Measurement of the tt¯ cross section and its ratio to the Z production cross section using pp collisions at sqrt(=) 13.6 TeV with the ATLAS detector

The inclusive top-quark-pair production cross section sigma_ttbar 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_ttbar = 850 +- 3_stat +- 18_syst +- 20_lumi pb. The ratio of the ttbar 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 ttbar cross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, R_ttbar/Z = 1.145 +- 0.003_stat +- 0.021_syst +- 0.002_lumi is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.