Simultaneous efficiency measurements of $b$- and $c$-jets in $t\bar{t}$ events from $\sqrt{s}=13.6$ TeV $pp$ collision data collected with the ATLAS detector

This paper reports simultaneous measurements of the efficiencies with which $b$-jets are identified, and $c$-jets are misidentified as $b$-jets, by the single-transformer-based GN2 tagger that is currently used in the ATLAS experiment at the LHC. The measurements use $t\bar{t}$ 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, together with the known $t\to bW$ and $W\to cs$ branching fractions. The data were collected from $\sqrt{s}=13.6$ TeV proton-proton collisions by the ATLAS detector and correspond to an integrated luminosity of 56 fb${}^{-1}$. Events are reconstructed using a kinematic likelihood technique which maps the jets to the $t\bar{t}$ decay products. The $b$-jet tagging efficiency is measured from jets arising from leptonically decaying top quarks and the $c$-jet mistagging efficiency is obtained from jets arising from hadronic $W$ decays. The efficiencies are measured as a function of jet transverse momentum in intervals of the tagging-variable distribution. The $b$-jet ($c$-jet) tagging efficiency measurements reach a precision of better than 2% (5%). Each $b$-jet efficiency is combined with an independent measurement using $t\bar{t}$ events where both $W$ bosons decay leptonically.