Search for a heavy charged Higgs boson decaying into a <em>W</em> boson and a 125 GeV Higgs boson, in final states with leptons and jets, with the ATLAS experiment at the LHC

This thesis presents a first search for a heavy charged Higgs boson produced in association with a top- and a bottom-quark, and decaying into a <em>W</em> boson and a 125 GeV Higgs boson decaying subsequently into a pair of b-quarks. The search is performed in final states with leptons and jets using proton-proton collision data at a center-of-mass energy of 13 TeV, recorded with the ATLAS detector during Run 2 of the LHC at CERN. This dataset corresponds to a total integrated luminosity of 140 fb^-1. Multivariate analysis techniques are used to reconstruct the charged Higgs boson candidate, and to improve the background rejection. The reconstructed invariant mass distribution of the charged Higgs boson candidate is scrutinised for an excess in data events in the mass range from 250 GeV to 3 TeV. No significant excess is observed, and 95% confidence level upper limits are set between 2.77 pb and 1.18 fb on the production cross section times branching ratio of the charged Higgs boson decays.