Searches for supersymmetry in events with three leptons at the ATLAS detector

This thesis presents searches for supersymmetry in $\sqrt{s} =7$ TeV and 8 TeV proton-proton collisions at the LHC using data collected by the ATLAS detector in 2011 and 2012. Events with exactly three electrons or muons and missing transverse energy were selected, since these can be a striking signature of supersymmetry with low Standard Model back- ground. Additional kinematic criteria were investigated and applied in order to increase the sensitivity to supersymmetric signals. Standard Model backgrounds were estimated by a combination of Monte Carlo simulations and data-driven techniques. The agreement between data and background predictions was validated before analysing data in the signal regions. No significant excess was recorded in any of the analyses performed: the results were therefore interpreted as lower bounds on the masses of supersymmetric particles in a variety of different model scenarios.