Measurement of the b-jet tagging efficiency using top quark pair events with ATLAS data

Many physics analyses with the ATLAS data at the LHC expect to have jets originating from b-quarks in the final state. Algorithms that allow to identify such jets are thus of great importance and it is crucial to study their performance directly in data by measuring the tagging efficiencies and fake rates. Since the top quark almost exclusively decays to a W boson and a b-quark, a sample of top quark pair events (tt ̄) is ideal for studying the b-tagging performance. Final states containing one or two leptons have been used to measure the b-tagging efficiency, either by count- ing the number of b-tagged jets, by exploiting the kinematics of top quark pair decays and flavour composition of studied sample or by applying a kinematic fit to extract a sample rich in b-jets. The calibration methods based on top quark pair events are especially important because they can provide measurements of the b-tagging efficiency also for jets with high transverse momentum. Three different methods using two statistically independent selections of tt ̄ events according to the single lepton and dilepton decays are presented. For all b-tagging algorithms calibrated, the scale factors measured with these methods are in good agreement with each other. The total uncertainties range from 5% to 15% for jet pT in the range from 25 GeV to 300 GeV.