Study of the associated production of the Higgs boson with a leptonically decaying W or Z boson in proton-proton collisions at $\sqrt{s}$ = 13 TeV

This thesis presents the study of the Standard Model Higgs boson production in association with a W boson decaying into a charged lepton and neutrino, followed by a Higgs boson decaying into a pair of W bosons, which subsequently decay into a pair of charged leptons, neutrinos and a pair of light quarks. The final state consists of two same charged leptons, moderate missing transverse energy and one or two jets. The full proton-proton collision dataset collected by the CMS detector in the years 2016-2018 (referred as "Run-II" dataset) at the center of mass energy, $\sqrt{s}$ = 13 TeV, corresponding to the integrated luminosity of 137 $fb^{-1}$, is used in this study. The pair of isolated leptons, selected to be of the same charge, greatly reduce the backgrounds from other physics processes having much larger production cross sections. Events are then categorized into 1 and 2-jets categories, and further sub-categorized into two muons (same flavor) or a pair of electron and muon (different flavor) to maximize the signal sensitivity. The dominant backgrounds to the signal regions are WZ, non-prompt processes originating from W+jets production and heavy flavor charm or beauty decay, and $W\gamma^{\star}$ with photon conversion giving two leptons in which one of the final state lepton is undetected. The WZ background normalization is estimated from data control region, and the non-prompt processes are modeled with a data-driven method. Other minor backgrounds are modeled from simulation. The instrumental background such as the electron charge mis-assignment is estimated from data and taken into account for backgrounds containing same charge electron and muon pair. The non-prompt background in the signal regions is reduced by using a multivariate technique, exploiting the kinematical difference between the non-prompt and prompt leptons. The associated WH production cross section times the $H \rightarrow W^{+}W^{-}$ branching fraction, is $\mu_{wh} = 0.89_{-0.62}^{+0.64} (stat)_{-0.78}^{+0.74} (exp)$ times the Standard Model expectation, corresponding to an observed significance of 0.9 standard deviation. The study of the measurement is dominated by statistical and systematic uncertainties arising from the background modeling and theoretical model. Additionally, using the same dataset, an inclusive measurement of the Higgs boson production in association with a leptonically decaying Z or W boson (leptonic VH, V=Z,W) is performed by using sample of $H \rightarrow W^{+}W^{-}$ event candidates in which at least one W boson decays leptonically. The final states considered in the measurement are the associated WH production resulting in final states with two same sign leptons, and three leptons, and the associated ZH production yielding three and four leptons. The measured signal strength modifiers are consistent with Standard Model expectation in general, with large deviation observed in the associated ZH production with three lepton final state due to the background fluctuation. The combined result for the leptonic VH production signal strength is $\mu_{VH} = 1.85^{+0.47}_{-0.44}$ times their respective Standard Model expectation, with the observed significance of 4.7 standard deviation. The precision of the measurement is limited by statistics, and systematic effects from the non-prompt backgrounds. The measurement is also performed in the exclusive phase space bins in the so-called Simplifed Template Cross Section (STXS) framework. The signal strength modifiers are determined for the two STXS bins, defined in different regions in the associated vector boson transverse momentum. The STXS measurement for the leptonic VH production signal strength modifier is $\mu_{p_{T}^{V}~<~150 GeV} = 2.65^{+0.72}_{-0.66}$ and $\mu_{p_{T}^{V}~>~150 GeV} = 1.56^{+0.97}_{-0.88}$, with an observed significance of 4.7 and 1.8 standard deviation.