Abstract
A technique is presented to measure the efficiency with which c -jets are mistagged as b -jets (mistaggingefficiency) using tt ¯ events, where one of the W bosons decays into an electron or muon and a neutrino and the other decays into a quark–antiquark pair. The measurement utilises the relatively large and known W → cs branching ratio, which allows a measurement to be made in an inclusive c -jet sample. The data sample used was collected by the ATLAS detector at √ s = 13 TeV and corresponds to an integrated luminosity of 139 fb−1. Events are reconstructed using a kinematic likelihood technique which selects the mapping between jets and tt ¯decay products that yields the highest likelihood value. The distribution of the b -tagging discriminant for jets from the hadronic W decays in data is compared with that in simulation to extract the mistagging efficiency as a function of jet transverse momentum. The total uncertainties are in the range 3–17%. The measurements generally agree with those in simulation but there are some differences in the region corresponding to the most stringent b -jet tagging requirement.