Measurement of production in association with additional $\textit{b}$-jets in the $\textit{eμ}$ final state in proton–proton collisions at = 13 TeV with the ATLAS detector

This paper presents measurements of top-antitop quark pair ( ) production in association with additional $\textit{b}$-jets. The analysis utilises 140 fb of proton–proton collision data collected with the ATLAS detector at the Large Hadron Collider at a centre-of-mass energy of 13 TeV. Fiducial cross-sections are extracted in a final state featuring one electron and one muon, with at least three or four $\textit{b}$-jets. Results are presented at the particle level for both integrated cross-sections and normalised differential cross-sections, as functions of global event properties, jet kinematics, and $\textit{b}$-jet pair properties. Observable quantities characterising $\textit{b}$-jets originating from the top quark decay and additional $\textit{b}$-jets are also measured at the particle level, after correcting for detector effects. The measured integrated fiducial cross-sections are consistent with predictions from various next-to-leading-order matrix element calculations matched to a parton shower within the uncertainties of the predictions. State-of-the-art theoretical predictions are compared with the differential measurements; none of them simultaneously describes all observables. Differences between any two predictions are smaller than the measurement uncertainties for most observables.