A wide range of proposed extensions to the SM, including supersymmetry, predict enhanced rates of collision events at the LHC containing two leptons of equal charge. Due to the short life-time of most theorized BSM particles, these leptons are typically produced directly at the interaction point. As the SM cross-section for prompt same-sign lepton pairs is relatively small, these events form an interesting signal to search for BSM physics at the LHC.
In this talk I will focus on a cross-section measurement for pompt same-sign muon pairs in ATLAS, performed on 2.3 fb-1 of 7 TeV data. The main challenge for such measurements is to reject and quantify the large background from muons produced in the decay of longer lived particles such as B- or D-mesons. This is usually done by imposing strong isolation requirements on the muons, a quantity which depends strongly on the event topology and therefore on the properties of possible new physics.
As an alternative I used a more model independent approach based on fitting the impact parameter distribution of the muons to determine the prompt fraction, leading to an inclusive fiducial cross-section measurement which is independent of muon isolation. I will discuss the validation of this method as well as compare the final result with those obtained with the isolation requirement.
