Search for Supersymmetric Electroweak Production and the Measurement of Higgs Production Cross-sections at the ATLAS Experiment

This thesis presents a search for electroweak production of supersymmetric particles in two-lepton and three-lepton final states. The search uses the data collected from 2015 to 2018 by the ATLAS experiment in $\sqrt{s} = 13$ TeV proton–proton collisions at the CERN Large Hadron Collider, which corresponds to an integrated luminosity of $139.9$ fb^{−1}. We present the results of this search using model-independent upper limits. We also present a measurement of the Higgs boson gluon–gluon fusion production cross-section, via the $H \to WW \to l\nu l\nu$ final state, we use the data collected in 2015 by the ATLAS experiment, which corresponds to an integrated luminosity of $36.1$ fb$^{−1}$. We measure the cross-section of gluon–gluon fusion Higgs production and subsequent decay via $WW$ to be $11.4^{+2.2}_{-2.1}$ pb (statistical and systematic). Finally, we outline prospects in future upgrade works, in the fast simulation of the Fast Tracker ATLAS subsystem and the development of digitisation software in the ATLAS Inner Tracker Strips Detector.