Search for same-sign dilepton signatures with the ATLAS detector

Abstract: This thesis presents the work I did for the inclusive search for anomalous production of same charge lepton-pairs, originating from the collision point, at sqrt(s) = 8 TeV with the ATLAS detector at the LHC using the 2012 data. I optimized the selection criteria for isolated electrons and muons, calculated the so-called "fake factors" and "fake factor" systematic uncertainties for the muon selection. The fake-factor is used to estimate the contribution in the region of interest due to muons coming from secondary vertices, i.e. background muons. Furthermore I extracted limits of the cross-section of new sources of isolated, same-sign leptons originating from the collision point. The isolation criteria and the limits are used in the paper on the search for anomalous production of same-sign lepton-pairs originating from the collision point, which will be published soon. I found that for this analysis the best background rejection while still achieving a good signal efficiency is accomplished by using isolation criteria based on both calorimeter and tracking information. The muon fake factors are calculated in a data-driven way using muon pairs having the same charge. The fake factors obtained using the new selection criteria for isolated muon tracks are higher than when the isolation criteria from the previous analysis is used, but the new criteria also accepts substantially more true same-sign muons originating from the collision point, resulting in a lower ratio of muons originating from secondary vertices to muons originating from the collision point in the region of interest. As we are working on publishing the results it was not possible to analyse the region of interest because of internal ATLAS restrictions. Hence it is not known whether the data agrees with the predictions from known physics processes. If no excess is found in the number of lepton pairs I will put limits on the cross section of new resonances. At the moment I am only allowed to calculate the expected limits.