Observation of Higgs Boson Production Through Vector Boson Fusion in the $WW^{*}$ Channel at the ATLAS Detector

This thesis presents an observation of vector-boson-fusion (VBF) production of Higgs boson in the $H{\rightarrow\,}WW^{\ast}{\rightarrow\,}\ell\nu\ell\nu$ decay channel. For this analysis, $\sqrt{s}=13$ TeV proton-proton collision data collected by the ATLAS detector between 2015 and 2018, corresponding to an integrated luminosity of $\mathrm{139~fb^{-1}}$, is used. Events with one electron (positron) and one anti-muon (muon), and missing transverse momentum, along with at least two forward jets in the final state, are selected for the study. A major part of the analysis concerns the estimation of backgrounds with misidentified leptons, which generally originate from the production of a $W$ boson and an associated object mistakenly identified as a lepton and are estimated with data-driven techniques. Signal sensitivity has been significantly improved by employing a deep neural network (DNN) in separating signal from background events. The significance of the data assuming the background-only hypothesis to be true has been observed to be 7.0 $\sigma$ (6.2 $\sigma$ expected), constituting the first observation of the VBF Higgs boson production in the $H{\rightarrow\,}WW^{\ast}{\rightarrow\,}\ell\nu\ell\nu$ decay channel. The product of the total VBF cross-section times the $H{\rightarrow\,}WW^{\ast}$ branching fraction is measured to be $0.85 \pm 0.10$(stat.)$^{+0.17}_{-0.13}$(syst.)~pb, compatible with the Standard Model prediction of $0.81\pm0.02$~pb.