Probing the Beyond the Standard Model Higgs Sector using the ATLAS Detector

A Higgs boson with a measured mass of 125 GeV has been observed by both the ATLAS and CMS experiments and found to be compatible with the predictions from the Standard Model (SM). The now consolidated Higgs sector has become a topic of considerable study as a potential portal to new physics. Several models beyond the SM (BSM) exist with non-trivial Higgs sectors and predict the existence of additional Higgs bosons yet to be observed. One such type of model is the Two-Higgs-Doublet-Model (2HDM) which extends the SM Higgs sector in a minimal way, while another type which considers supersymmetry is the Minimal Supersymmetric Standard Model (MSSM). The implications of both models are considered in the thesis. This thesis presents two searches for additional Higgs bosons predicted by BSM models. One search is for heavy neutral Higgs bosons decaying to a pair of tau leptons. The second search is for charged Higgs bosons decaying to a tau lepton and a neutrino. No significant excess is observed in either analysis and regions of the parameter space of the MSSM are excluded. This thesis also presents a reinterpretation of the measurement of the 125 GeV Higgs boson cross-section in the context of the 2HDMs and MSSM. The measured 125 GeV Higgs boson cross-sections are found to be compatible with the predictions of the SM, and are used to constrain the 2HDM and MSSM parameter space. Results from the direct searches, in terms of model-independent cross-section limits, and the 125 GeV Higgs boson cross-section measurements are shown to provide complementary constraints on the MSSM parameter space

Search for dark matter produced in association with bottom or top quarks in √s = 13 TeV pp collisions with the ATLAS detector

A search for weakly interacting massive particle dark matter produced in association with bottom or top quarks is presented. Final states containing third-generation quarks and miss- ing transverse momentum are considered. The analysis uses 36.1 fb−1 of proton–proton collision data recorded by the ATLAS experiment at √s = 13 TeV in 2015 and 2016. No significant excess of events above the estimated backgrounds is observed. The results are in- terpreted in the framework of simplified models of spin-0 dark-matter mediators. For colour- neutral spin-0 mediators produced in association with top quarks and decaying into a pair of dark-matter particles, mediator masses below 50 GeV are excluded assuming a dark-matter candidate mass of 1 GeV and unitary couplings. For scalar and pseudoscalar mediators produced in association with bottom quarks, the search sets limits on the production cross- section of 300 times the predicted rate for mediators with masses between 10 and 50 GeV and assuming a dark-matter mass of 1 GeV and unitary coupling. Constraints on colour- charged scalar simplified models are also presented. Assuming a dark-matter particle mass of 35 GeV, mediator particles with mass below 1.1 TeV are excluded for couplings yielding a dark-matter relic density consistent with measurements