From luminosity to cross-section measurements - Inclusive measurement of the $t\bar{t}$ cross-section using proton-proton collisions at $\sqrt{s}=13.6$ TeV with the ATLAS detector

The inclusive $t\bar{t}$ cross-section measurement at a centre-of-mass of 13.6 TeV is presented using 29 fb$^{-1}$ of data recorded by the ATLAS detector at the Large Hadron Collider in 2022. For a cross-section measurement it is necessary to count the events and measure the luminosity of the collider. Additionally, in this measurement, the luminosity uncertainty is the largest uncertainty. The luminosity determination for the 2022 dataset, crucial for numerous ATLAS physics analyses, and in particular for this one, follows the established procedure from Run 2. This methodology is based on three steps: the absolute luminosity calibration using van der Meer scans, the calibration transfer from the special beam conditions during the van der Meer scans to the physics data-taking conditions, and the stability of the luminosity determination. For every step the details of the corrections applied and how the uncertainties are estimated are explained. The preliminary luminosity uncertainty for the 2022 dataset amounts to 2.2\%, where the main uncertainties arise from the calibration transfer part and the limited knowledge of non-factorisation effects. The inclusive $t\bar{t}$ cross-section is determined using the $b$-jet counting method in the dilepton final state. This method obtains the cross-section and the $b$-jet efficiency simultaneously using $t\bar{t}$ events with exactly one or exactly two $b$-jets. Through a simultaneous profile likelihood fit of the two yields, the value of the measured $t\bar{t}$ cross-section is found. The measured inclusive $t\bar{t}$ cross-section value is $\sigma_{t\bar{t}} = 850 \pm 3 \mathrm{(stat.)}\pm 18\mathrm{(syst.)}\pm 20\mathrm{(lumi.)}$ pb, which is 1.5$\sigma$ lower than the Standard Model prediction. Additionally, the ratio of the $Z$ production in fiducial phase space and the inclusive \ttbar cross-section is presented. This ratio is sensitive to the gluon-to-quark PDF ratio and achieves good precision thanks to the cancellation of various systematic uncertainties. The ratio is found to be $R_{t\bar{t}/Z} = 1.145 \pm 0.003 \mathrm{(stat.)}\pm 0.021 \mathrm{(syst.)}\pm 0.002\mathrm{(lumi.)}$, consistent with the Standard Model prediction. Furthermore, the investigation includes an energy ratio of the inclusive \ttbar cross-section between 13.6 TeV and 13 TeV, amounting to $R_{13.6/13} = 1.0253 \pm 0.0347$, which is 2.4 standard deviations away from the predicted value. These comprehensive measurements contribute to our understanding of fundamental particle interactions and validate the Standard Model predictions, underlining the significance of precise luminosity determination and cross-section measurements in advancing the knowledge of particle physics.