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The Search for Exotic Diboson Production in the Semileptonic Channels with the ATLAS Detector

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The Search for Exotic Diboson Production in the Semileptonic Channels with the ATLAS Detector The Search for Exotic Diboson Production in the Semileptonic Channels with the ATLAS Detector by Robert Les A thesis submitted in conformity with the requirements for the degree of Doctor of Philosophy Graduate Department of Physics University of Toronto c Copyright 2020 by Robert Les Abstract The Search for Exotic Diboson Production in the Semileptonic Channels with the ATLAS Detector Robert Les Doctor of Philosophy Graduate Department of Physics University of Toronto 2020 This thesis presents several studies in the search for new physics in the production of elec- 1 troweak gauge bosons pairs with 36 fb− of proton-proton collisions measured by the ATLAS detector. Processes with electroweak gauge bosons in the final state are sensitive to new physics which alter the electroweak or Higgs sector of the Standard Model. The studies are conducted in the \semileptonic" decay channels, where one of the electroweak gauge bosons decays hadron- ically and the other decays leptonically. The groundwork of these studies is established in a search for a new resonant particle which can decay on-shell to pairs of W=Z bosons in the `νqq channel. No significant excess of data with respect to the background prediction was observed. Therefore, upper limits at the 95% CLs confidence level are placed on the possible resonant masses in a strongly coupled Heavy Vector Triplet model and on Bulk Randall-Sundrum Gravi- tons at 3 TeV and 1.7 TeV, respectively. These results are expanded upon with a dedicated study on an ATLAS-wide combination of resonant diboson and dilepton search results. The combination improves the 95% CLs upper excluded mass values to 5.5 TeV and 2.1 TeV, respectively. In conjunction, a search for non-resonant new physics is conducted through a measurement of the electroweak vector-boson scattering in all semileptonic channels. Evidence for the process was observed at a significance of 2:7σ and a fiducial cross-section measure- +15:9 ment of σ = 45:1 ± 8:6 (stat:) 14:6 (syst:) fb was extracted, consistent with the Standard Model − prediction. Lastly, both the prospects of the `νqq resonance search and the measurement of 1 vector-boson scattering in the High-Luminosity LHC era were evaluated. With 3000 fb− of proton-proton data, the upper mass limits on new resonances in the `νqq channel are expected to increase by 1.3 - 2 TeV depending on the benchmark model and the vector-boson scattering cross-section is expected to be measured at the percent level. ii Acknowledgements This work is possible due to everyone I have encountered in my life. I view myself as the sum of my parts, with every interaction forming the person I have become (and will become). Of course some have affected me in a more positive and relevant way in the context of this work. I would like to thank the faculty of the Physics Department at the University of Toronto for guiding my physics studies from the undergraduate and graduate level. Particular praise needs to be given to the Experimental High Energy Physics group of Pekka Sinervo, Robert Orr, Pierre Savard, Peter Krieger, Richard Teuscher, and my supervisor William Trischuk, who each taught me much in the field and provided guidance in the direction of this work. Double praise is given to Pekka Sinervo, Pierre Savard, Peter Krieger, William Trischuk, Bob Holdom, and Philippe Di Stefano who spent time out of their busy days to read this strenuous document. In a similar vein I would like to thank all the undergraduate students I have taught over these short years. By teaching I have learned to contextualize and refine my knowledge of physics topics, making me both a better physicist and communicator of science. This work could not be accomplished without the members of the ATLAS collaboration. Only a small subset of the world can claim to work on a project with \3000 of their best friends" as my supervisor would say. Through the hard work of each individual, this thesis and many publications are made possible. I provide additional praise to Viviana Cavaliere, Lailin Xu, Kalliopi Iordanidou, Tatsumi Nitta, and many others who were invaluable \partners in crime" in getting various publications released. Additionally, I would like to thank Vincent Pascuzzi and Kyle James Read Cormier for many fruitless discussions on the thesis writing procedure, and for informative comments on various drafts. I would never have been able to complete this work without the funding provided by the National Scientific Research Council of Canada (NSERC) and the University of Toronto. By inference, I would like to thank all Canadian tax-payers, a fraction of whom I know care about these types of work. I would also like to thank my mother, who always wanted me to be a doctor (just not this kind), and my father who will probably be the only non-physicist who will attempt to read this. Lastly I would like to thank Claudia Palermo, both my muse and source of insanity. iii Contents 1 Introduction 1 2 Theory 4 2.1 Introduction to Quantum Field Theories.......................4 2.2 Standard Model.....................................7 2.2.1 Fermionic Matter Content...........................9 2.2.2 Gauge Fields.................................. 10 2.2.3 Spontaneous Symmetry Breaking....................... 11 2.2.4 Vector Boson Scattering............................ 13 2.3 Beyond the Standard Model.............................. 14 2.3.1 Extended Higgs Sectors............................ 15 2.3.2 Extended Gauge Sectors............................ 18 2.3.3 Warped Extra Dimensions........................... 21 2.3.4 Effective Field Theories............................ 23 3 The ATLAS experiment 26 3.1 Large Hadron Collider................................. 26 3.1.1 Luminosity Measurements........................... 29 3.2 The ATLAS Detector................................. 31 3.2.1 Inner Tracking Detector............................ 32 3.2.2 Calorimeters.................................. 36 3.2.3 Muon Spectrometer.............................. 42 3.2.4 Forward Detectors............................... 46 3.2.5 Trigger and Data Acquisition System.................... 47 3.2.6 Data Quality.................................. 51 4 Detector Simulation and Event Reconstruction 53 4.1 Event Generation.................................... 54 4.2 Detector Simulation.................................. 56 4.3 Object Reconstruction................................. 57 4.3.1 Tracks...................................... 58 iv 4.3.2 Jets....................................... 60 4.3.3 Electrons.................................... 69 4.3.4 Muons...................................... 71 4.3.5 Missing Transverse Energy.......................... 73 5 VV ! `νqq Resonance Search with 36 fb−1 76 5.1 Signal and Background Simulation.......................... 78 5.1.1 Signal Models.................................. 78 5.1.2 Background Processes............................. 79 5.2 Event Selection..................................... 81 5.2.1 Common Selection............................... 82 5.2.2 VBF Channels................................. 83 5.2.3 Merged Selection................................ 84 5.2.4 Resolved Channel............................... 87 5.2.5 Background Determination.......................... 88 5.2.6 Event Orthogonality Strategy......................... 89 5.3 Systematic Uncertainties................................ 90 5.3.1 Experimental Uncertainties.......................... 90 5.3.2 Modeling Uncertainties............................ 92 5.4 Exclusion Limits on Benchmark Models....................... 94 6 Diboson+Dilepton Resonance Combination with 36 fb−1 101 6.1 Signal Models...................................... 102 6.2 Orthogonality...................................... 104 6.2.1 VV and V h Overlap.............................. 105 6.2.2 Dilepton and Diboson Overlap........................ 107 6.3 Correlation Scheme................................... 107 6.4 Exclusion Limits on Benchmark Model........................ 109 6.4.1 Asymptotic vs Toy Limit Comparison.................... 117 6.5 Exclusion Limits on HVT Coupling Parameters................... 119 7 Semileptonic VBS search with 36 fb−1 124 7.1 Signal and Background Simulation.......................... 126 7.2 Event Selection..................................... 127 7.2.1 Trigger and Lepton Selection......................... 128 7.2.2 Signal Jet Selection.............................. 129 7.2.3 VBS Jet Selection............................... 130 7.2.4 Background Determination.......................... 130 7.3 BDT Optimization................................... 132 7.4 Systematics....................................... 135 v 7.5 Results.......................................... 136 7.5.1 Search for Electroweak V V jj Production.................. 140 7.5.2 Fiducial Cross-section Measurement..................... 140 8 Semileptonic Diboson Study Prospects at the HL-LHC 145 8.1 Analysis Procedure................................... 146 8.2 Event Selection..................................... 147 8.2.1 Object Level Differences............................ 147 8.2.2 VBS Search Differences............................ 148 8.2.3 Kinematic Distributions............................ 148 8.3 Systematics....................................... 148 8.4 Prospect Results.................................... 151 8.4.1 Resonance Search Prospect Results...................... 154 8.4.2 VBS Search Prospects Results........................ 155 9 Summary 157 Appendices 159 A Statistical Framework 160 A.1 Profile-likelihoods...................................
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