Measurement of btagging efficiency using ttbar events

Jet flavour tagging is one of the key ingredients of the diverse physics program of the CMS experiment. Various flavour taggers based on the lifetime and semileptonic decay of the heavy quarks have been implemented. They use jets, tracks, leptons and/or vertices. We exploit the \ttbar samples collected at $\sqrt {s}=7$ TeV running of the LHC during the first half of 2011, to extract the efficiencies for tagging the b-quark and c-quark jets. Event samples identified as semileptonic and fully-leptonic decays of the $t\overline t$ are used. Different techniques based on extraction of high purity b-flavoured jets, or on requiring the consistency between the observed and expected number of tags in the events are used to extract the performance of the heavy flavour algorithms directly from data and measure the differences between data and simulation.