Measurement of $t\bar{t}$ and $Z$-boson cross sections and their ratio using $pp$ collisions at $\sqrt{s} = 13.6$ TeV with the ATLAS detector

The inclusive top-quark-pair production cross-section $\sigma_{t\bar{t}}$ and fiducial $Z$-boson production cross-section $\sigma_{Z\to\ell\ell}^{\mathrm{fid.}}$ have been measured in proton–proton collisions at $\sqrt{s} = 13.6$ TeV, using 11.3 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 $\sigma_{t\bar{t}} = 859 \pm 4 \mathrm{(stat.)}\pm 22\mathrm{(syst.)}\pm 19\mathrm{(lumi.)}$ pb. The $Z$-boson production cross-section in a fiducial phase space is measured in $e^+e^-$ and $\mu^+\mu^-$ final states. The measured cross-section is $\sigma_{Z\to\ell\ell}^{\mathrm{fid.}} = 751 \pm 0.3 \mathrm{(stat.)}\pm 15\mathrm{(syst.)}\pm 17\mathrm{(lumi.)}$ pb. Additionally, a measurement of the ratio of the $t\bar{t}$ and the $Z$-boson production cross-sections, which benefits from cancellation of several systematic uncertainties, is reported. The result for the ratio, $R_{t\bar{t}/Z} = 1.144 \pm 0.006 \mathrm{(stat.)}\pm 0.022 \mathrm{(syst.)}\pm 0.003\mathrm{(lumi.)}$ is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.