Searches for New Physics in final states containing leptons, b-jets and MET at the ATLAS detector

This Thesis documents analyses targeting BSM processes in final states containing leptons, $b$-jets and missing transverse energy at the ATLAS detector, as well as studies relevant for the high luminosity upgrade of ATLAS. The first analysis presented targets the pair-production of a supersymmetric chargino and next-to-lightest neutralino, decaying via a $W$ boson and SM-like Higgs boson. This analysis used 36.1 fb$^{-1}$ of ATLAS $pp$ data taken between 2015-2016 at a centre-of-mass energy ($\sqrt{s}$) of $13~\textrm{TeV}$, and placed exclusion limits on chargino/next-to-lightest neutralino masses up to 550 GeV for a massless neutralino. An updated search is also presented, using 139 fb$^{-1}$ data taken between 2015-2018, extending the sensitivity up to chargino/next-to-lightest neutralino masses up to 740 GeV. Preliminary studies using machine learning classifiers to select this signal from the dominant Standard Model backgrounds are presented, showing promising classification performance. In addition, a search for dark matter produced in association with a single top quark is presented. The result of this analysis is interpreted in the context of an extended Higgs sector model, where a pseudoscalar mediator couples the dark matter to the Higgs sector. The analysis used 139 fb$^{-1}$ of data taken between 2015-2018, collected at $\sqrt{s} = 13$ TeV. The process was excluded for charged Higgs masses, originating from an extended Higgs sector New Physics model, up to 1250 GeV for a mediator mass of 100 GeV. The complimentary sensitivity of this analysis to dark matter produced in association with both a single top quark or a pair of top quarks was evaluated, excluding charged Higgs masses up to 1400 GeV. Finally, work performed for the upcoming luminosity upgrade is presented. Performance studies of pixel modules for the ATLAS inner tracker are shown using data collected in test beams at DESY. The sensitivity to chargino/next-to-lightest neutralino pair-production at the upgraded ATLAS detector is evaluated using the expected dataset of 3000 fb$^{-1}$ taken at $\sqrt{s} = 14$ TeV. Using machine learning classifiers, the expected exclusion limits on the chargino/next-to-lightest neutralino are set at 1280 GeV for a massless neutralino.