New Physics at the LHC: Direct and Indirect Probes
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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 ! and measurement of its branching ratio, uses 25 fb1 of ps = 7 and 8 TeV data recorded during 2011 and 2012. After observing a small number of these decays, a branching ratio of B(Bs ! ) = (0:9+1:1 ��0:8) 10��9 is measured, assuming non-negative event yields. This is compatible with the Standard Model at the 2 level. The second, a search for direct pair production of the supersymmetric top quark partner, is performed using 36.07 fb��1 of p 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 2 . Limits are set on the stop mass, excluding up to m~t1 = 940 GeV for values of m~ 01 below 160 GeV, assuming a 100% branching fraction to ~t1 ! t~ 01 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.
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