Measurements of b-jet tagging efficiency with the ATLAS detector using t t^¯ events at ^√s = 13 TeV

The efficiency to identify jets containing b-hadrons (b-jets) is measured using a high purity sample of dileptonic top quark–antiquark pairs (tt̄) selected from the 36.1 fb⁻¹ of data collected by the ATLAS detector in 2015 and 2016 from proton–proton collisions produced by the Large Hadron Collider at a centre-of-mass energy √s = 13 TeV. Two methods are used to extract the efficiency from tt̅ events, a combinatorial likelihood approach and a tag-and-probe method. A boosted decision tree, not using b-tagging information, is used to select events in which two bjets are present, which reduces the dominant uncertainty in the modelling of the flavour of the jets. The efficiency is extracted for jets in a transverse momentum range from 20 to 300 GeV, with data-to-simulation scale factors calculated by comparing the efficiency measured using collision data to that predicted by the simulation. The two methods give compatible results, and achieve a similar level of precision, measuring data-to-simulation scale factors close to unity with uncertainties ranging from 2% to 12% depending on the jet transverse momentum.