Search for Supersymmetry in Multileptonic Final States with Collimated $\tau$ Pairs with the ATLAS Detector at the LHC

The search for supersymmetry (SUSY) is a major part of the ATLAS physics program. Due to the low Standard Model background the search for four-lepton final states provides excellent sensitivity to R-parity violating SUSY models where the lightest supersymmetric particle (LSP), produced in pairs, decay into two charged leptons and a neutrino. For LSP decays into hadronically decaying $\tau$ pairs, however, the current analysis is not sensitive if the mass difference between the LSP and the next heavier supersymmetric particle is large, because the $\tau$ jet become highly collimated and the standard $\tau$ pair reconstruction method is not able to resolve them. A new specialized collimated $\tau$ pair reconstruction method has been developed for LHC run-2 at 13~TeV center-of-mass energy. In this thesis, the new method is evaluated and optimized for the search for SUSY in four-lepton final states. It is shown, that exclusion limits can be extended to lower LSP masses with the new di-$\tau$ reconstruction. Furthermore, the sensitivity of the four-lepton search to R-parity conserving scenarios is evaluated. It is shown, that the sensitivity to such scenarios can be increased with a new optimized signal region.