Measurement of the $t\bar{t}$ cross section and its ratio to the Z production cross section using pp collisions at $\sqrt{s}$ = 13.6 TeV with the ATLAS detector

The inclusive top-quark-pair production cross section $σ_{t\bar{t}}$ 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 $σ_{t\bar{t}}$ = 850±3(stat.)±18(syst.)±20(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 $μ^+μ^−$ 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±0.003(stat.)±0.021(syst.)±0.002(lumi.) is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.