Search for scalar top quarks decaying into scalar tau leptons with ATLAS at $\sqrt s $= 8 TeV

The Standard Model of particle physics (SM), completed by the discovery of the Higgs boson in 2012 at the Large Hadron Collider, provides a description of the known particles and theirstrong, weakandelectromagneticinteractions. The Standard model is a quantum field theory incorporating both quantum mechanics and special relativity and encapsulates in its mathematical formulation the known fundamental symmetries of nature. All the known matter is composed of elementary half-spin particles (fermions) whose interactions are mediated by integer spin particles (bosons). The SM theory, however, is thought to be far from being complete. In fact it only incorporates three of the four fundamental interactions leaving gravity out of the picture. In addition it is unable to explain a series of observations such as the presence of Dark Matter in the Universe and the matter-antimatter asymmetry. Among several theories that tried to solve the SM shortcomings, Supersymmetry (SUSY) is a theory which extends the symmetries of the SM theory allowing the interchange of fermions into bosons, predicting the existence of a superpartner for each SM particle. The fact that no superparticles have been observed yet implies that Supersymmetry, if it exists, has to be a broken symmetry at the energy scale we have been able to probe so far and the superpartner particles are thought to be heavier than their SM counterpart. The search for Supersymmetry has been carried out for many years now in a large variety of possible scenarios and at various particle colliders with different initial states. The current most powerful particle collider is the Large Hadron Collider (LHC), a two-ring superconducting, circular proton or heavy ion collider located at CERN, across the border between Switzerland and France, with a design proton-proton collision centre of mass energy of √s = 14 TeV. The "A Toroidal LHC ApparatuS" (ATLAS) experiment is one of the four experiments which records the collisions delivered by the LHC. During 2012 it recorded a 20.3 fb−1 dataset of p − p collisions delivered at a centre of mass energy of √ s = 8 TeV and in 2015 a dataset of 3.2 fb−1 at √s = 13 TeV. The LHC is foreseen to run for many years from now and an upgrade to the High-Luminosity LHC (HL-LHC) is foreseen in 2026, bringing the expected size of the collected p − p collisions dataset by ATLAS to 3000 fb−1. Therefore, the LHC and ATLAS provide an excellent experimental setup to probe the existence of physics beyond the Standard Model in the so far unexplored phase space. However, the quality of the results of an experimental physics analysis does not depend only on the size of the collected data or the centre of mass energy. Within the ATLAS experiment a lot of attention is dedicated to the understanding of the experimental setup in order to provide the best possible reconstruction chain from raw data recorded by the detector to final high-level physics analyses. The study and the improvement of the performance of the detector is a fundamental area of research whose results have a great impact on the whole experiment and its physics program.