Search for Supersymmetry with the ATLAS detector at the LHC

The search of SUSY is an important part of the physics program at CERN's LHC. The large amount of data successfully collected by the experiments so far, provide a unique opportunity to explore SUSY with novel analysis techniques. If particles predicted by SUSY exist and are not extremely massive, they must be produced at LHC collision events and could be hiding in data collected by the ATLAS and CMS detectors. However, unlike most processes at the LHC, which are governed by strong force interactions, electroweak superpartners would be created through the much weaker electroweak interaction, thus lowering their production rates. Further, most of these new SUSY particles are expected to be unstable and can be searched by tracing their decay products { typically into a known SM particle and a LSP, which could be stable and non-interacting, thus forming a natural dark matter candidate. Despite the low production cross-section of electroweak SUSY processes, events with many leptons in the nal state are very interesting since SM processes mimicking them are very rare. ATLAS searched for such multi-lepton events and presented the results in terms of the number of events from new physics processes with a four-lepton signature, and also in terms of RPV and RPC SUSY models. All data yields are found to be consistent with SM expectations and results are thus used to set upper limits on the event yields from processes beyond the SM. Stringent exclusion limits are set in simplied models of GGM SUSY with higgsino particles, which are assumed to decay into either Higgs or Z bosons. In RPV simplied models with decays of the LSP to charged leptons, exclusions are placed on the mass of wino, slepton and gluino masses up to the TeV scale, extending signicantly the lower limit results set by previous ATLAS searches. Until recently, an important superpartner of the tau slepton, the "stau", had yet to be searched for beyond the exclusion limit of around 90 GeV found at the LHC's predecessor at CERN, the LEP collider. A light stau, if it exists, could play a role in neutralino co-annihilation, moderating the amount of dark matter in the visible universe, which otherwise would be too abundant to explain astrophysical measurements. The search for a light stau is experimentally challenging due to its extremely low production rate in LHC pp collisions, requiring advanced techniques to reconstruct the SM tau leptons it can decay into. In fact, during the LHC Run 1 data taking period, only a narrow parameter region around a stau mass of 109 GeV and a massless lightest neutralino could be excluded by LHC experiments. In LHC Run 2, ATLAS search eorts target the direct production of a pair of staus, each decaying into one tau lepton and one invisible LSP. The ATLAS data do not reveal hints for stau pair production and thus new exclusion limits are set on the mass of staus using dierent assumptions on the presence of both possible stau types (left and right, referring to the two dierent spin states of the tau partner lepton). The limits obtained supersede signicantly the LEP results and are in fact the strongest obtained so far in collider experiments. Overall, both sets of results place strong constraints on important supersymmetric scenarios, which will guide future ATLAS searches. Further, they illustrate the benets brought by advanced analysis techniques, which help improve the sensitivity to new physics phenomena at colliders in the future.