A search for charged Higgs boson with the ATLAS detector at the LHC

This dissertation considers the search for a charged Higgs boson (H±) in proton-proton collisions with centre-of-mass energies of 7 TeV and 8 TeV, using data collected by the ATLAS experiment at the Large Hadron Collider at CERN. The discovery of such a charged Higgs boson would be a clear indication for physics beyond the Standard Model. The first data collected during the LHC Run-I is analysed, searching for a heavy charged Higgs bosons (above the top quark mass). This study covers the gb -> tH± ->ttb where one top-quark decays leptonically and the other decays hadronically. A significant background originating from tt ̄ processes, in addition to others, may conceal the charged Higgs decay in the data. Methods using boosted decision trees to con- strain such backgrounds and provide signal to background discrimination are presented. Upper limits on the production cross section times branching ratio of H± are derived for the mass range of 200 GeV <=mH± <= 600 GeV for six MSSM benchmark scenarios.