Measurement of the c-jet mistagging efficiency in $t\bar{t}$ t t

A technique is presented to measure the efficiency with which c -jets are mistagged as b -jets (mistagging efficiency) using $t\bar{t}$ 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 $W\rightarrow cs$ W → c s 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 $\sqrt{s} = 13$ s = 13   $\text {TeV}$ TeV and corresponds to an integrated luminosity of 139 fb $^{-1}$ - 1 . Events are reconstructed using a kinematic likelihood technique which selects the mapping between jets and $t\bar{t}$ 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.