Search for direct scalar top pair production in final states with two tau leptons in pp collisions at $\sqrt{s}=8$\ TeV with the ATLAS Detector at the Large Hadron Collider

The ATLAS Experiment at the CERN Large Hadron Collider is a particle physics experiment to study fundamental particles and their interactions at very high energies. Supersymmetry is a theory of new physics beyond the Standard Model of particle physics. A search for directly produced pairs of the supersymmetric partner of the top quark was performed using 20 fb$^{-1}$ of proton--proton collision data at a centre of mass energy of $8$ TeV taken in 2012. The search targeted a model where the supersymmetric partner of the top quark (``scalar top'') decays via the supersymmetric partner of the tau lepton (``scalar tau'') into the supersymmetric partner of the graviton (``gravitino''). Scalar top candidates were searched for in $pp$ collision events with either two hadronically decaying taus, two light leptons (electrons or muons), or one hadronically decaying tau and one light lepton. The numbers of events passing the analysis selection criteria agree with the Standard Model expectations. Exclusion limits at the $95\%$ confidence level were set as a function of the scalar top and scalar tau masses. Depending on the scalar tau mass, ranging from the $87$ GeV limit set by the LEP experiments to a few GeV below the scalar top mass, lower limits between $490$ GeV and $640$ GeV were placed on the scalar top mass within the model considered.