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Search for Supersymmetric Pseudo-Goldstini at √ S = 13 Tev Search for Supersymmetric p Pseudo-Goldstini at s = 13 TeV with the ATLAS Detector A Dissertation Presented to The Faculty of the Graduate School of Arts and Sciences Brandeis University Department of Physics Craig Blocker, Martin A. Fisher School of Physics, Advisor In Partial Fulfillment of the Requirements for the Degree Doctor of Philosophy by David Michael Dodsworth February, 2021 This dissertation, directed and approved by David Michael Dodsworth's committee, has been accepted and approved by the Graduate Faculty of Brandeis University in partial fulfillment of the requirements for the degree of: DOCTOR OF PHILOSOPHY Eric Chasalow, Dean Graduate School of Arts and Sciences Dissertation Committee: Craig Blocker, Martin A. Fisher School of Physics, Chair James Bensinger, Martin A. Fisher School of Physics Andy Haas, Department of Physics, New York University c Copyright by David Michael Dodsworth 2021 You are a stolen sigh-between- -naps. A mind, unwrapped You are time's relative. Kind, and kinder to a faulty space Patient inter-action; conferred|interred Your courage unreserved. Grace is eternal, a kin to gratefulness. Aching to ward the whole I will always love you. iv Acknowledgments I'd first like to thank Craig Blocker, my advisor. This thesis would not be possible without him, both as an active collaborator and as a sanity check for my own ideas. I'm thankful for his patience throughout my PhD and for his thoughtful, insightful, and considerate input at every step. Thank you to Jim Bensinger for being on my committee and for his general support throughout my time at Brandeis|whether that was during my advanced exam, at alignment meetings, or over dinner in Geneva. I'd also like to thank Andy Haas for serving on my committee and his generous mentorship at the beginning of my particle physics journey, all those years ago. Thank you to my parents, Andrew Dodsworth and Shirley Grace Dodsworth, for their unquestioning support (in all its forms) and for giving me the space and resources to find my own path. Thank you to my grandmother Doris Wallis for the happy childhood memories that I remember fondly. Thank you to my siblings Duncan, Emma, Michael, Peter, Steven, and Vicky for being there when it counts, as well as all my family for being a refuge during Christmas breaks. Thank you to Kelsey O'Connor, my colleague and ally throughout our PhDs, who has become a lifelong friend. Thank you for the shared experiences and for being my first friend in an unfamiliar place. Thank you as well to Graham Stoddard for his companionship v during the summer and for opening up his home to me when I was commuting. Thank you to Lu An and Isabella Soldner-Rembold for their calming perspectives and efforts in keeping me sociable. Thank you to Siyuan Sun for showing me the ropes at CERN, our candid conversations about life and philosophy, and for my initial foray into bouldering. Thank you to Joanna Robaszweski for succor during tough times and to Aparna Baskaran for taking the time to help me with statistical mechanics. Thank you to James Cox, Chris Daly, Michela Grant, Jennifer Hadley, Alex L^e,Duncan Leggat, Kevin Lee, Shabria Ray, Tricia Tillman, Hugo Wainwright, Isabel Wheeler, and Caz Yang for their enjoyable company and interesting discussions. Thank you to old friends Jay Blake, Will Bovill, Kathy George, Nikki George, Jen Johnson, Harrison Louca, Carl Shrimpton, and Charlie Shute for keeping me grounded and connected. Thank you to the Brandeis Melee group (Tazio De Tomassi, Sam Ruditsky, Max Everson, Kevin Loew, Kevin Wei, David Heaton, Henry Goodridge, Jon Maeda, Kei Isobe, Dylan Quinn, and Yuhua Ni) for fond memories and a welcome distraction from work. Thank you to my new family in Dallas|Diantha Pyle, Phillip Pyle, Davonda Parker, MaToya Parker, Shayla Parker, Walter Parker, and Mary Lofton|for their support, encouragement, and welcoming me with open arms. Finally, I'd like to thank Marcus Pyle. Thank you for the adventures we've had across the globe, thank you for inspiring me and motivating me to go beyond what I thought I could accomplish, and thank you for continuing to surprise and delight me with your love of life and learning. You were there during the hard and uncertain times, and are the source of so much joy and satisfaction. I count myself lucky to have found you and I am proud to share my journey through life with you. vi Abstract p Search for Supersymmetric Pseudo-Goldstini at s = 13 TeV with the ATLAS Detector A dissertation presented to the Faculty of the Graduate School of Arts and Sciences of Brandeis University, Waltham, Massachusetts by David Michael Dodsworth This thesis presents a search for events in a final state containing two opposite-sign, same- miss flavor leptons, two isolated photons, and significant missing transverse momentum, ET . The final state signature is motivated by slepton pair-production in models describing exten- sions to the Minimal Supersymmetric Standard Model, with sleptons decaying to a neutralino and lepton, and neutralinos decaying to a photon and one of two supersymmetric particles| the massless, true goldstino and the massive pseudo-goldstino. This work is a novel analysis and is performed with the full Run-II ATLAS dataset, consisting of 139 fb−1 of proton-proton p collision data, at a center-of-mass energy s = 13 TeV. No significant excess is observed, and results are presented as exclusion limits at the 95% confidence level as a function of the neutralino-to-pseudo-goldstino branching ratio for each signal sample. vii Contents Abstract vii 1 Introduction1 2 Theoretical Motivation3 2.1 A Brief History of Particle Physics.......................3 2.2 The Standard Model...............................6 2.3 Supersymmetry.................................. 14 2.4 Pseudo-Goldstini and the mssm-goldstini Model................ 18 3 Analysis Search Strategy 24 4 Experimental Apparatus 28 4.1 The Large Hadron Collider............................ 28 4.2 The ATLAS Detector............................... 33 5 Object Reconstruction at ATLAS 42 5.1 Track and Vertex Reconstruction........................ 42 5.2 Electrons and Photons.............................. 43 5.3 Muons....................................... 48 5.4 Object Isolation.................................. 51 5.5 Hadronic Jet Reconstruction........................... 53 miss 5.6 ET Reconstruction............................... 55 5.7 Overlap Removal................................. 58 5.8 The Trigger System................................ 60 6 Monte Carlo Event Simulation 63 6.1 Signal Monte Carlo Generation......................... 63 6.2 Background Monte Carlo Generation...................... 64 6.3 Scale Factor Reweighting............................. 66 viii CONTENTS 7 Physics Object Definitions 68 7.1 Baseline and Signal Electrons.......................... 68 7.2 Baseline and Signal Muons............................ 69 7.3 Photons...................................... 70 7.4 Baseline Jets................................... 72 7.5 Signal b-Jet Veto................................. 72 7.6 Baseline Taus................................... 73 8 Event Selection 74 8.1 Preselection.................................... 75 8.2 Channel Selection................................. 76 9 Background Estimation 90 9.1 Background Types and Relevance........................ 91 9.2 Jets Misidentified as Leptons........................... 92 9.3 Different-Flavor Lepton Pair Backgrounds................... 93 9.4 Jets Misidentified as Photons.......................... 93 9.5 ``γγ Normalization Scaling............................ 97 10 Systematic and Statistical Uncertainties 104 10.1 Electron and Photon Uncertainties....................... 105 10.2 Muon Uncertainties................................ 106 10.3 Jet Uncertainties................................. 107 10.4 Flavor-Tagging Uncertainties........................... 108 miss 10.5 Pileup, Luminosity, and ET Uncertainties.................. 108 10.6 Statistical Uncertainties............................. 109 11 Results and Conclusion 111 11.1 Exclusion Limit Calculation on Signal Samples................. 114 ix List of Tables 2.1 Global symmetries, local gauge symmetries, and their resulting conserved quantities...................................... 11 3.1 Parameter combinations in the mssm-goldstini signal samples......... 27 4.1 List of all accelerators in the CERN complex, with the center-of-mass energies to which proton beams are accelerated...................... 29 4.2 List of all experiments present at the LHC, with a brief description. References to the experiments' technical design reports and/or proposals are included.. 29 5.1 [87] A list of all electron discriminating variables used by the ATLAS iden- tification algorithms. The Rejection column indicates whether the variable offers discriminating power between light-flavor (LF) jets, heavy-flavor (HF) jets, or photon conversions; the Usage column indicates a direct selection cut (C) or use as part of the likelihood function (LH)............... 46 5.2 [88] A list of all photon discriminating variables used by the ATLAS identifi- cation algorithms, for both Loose and Tight photon identification....... 47 5.3 List of dielectron triggers used in this analysis for each year of data-taking (NB: The logical OR of two triggers were used for 2017 and 2018, except for runs B5-B8 where the sole trigger denoted by y was used, due to partial pre-scaling)..................................... 61 5.4 List of dimuon triggers used in this analysis for each year of data-taking..
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