Measurement of the Cross Section and its Ratio to the Production Cross Section Using Collisions at TeV with the ATLAS detector

The inclusive top-quark-pair production cross section ${\sigma }_{t\overline{t}}$ 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 ${\sigma }_{t\overline{t}}=850±3\left(\mathrm{stat}.\right)±18\left(\mathrm{syst}.\right)±20\left(\mathrm{lumi}.\right)$ pb. The ratio of the $t\overline{t}$ 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 $t\overline{t}$ cross section, thanks to the cancellation of several systematic uncertainties. The result for the ratio, ${R}_{t\overline{t}/Z}=1.145±0.003\left(\mathrm{stat}.\right)±0.021\left(\mathrm{syst}.\right)±0.002\left(\mathrm{lumi}.\right)$ is consistent with the Standard Model prediction using the PDF4LHC21 PDF set.