Measurement of the Cross Section for the Process $pp \to \gamma^{∗}/Z \to \tau^{+}\tau^{-}$ and a Search for Neutral MSSM Higgs Bosons with the ATLAS Detector

The search for Higgs bosons is among the primary goals of the ATLAS experiment. The minimal supersymmetric extension to the Standard Model contains three neutral Higgs bosons which are detectable through their decays into pairs of tau leptons. A major background for this discovery channel in the low mass region $M_{A} = 100-150 GeV$ arises from $\gamma^{*}/Z \to \tau^{+}\tau^{-}$ events. In this thesis, the cross section of the process $pp \to \gamma^{*}/Z \to \tau^{+}\tau^{-}$ is measured by investigating the fully leptonic final state. Therefore, $L = 35.5 pb^{-1}$ of data from proton-proton collisions recorded by the ATLAS detector at a centre-of-mass energy of 7TeV are analysed. The result obtained is $\sigma(pp \to \gamma^{*}/Z \to \tau^{+}\tau^{-})= 1.05 \pm 0.13(stat.)\pm 0.10(syst.)\pm 0.04(lumi.)nb$ and is in agreement with the theoretical prediction. Furthermore, the discovery channel $pp \to h/H/A \to \tau^{+}\tau^{-}$ is analysed in the purely leptonic final state for the low mass region $M_{A}=100-150 GeV$. An optimisation of the event selection by the usage of multivariate techniques is investigated. The optimal selection strategy is applied on $L = 1.06 fb({-1}$ of data recorded by the ATLAS detector. As the statistical analysis does not yield a significant deviation from the Standard Model expectation, exclusion limits in the $M_{A}\times tan \beta$ parameter space of the MSSM are derived. In the context of the $m_{h}^{max}$ scenario, this thesis concludes that values of $tan \beta > 32-51$ depending on the parameter $M_A$ are excluded at a 95% confidence level.