UBC Theses and Dissertations
New physics hunt at the Large Hadron Collider with the ATLAS detector : search for heavy exotic resonances and upgrade of the Transition Radiation Tracker DAQ system Wong, Vincent Wai Sum
Many theories beyond the Standard Model suggest new particles at the TeV energy scale, that could be produced in proton-proton collisions delivered by the CERN Large Hadron Collider and collected by the ATLAS experiment. Since 2016, the accelerator has surpassed its expected value of luminosity by up to a factor of two. The data acquisition system of the ATLAS Transition Radiation Tracker had to be upgraded to meet the demand from the challenging beam conditions and the higher trigger rate in Run II beyond the original design of the tracking detector. The improvements developed in the hardware and firmware of the DAQ system are documented in this dissertation, with a study to evaluate the performance of the system. This dissertation also presents two searches for new massive bosons at the TeV scale. The first analysis searches for heavy resonances decaying into a hadronic Z/W/Higgs boson and a photon. The boosted Z/W/Higgs boson is identified using large-radius jet mass and substructure informations. The analysis is based on 36 inverse femtobarns of √s = 13 TeV proton–proton collision data, collected with the ATLAS detector in Run II of the Large Hadron Collider. No significant deviations from the Standard Model prediction is observed. Upper limits are set on the signal cross section multiplied by the branching fraction of resonance for the three different diboson final states at 95% confidence level, excluding those productions at 10–0.2 femtobarns in the resonance mass range of 1–6.8 TeV. The second analysis searches for scalar leptoquark pair productions, where each leptoquark decays into a top quark and an electron or a muon. The search sensitivity is optimized for high leptoquark masses, at which the hadronic decay products of each top quark are contained within a large-radius jet. The analysis exploits the full Run II dataset that corresponds to an integrated luminosity of 139 inverse femtobarns. No significant excess of events is found. Lower mass limits on leptoquarks decaying into electron–top-quark or muon–top-quark pair are set to 1.48 TeV and 1.47 TeV at 95% confidence level.
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