Measuring the b-jet tagging efficiency using top anti-top events with ATLAS data

Many physics analyses with the ATLAS detector 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 understand their performance with data-driven measurements of efficiencies and fake rates. Since the top quark almost exclusively decays to a W boson and a b-quark, a sample of top anti-top events 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 counting the number of b-tagged jets or by applying kinematical fit methods to extract a pure sample of b-jets. The calibration methods based on top anti-top events are especially important because they can provide measurements of the b-tagging efficiency also for jets with high momentum.