Kotwal, Ashutosh VSen, Sourav2021-01-122021-01-122020https://hdl.handle.net/10161/22178<p>This thesis presents an observation of vector-boson-fusion (VBF) production of Higgs boson in the H→WW*→l?l? decay channel. For this analysis, √s = 13 TeV proton-proton collision data collected by the ATLAS detector between 2015 and 2018, corresponding to an integrated luminosity of 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 σ (6.2 σ expected), constituting the first observation of the VBF Higgs boson production in the H→WW*→l?l? decay channel. The product of the total VBF cross-section times the H→WW* branching fraction is measured to be 0.85 土 0.10(stat.) ^{+0.17}_{-0.13}(syst.) pb, compatible with the Standard Model prediction of 0.81 土 0.02 pb.</p>PhysicsDissertation