Search for charged Higgs Bosons decaying via $H^+ \to \tau_{lep.} + \nu$ in $t\bar{t}$ events at $\sqrt{s}=7 TeV$ in ATLAS

This thesis for the degree of Master of Science by Research presents the theory, methodology and results of a search for charged Higgs bosons decaying via ${H^+ \to \tau_{lep.} + \nu}$ using single lepton and two lepton channels in ${t}$ quark pair (${t\bar{t}}$) events with a leptonically decaying ${\tau}$ in the final state based on ${1.03\textrm{ fb}^{-1}}$ of proton-proton collision data at centre of mass energy ${\sqrt{s}=7\textrm{ TeV}}$ from ATLAS, an experiment of the LHC, with special attention given to the statistical analysis and calculation involved in the limit setting process. For the single lepton channel, the expected number of Standard Model-like\linebreak${t\bar{t} \to b\bar{b}W^{+}W^{-}}$ background events lies between $0.99$ and $1.03$ times the Standard Model prediction, with uncertainties in the range ${2\%}$ -- ${3\%}$. For the two lepton channel, the expected number of Standard Model-like background events lies between $0.99$ and $1.03$ times the Standard Model prediction, with uncertainties in the range ${5\%}$ -- ${25\%}$. Assuming the branching fraction ${\mathcal{B}\left(H^{+} \to \tau\mu\right)=1}$, the upper limits on the branching fraction ${\mathcal{B}\left(t \to bH^{+}\right)}$ at the $95\%$ confidence level are between ${5.2\%}$ and ${14.1\%}$ for charged Higgs boson masses in the range ${90\textrm{ GeV}\leq m_{H^{+}}\leq 160\textrm{ GeV}}$. In the context of the ${m^{\textrm{max.}}_{h}}$ scenario of the MSSM, values of ${\tan\beta}$ greater than ${30}$--${56}$ are excluded in the mass range ${90\textrm{ GeV}\leq m_{H^{+}}\leq 140\textrm{ GeV}}$. The compatibility with background of the combination of the single lepton and two lepton channels ranges between ${26\%}$ and ${50\%}$, as measured by ${p_{0}}$ values. Hence, no indication of a ${H^{+}}$-like excess is found.