Calibrating the b-Tag Efficiency and Mistag Rate in 35 pb^{-1} of Data with the ATLAS Detector

The b-tag efficiency and mistag rate of two b-tagging algorithms have been measured with a number of complementary methods using data from the ATLAS detector. Two of the b-tag efficiency measurements are based on a sample of jets containing muons while the remaining three measurements make use of a sample of ttbar events. Two of the ttbar methods make a simultaneous measurement of the b-tag efficiency and the ttbar production cross section. The measurement of the mistag rate is performed on an inclusive jet sample and includes two methods, one which uses the invariant mass spectrum of tracks associated with reconstructed secondary vertices to separate light- and heavy-flavour jets, and one which is based on the rate at which secondary vertices with negative decay length significances, or tracks with negative impact parameter significances, are present in the data. The measurements of the b-tag efficiency and mistag rate are provided in the form of jet-pt dependent scale factors that correct the b-tagging performance in simulation to that observed in data. Good consistency is observed between the results of all methods. The b-tag efficiency scale factor is found to be close to one in all bins of jet pt. The precision of the measurement depends on the chosen method. As an example, the relative uncertainties from the muon-based methods range from 5% to 14% depending on jet-pt bin and tagging algorithm. For light-flavour jets, the simulation underestimates the tag rate by up to 30% in the lowest jet-pt bins, with the agreement improving for higher jet pt.