Hunting the Charged Higgs Boson with Lepton Signatures in the ATLAS Experiment

This thesis presents searches for a charged Higgs boson ($H^\pm$) in proton-proton collisions with center-of-mass energies of 7 TeV and 8 TeV, using data collected by the ATLAS experiment at the Large Hadron Collider at CERN. Multiple search channels are used with the common characteristic of at least one charged lepton (electron or muon) that effectively reduces the multi-jet background and is used for efficient triggering. Charged Higgs bosons decaying to a tau lepton and a neutrino are searched for using final states with two charged leptons, or one charged lepton and a hadronically decaying tau. A significant background originates from quark- or gluon-initiated jets that may be misidentified as hadronic tau decays. Methods to estimate this background are presented, including a largely data-driven matrix method. Signal processes with a charged Higgs boson mass below or above that of the top quark are considered. With the dataset collected at a center-of-mass energy of 8 TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$, upper limits at 95% confidence level are placed on the branching fraction $B(t \rightarrow H^\pm) \times B(H^\pm \to \tau\nu)$ in the range 1.1-0.3% for charged Higgs boson masses between 80 GeV and 160 GeV, as well as on the top-quark associated $H^\pm$ production cross section in the range 0.93-0.03 pb for charged Higgs boson masses between 180 GeV and 1 TeV.