Search for supersymmetry in events with two same-sign leptons or three leptons with the ATLAS detector at $\sqrt{s}$ = 13 TeV

Supersymmetry is one of the possible extensions of the Standard Model of particle physics and has already been extensively studied during the recent years. In many supersymmetric models, the lightest supersymmetric particle, which is typically the lightest neutralino $\widetilde{\chi}_{1}^{0}$, is stable. Thus, it can be a promising candidate for dark matter. This thesis presents a search for supersymmetric phenomena in final states with either two leptons of the same electric charge or three leptons, together with jets and missing transverse energy. While this event signature is present in various supersymmetric scenarios, Standard Model processes leading to such event signatures have typically very low production rates. Therefore, this analysis benefits from a small Standard Model background in the signal regions leading to a good sensitivity, in particular for scenarios with compressed mass spectra of the supersymmetric parameter space. The search was performed with the full dataset collected at $\sqrt{s}$ = 13 TeV with the ATLAS detector during the years 2015 and 2016, corresponding to a total integrated luminosity of 36.1 fb$^{-1}$. No significant excess above the Standard Model prediction has been observed. Accordingly, the results were used to set either model-independent upper limits on new physics signals or interpreting them in the context of a large variety of supersymmetric benchmark processes. The model-dependent interpretations allowed to set exclusion limits on the superpartner masses in new supersymmetric scenarios as well as improving existing limits obtained from previous searches.