Searches for Supersymmetry in Di-Lepton Final States with the ATLAS Detector at √s = 7 TeV

The Standard Model (SM) of particle physics has been shown to describe, to great precision, many of the phenomena observed in Nature. With the recent discovery of the Higgs boson even the last missing piece of the SM is confirmed experimentally. Nevertheless, the SM has its limitations. It does not contain an explanation of the dark matter and dark energy, proven to constitute most of our Universe, nor does it incorporate the gravitational force. The SM also contains several parameters whose values are not predicted by the theory and therefore need to be measured experimentally. The need for developing an extension of the SM, in order to answer all its questions and puzzles, has occupied theorists for many decades already. One of the most believed theories is Supersymmetry (SUSY). It postulates that every SM particle has a supersymmetric partner, similar in all respect except 1/2 difference in spin. As no such particles are yet discovered SUSY needs naturally to be a broken symmetry, making the SUSY particles somewhat heavier than their SM partners. SUSY, with its large number of parameters, gives a wide spread in possible phenomenological models, utterly interesting for experimental particle physicists. One of the main topics of the ATLAS experiment at the Large Hadron Collider (LHC) is to reveal if Nature is indeed supersymmetric or not. This thesis concentrates in particular on the searches for direct slepton and gaugino production in final states with two leptons and missing transverse energy using the pp collision data collected by the ATLAS experiment during 2011 at a center of mass energy of 7 TeV. However, before searching for the unknown one needs to understand what is known. The LHC operates at energies larger than any other particle physics experiment ever has done before. All SM processes, possibly serving as backgrounds to the searches for new physics, must therefore be properly understood. A comprehensive study regarding the estimation of the SM backgrounds to SUSY searches stemming from fake leptons, i.e. leptons from decays of heavy- and light-flavoured hadrons and conversion processes, using the Matrix Method, is covered in this thesis. No excess of events above what was expected from the SM was observed, and upper limits on the cross-section and masses of new phenomena and particles have consequently been calculated, extending the existing limits.