Development of a data based algorithm for measuring the mistag rate of $b$-quarks at the ATLAS experiment

The ability to identify jets containing $b$-hadrons is important for the high-$p_{T}$ physics programme of a general-purpose experiment at the LHC such as ATLAS. Three tagging algorithms taking advantage of the accurate measurement of the transverse impact parameter of charged tracks with respect to the reconstructed primary vertex measured in the ATLAS Inner Detector have been developed and are particularly important for early data. One quantity to characterise the performance of these algorithms is the mistag rate. In order to determine the mistag rate using data measured with the ATLAS detector, an algorithm is developed and tested. The results of these tests prove the reliability of developed algorithm. Subsequently, the mistag rate is measured in the early 7 TeV collision data recorded in 2010, corresponding to an integrated luminosity of about 15 nb$^{-1}$. The results of the mistag rate determination shows, that mistag rates in simulated data are underestimated.