Search for New Physics in Fully Hadronic Di-tau Final States with the ATLAS Detector

Searches for new physics are one of the primary goals of the ATLAS experiment. Many extensions of the Standard Model predict new heavy gauge bosons, like $Z'$ bosons originating for example from an additional $\text{U}(1)'$ gauge group. In this thesis the decay of $Z'$ bosons into hadronically decaying $\tau^+\tau^-$ pairs is investigated. Data collected by the ATLAS detector during data taking in 2012 in proton-proton collisions at $\sqrt{s}=8~\text{TeV}$ is analysed, corresponding to $20.34~\text{fb}^{-1}$ of integrated luminosity. Systematic uncertainties on the tau lepton identification are studied at high transverse momenta, as expected in the decay of a heavy $Z'$. In order to obtain signal events for arbitrary $Z'$ masses and coupling structures, reweighting of $Z/\gamma^*\rightarrow\tau\tau$ to $Z'\rightarrow\tau\tau$ signal events is preformed and evaluated against simulated $Z'\rightarrow\tau\tau$ events. A new method for multi-jet background estimation is introduced for the fully hadronic $Z'\rightarrow\tau\tau$ process.