The inclusive top-quark-pair production cross section $\sigma_{ t\bar{ t}}$ and its ratio to the $Z$-boson production cross section have been measured in proton--proton collisions at $\sqrt{ s} = 13.6$ TeV, using 29 fb${ }^{ -1}$ of data collected in 2022 with the ATLAS experiment at the Large Hadron Collider. Using events with an opposite-charge electron-muon pair and $b$-tagged jets, and assuming Standard Model decays, the top-quark-pair production cross section is measured to be $\sigma_{ t\bar{ t}} = 850 \pm 3\mathrm{ (stat.)}\pm 18\mathrm{ (syst.)}\pm 20\mathrm{ (lumi.)}$ pb. The ratio of the $t\bar{ t}$ and the $Z$-boson production cross sections is also measured, where the $Z$-boson contribution is determined for inclusive $e^+e^-$ and $\mu^+\mu^-$ events in a fiducial phase space. The relative uncertainty on the ratio is reduced compared to the $t\bar{ t}$ cross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, $R_{ t\bar{ t}/Z} = 1.145 \pm 0.003\mathrm{ (stat.)}\pm 0.021\mathrm{ (syst.)}\pm 0.002\mathrm{ (lumi.)}$ is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.
Categories: hep-ex