Search for charged Higgs bosons decaying into top and bottom quarks with single-lepton final states using $pp$ collisions collected at a centre-of-mass energy of 13 TeV by the ATLAS detector

This thesis presents a search for charged Higgs bosons produced in proton–proton collisions at a centre-of-mass energy of 13 TeV, using 36.1 fb$^{−1}$ of data collected by the ATLAS detector at the LHC in 2015 and 2016. The existence of charged Higgs bosons is predicted by various theories Beyond the Standard Model and it is motivated by the inadequacy of the Standard Model to explain some observed experimental phenomena. The work focuses on charged Higgs bosons heavier than the top quark and decaying via $H$± → $tb$. The production in association with a top and a bottom quark ($pp$ → $tbH$$^{±}$) is investigated in the mass range between 200 and 2000 GeV. A final state containing one charged lepton and jets is considered. Multiple kinematic variables are combined using a boosted decision tree (BDT) in order to separate signal and background. The output of the BDT is used to perform a profile likelihood fit of the Monte Carlo predictions to the observed data. No significant excess of events above the expected Standard Model background is observed, therefore upper limits are set for the cross-section of the charged Higgs boson production times the branching fraction of its decay.Limits are also provided for the tanβ parameter of the MSSM, in the m$^{mod-}_{h}$ and hMSSM benchmark scenarios. The work improves the reach of all previous searches, including for the first time masses ranging up to 2000 GeV.