Measurement of the tt^- cross section and its ratio to the Z production cross section using pp collisions at √s = 13.6 TeV with the ATLAS detector

The inclusive top-quark-pair production cross section σ_tt^- and its ratio to the Z-boson production cross section have been measured in proton–proton collisions at √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_tt^- = 850 ± 3 (stat. ) ± (18 (syst.) ± 20 (lumi) pb. The ratio of the tt^- 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 tt^- cross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, R_{tt- / Z} =  1.145 ± 0.003 (stat.) ± 0.021 (syst.) ± 0.002 (lumi)   is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.