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Title: New physics at the LHC : direct and indirect probes
Author: Lewis, Dave
Awarding Body: Queen Mary University of London
Current Institution: Queen Mary, University of London
Date of Award: 2017
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This thesis presents the results for two searches for new physics performed with the ATLAS experiment. The first, a search for the rare B-meson decay Bs → μμ and measurement of its branching ratio, uses 25 fb⁻¹ of √s = 7 and 8 TeV data recorded during 2011 and 2012. After observing a small number of these decays, a branching ratio of B(Bs → μμ) = (0.9⁺¹·¹₋₀.₈) x 10⁻⁹ is measured, assuming non-negative event yields. This is compatible with the Standard Model at the 2σ level. The second, a search for direct pair production of the supersymmetric top quark partner, is performed using 36.07 fb⁻¹ of √s = 13 TeV data recorded during 2015 and 2016. Final states with a high jet multiplicity, no leptons and large missing transverse momentum are selected to target these decays, with several signal regions designed to cover a wide range of particle masses. No excess is observed, with all signal regions being compatible with the Standard Model within 2σ. Limits are set on the stop mass, excluding up to mt̃1 = 940 GeV for values of mx̃⁰₁ below 160 GeV, assuming a 100% branching fraction to t̃1 → tX̃⁰₁ decays. In addition two reinterpretations of this data are presented, for a gluino-mediated stop production scenario and a direct dark matter production scenario. No excess is observed for either model, and limits are set on the mass of the relevant particles. Finally a viability study into using machine learning techniques to improve on existing SUSY search methods has been performed, with the initial results proving promising.
Supervisor: Not available Sponsor: Science and Technology Facilities Council
Qualification Name: Thesis (Ph.D.) Qualification Level: Doctoral
EThOS ID:  DOI: Not available
Keywords: Physics ; Large Hadron Collider ; CERN ATLAS