Measurement of Associated Production of Higgs Bosons Decaying to Pairs of $W$ Bosons with the ATLAS Detector at the Large Hadron Collider

This thesis describes the measurement of Higgs boson production in association with a vector boson, $WH$/$ZH$, using the $H\rightarrow WW^\ast\rightarrow\ell\nu\ell\nu$ and $H\rightarrow WW^\ast\rightarrow\ell\nu q\bar{q}$ decay channels, where $\ell$ is an electron or a muon. The analysis uses proton-proton collision data at centre-of-mass energy $13\,\textrm{TeV}$ delivered by the Large Hadron Collider and recorded by the ATLAS detector between 2015 and 2018, corresponding to an integrated luminosity of $139\,\textrm{fb}^{-1}$. The analysis is performed in several channels across 2-, 3-, and 4-lepton final states and utilizes a diverse set of multivariate techniques for signal extraction. The $WH$ and $ZH$ production cross sections times the $H\rightarrow WW^\ast$ branching ratio are measured to be $0.13^{+0.08}_{-0.07}\textrm{(stat.)}^{+0.05}_{-0.04}\textrm{(sys.)}\,\textrm{pb}$ and $0.31^{+0.09}_{-0.08}\textrm{(stat.)}\pm0.03\textrm{(sys.)}\,\textrm{pb}$, respectively, in agreement with the Standard Model expectations.