Search for supersymmetry in multilepton events with the ATLAS detector

A search for Supersymmetry in events with three leptons (electrons or muons) and miss- ing transverse momentum is presented. The observation of a significant excess of events with three leptons in the final state with respect to the prediction of the Standard Model would be a hint of New Physics. A sample with an integrated luminosity of L = 2 : 06 fb-1 of proton-proton collision data at a centre-of-mass energy of p s = 7 TeV delivered by the LHC and recorded by the ATLAS detector in 2011 is used. Special focus is placed on the composition of the Standard Model background and the measurement of misidentifica- tion rates of electrons and muons. The misidentification rates are determined from Monte Carlo simulated samples. The misidentification rates obtained for electrons yield values between 20% and 6%, depending on the transverse momentum and the pseudorapid- ity of the electrons, while for muons values between 42% and 10%, depending on the transverse momentum of the muons, are obtained. Two different selections are inves- tigated, one selection vetoing events with the presence of Z bosons, the other selection requiring the presence of a Z boson. The observations of 32 events with an expectation of 26 5 events due to processes of the Standard Model and 95 events with an expectation of 72 15 events, respectively, are interpreted in a phenomenological Minimal Supersym- metric Standard Model and in simplified supersymmetric models. Limits are placed in the mass parameter space of these models.