Measuring the b-tag efficiency in a top-pair sample with 4.7 fb^-1 of data from the ATLAS detector

Many physics analyses in ATLAS depend on the reconstruction of b-jets. Aside from reliable b-tagging algorithms, an appropriate description of the b-tagging efficiencies based on measurements with data is essential for correctly modelling the measurements in Monte Carlo simulation. Furthermore, the systematic uncertainties need to be estimated realistically. Since many analyses at a centre of mass energy 7 TeV include high p_T jets, it is desirable to calibrate the b-tagging algorithms at comparable jet energies. With the large data sample collected in 2011 of 4.7 fb^-1, new calibration methods based on top-pair events have been developed. The measurements of the b-tagging efficiency are provided in the form of jet transverse momentum dependent scale factors that correct the b-tagging performance in simulation to that observed in data. For all b-tagging algorithms calibrated, the scale factors measured with the various top-pair based methods are in good agreement with each other. The total uncertainties range from 5 % to 15 % for jet transverse momentum in the range 25 GeV to 300 GeV.