Exclusive search for supersymmetry with same-flavour di-lepton final states with the ATLAS detector

Supersymmetry (SUSY) is one of the most promising extensions of the Standard Model of particle physics. It introduces a new symmetry between fermions and bosons by adding a bosonic superpartner to each SM fermion and a fermionic one to a each SM boson. If an excess of SUSY like signal is observed, SUSY particle properties (e.g. masses or mass dierences) must be measured in order to determine the underlying SUSY parameters. Therefore, exclusive SUSY decay cascades with two leptons in the nal state are isolated by the avour subtraction method, in order to t the endpoint of the invariant mass distribution of these leptons and determine SUSY particle mass dierences. This analysis uses a data sample collected during the rst half of 2011, corresponding to an integrated luminosity of 1 fb􀀀1 of p s = 7TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider. Since no signicant same avour excess is observed, the variable S, which is the measure of a same-avour excess, is used to determine model-independent and model-dependent limits for dierent SUSY scenarios. The tightest limits can be set for models expecting exactly two opposite-sign same-avour leptons and missing transverse momentum larger than 250 GeV. Assuming no combinatorial SUSY background events from dierent decay chains (``0), models with di-lepton decays (``) with SUSY same-avour excess Ss 4:5 can be excluded at 95% CL. Considering a combinatorial SUSY background contribution with a ratio BR(``0)=BR(``) = 50% (100%) models with Ss 5:5 (6.7) can be excluded at 95% CL. For the GMSB model with a slepton NLSP, this translates into a limit of the GMSB parameter = 40TeV exceeding the current LEP limits.