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 𝜎𝑡̄𝑡 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 𝜎𝑡̄𝑡 = 850 ± 3(stat.) ± 18(syst.) ± 20(lumi.) pb. The ratio of the 𝑡̄𝑡 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 𝑡̄𝑡 cross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, 𝑅𝑡̄𝑡∕𝑍 = 1.145 ± 0.003(stat.) ± 0.021(syst.) ± 0.002(lumi.) is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.