Flavour tagging calibration TOP BTAGGING 2014 2014 This note presents a new method to derive the $b$-jet tagging efficiency from data using a combinatorial likelihood approach applied to dileptonic $t\bar{t}$ events. Calibration results are obtained for the $pp$ collision data collected by the ATLAS experiment at the Large Hadron Collider at $\sqrt{s} = 8$ TeV, corresponding to an integrated luminosity of 20.3 fb$^{-1}$. The $b$-jet tagging efficiency measurements are provided in the form of jet transverse momentum dependent scale factors which correct the $b$-tagging performance in simulation to that observed in data. The scale factors have been measured in the range 20-300 GeV and have a total uncertainty $\sim 2\%$ for jets with transverse momenta around 100 GeV. This represents a significant improvement in precision compared to alternative calibration methods.