Measurement of the c-jet mistagging efficiency in t ¯t events using pp collision data at √s = 13 TeV collected with the ATLAS detector

A technique is presented to measure the efficiency with which c-jets are mistagged as b-jets (mistagging efficiency) using t ¯t 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 t ¯t 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.