Searching for new physics in events with three leptons in ATLAS

We investigate the potential of the ATLAS detector to discover trilepton final state signatures from decays of supersymmetric particles at the LHC. An inclusive and an exclusive trilepton search have been conducted for a range of different mSUGRA scenarios, in the focus point region, in the bulk region, and at the edge of the current experimental limits on SUSY. It is found that a simple inclusive selection, based on the presence of three leptons and at least one high-$p_T$ jet in the final state, has the potential to be an excellent candidate for an early physics programme at the LHC. The exclusive trilepton selection, optimised for the focus point region, relies on stringent lepton track isolation and also includes cuts on variables such as transverse missing energy and on the hadronic activity in the event. In a heavy SUSY scenario, where all scalar and coloured sparticles may be too heavy to be observed at the LHC, hadronically quiet trilepton final states could be amongst the few accessible channels to discover SUSY.