Measurement in the Di - Lepton Decay Channel

— We present simulations of the production cross-section measurement of top-antitop pairs in the di-leptonic decay channel with the ATLAS detector. This study uses the Commissioning Service Challenge (CSC) data, which is the latest and centrally produced Monte-Carlo data set to validate the detector simulation before the actual data taking. The signal process was generated with MC@NLO [1] and important background processes were studied. A cut and count method and two likelihood methods were employed to measure the cross section and important systematic effects were investigated. The expected statistical and systematic errors for a luminosity of 100 pb −1 are also given. 1. – Di-lepton signature and background processes The characteristic di-lepton signature is given by two opposite charged leptons(1) from the W decay and at least two jets from the decay of the two b-quarks as well as a large amount of missing transverse energy (/ E T) from the neutrinos. The leptonically decaying Z in Drell-Yan events is an important background process for the di-leptonic subchannels with same flavor leptons. A very similar signature to the signal is produced by di-boson (W/Z) decays. Background processes that arise by misidentified leptons are mainly t ¯ t lepton+jets, W +jets and QCD background events. Also single top events have been investigated. 2. – Cross section measurement methods 2. 1. Cut and count method. – A simple cut based analysis on the lepton and jet p T and on / E T was optimized for the best S/ √ S + B. Events with same flavored leptons and (1) The lepton identification is decribed in Ref. [2]. Additionally isolation from energy depo-sitions in a hollow cone of ∆R < 0.2 of 6 GeV is required for electrons and isolation from reconstructed jets within ∆R < 0.2 is required for muons.