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 differences) must be measured in order to determine the underlying SUSY parameters. Therefore, exclusive SUSY decay cascades with two leptons in the final state are isolated by the flavour subtraction method, in order to fit the endpoint of the invariant mass distribution of these leptons and determine SUSY particle mass differences. This analysis uses a data sample collected during the first half of 2011, corresponding to an integrated luminosity of 1 fb-1 of √(s)=7 TeV proton-proton collisions recorded with the ATLAS detector at the Large Hadron Collider. Since no significant same flavour excess is observed, the variable S, which is the measure of a same-flavour excess, is used to determine model-independent and model-dependent limits for different SUSY scenarios. The tightest limits can be set for models expecting exactly two opposite-sign same-flavour leptons and missing transverse momentum larger than 250 GeV. Assuming no combinatorial SUSY background events from different decay chains (ll'), models with di-lepton decays (ll) with SUSY same-flavour excess Ss ≥4.5 can be excluded at 95% CL. Considering a combinatorial SUSY background contribution with a ratio BR(ll')/BR(ll)=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 Λ=40 TeV exceeding the current LEP limits.