Search for the electroweak production of supersymmetric particles in three-lepton events at the ATLAS detector with focus on compressed mass spectra

This thesis presents a search for the electroweak production of supersymmetry using the dataset taken by the ATLAS detector at the Large Hadron Collider with √8 = 8 TeV during 2012. Events with three leptons are selected and required to satisfy additional kinematic criteria that define optimised signal regions. In these signal regions, Standard Model processes are discriminated against, whilst retaining a large fraction of events produced by specified compressed supersymmetry scenarios. Compressed refers to near massdegeneracy between the decaying gauginos and the final state gauginos. The expected number of Standard Model events are estimated using a combination of Monte Carlo and data-driven methods, where the predictions are tested against data in specifically designed validation regions. Exclusion limits are then set at 95% confidence level (CL) on via ℓ`L- and via WZ-decay scenarios for the decaying charginos and neutralinos. With the ℓ`L halfway between the decaying charginos and neutralinos and the final state neutralinos, there is a new sensitivity up to 250 GeV. In scenarios with large mass splitting, the decaying chargino and neutralino masses are excluded up to 740 GeV. Looking forward to the 13 TeV data-taking, a search for the electroweak production of supersymmetry with threelepton final states is presented, with a first look at an optimisation strategy to improve sensitivity to charginos and neutralinos.