me here. \Thanks" to the other ATLAS folks I've had the pleasure of working with as an ATLAS graduate student, namely Sergei Chekanov, John Chapman, Zach Marshall, Heather Grey, Emmanuel Monnier, Martin Aleksa, Nikiforos Nikiforou, and Tommaso Lari (and the more than two dozen others). You've all been pivotal in helping advance my career in ATLAS, and can't help but think I wouldn't be where I am today without your support and guidance. Cheers also to the awesome people in the ATLAS Simulation and JDM groups, as well as the LAr Calorimeter Operations team { it's been a hell of a good time! I'd like to also give a special shout-out to Osama \Uncle Hass" AbouZeid for being a great friend since almost the beginning of my studies in Toronto, and for being the godfather of my second daughter. To the rest of ATLAS Toronto: it's really been a pleasure getting to know you and to work with you. I sincerely wish you all the best. I need to of course also thank my family { the Pascuzzis, Edens, and Angers { for always showing interest in my work and supporting me (emotionally and sometimes financially) through what seems like a career as a professional student. Dad, you taught me how to be a man, and have always demonstrated the qualities of an incredible father and role model. Ma, you taught me how to love unconditionally, and that it's OK to constantly worry about your kids (no matter how old they are). Nicky P., you taught me what brotherhood and true friendship is all about. I love you all. Last, but not least, I thank my beloved and devoted wife, Julie Marie Carmelle Pascuzzi, who has put up with more than anyone could ever imagine (sorry, Ma!). You've loved me, always supported me, and took care of me through all of this { no matter what. You gave me the two (three) most important things in my life, our daughters Carmella and Doralie (and puppy, Ada), and have shown me what true love is. We've been through so much together over the past decade, and I can't wait to experience whatever our future together has in store. I hope we all enjoy California! Much love, Vince (aka Vinny P., aka vrp) iv Contents Abstract ii Contents v 1 Introduction 1 2 Some aspects of particle physics3 2.1 A brief introduction to the Standard Model . .4 2.1.1 Fundamental particles and forces . .4 2.1.2 Symmetries and gauge theories . .5 2.1.3 Spontaneous symmetry breaking (aka \hidden" symmetries) . .8 2.1.4 Quantum chromodynamics . 12 2.2 Physics beyond the Standard Model . 14 2.2.1 The hierarchy and fine-tuning problems . 14 2.2.2 Dark Matter . 15 2.2.3 The Technicolor Model . 16 2.2.4 The Simplified Dark Matter Model . 17 2.3 Proton-proton collisions . 18 2.3.1 Anatomy of a collision . 18 2.3.2 Hard-scattering . 20 2.3.3 Hadronisation . 22 3 The Large Hadron Collider 25 3.1 Machine operation . 25 3.2 The accelerator chain . 26 4 The ATLAS Experiment 32 4.1 Physics and detector requirements . 33 4.2 Magnet system and magnetic field . 35 4.2.1 Central solenoid magnet . 35 4.2.2 Toroid magnets . 36 4.3 Inner detector . 36 4.3.1 Pixel detector . 37 4.3.2 Semiconductor Tracker . 39 4.3.3 Transition Radiation Tracker . 39 v 4.3.4 Beam Conditions Monitor . 40 4.4 Calorimetry . 41 4.4.1 Electromagnetic calorimeters . 42 4.4.2 Hadronic calorimeters . 45 4.4.3 LAr signal and readout methodology . 49 4.5 Muon Spectrometer . 50 4.5.1 Monitored Drift Tubes . 51 4.5.2 Cathode Strip Chambers . 52 4.5.3 Resistive Plate Chambers . 52 4.5.4 Thin Gap Chambers . 53 4.6 The LUCID detector . 54 4.6.1 Detector design and algorithms . 54 4.6.2 Calibration transfer for final luminosity determination . 55 4.7 Trigger and data acquisition . 56 5 Dataset and object reconstruction 58 5.1 Dataset and data quality . 58 5.1.1 Dataset . 58 5.1.2 Data quality . 59 5.1.3 Pile-up . 60 5.2 Clustering algorithms . 62 5.2.1 Sliding-window calorimeter clustering . 62 5.2.2 Topological calorimeter clustering . 63 5.2.3 Adaptation of the 4-2-0 method for electrons . 63 5.2.4 Superclusters . 64 5.3 Object reconstruction and calibration . 65 5.3.1 Tracks . 65 5.3.2 Electrons . 67 5.3.3 Muons . 70 5.3.4 Jets . 73 6 The dijet+lepton analysis 79 6.1 Introduction . 79 6.2 Analysis strategy . 82 6.2.1 Background estimate performance metrics . 83 6.3 Standard Model backgrounds . 85 6.3.1 Multijet background . 85 6.3.2 Top quark background . 85 6.3.3 W + jets background . 86 6.4 Monte Carlo simulations and physics modelling . 86 6.4.1 Event generation . 87 6.4.2 Simulating the ATLAS detector . 88 6.4.3 Background Monte Carlo . 89 6.4.4 Signal modelling and Monte Carlo . 90 vi 6.5 Event selection . 92 6.5.1 Triggering events . 92 6.5.2 Object definitions and event preselection . 94 6.6 Final selection and definitions of analysis regions . 100 6.6.1 Signal leptons . 100 6.6.2 Signal jets and determining the minimum mjj .................... 100 6.6.3 Signal region definition . ..
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