Measurement of the $t\bar{t}$ production cross-section and lepton differential distributions in $e\mu$ dilepton events from $pp$ collisions at $\sqrt{s}=13$ TeV with the ATLAS detector

The inclusive top quark pair ($t\bar{t}$) production cross-section $\sigma_{t\bar{t}}$ has been measured in proton-proton collisions at $\sqrt{s}=13$ TeV, using 36.1 fb$^{-1}$ of data collected in 2015-16 by the ATLAS experiment at the LHC. Using events with an opposite-charge $e\mu$ pair and one or two $b$-tagged jets, the cross-section is measured to be: \begin{equation}\nonumber \sigma_{t\bar{t}} = 826.4 \pm 3.6\ \mathrm{(stat)} \pm 11.5\ \mathrm{(syst)} \pm 15.7\ \mathrm{(lumi)} \pm 1.9\ \mathrm{(beam)}\ \mathrm{pb}, \end{equation} where the four uncertainties are due to data statistics, experimental and theoretical systematic effects, the integrated luminosity, and the LHC beam energy, giving a total uncertainty of 2.4%. The result is consistent with state-of-the-art theoretical QCD calculations. It is used to determine the top quark pole mass via the dependence of the predicted cross-section on $m_{t}^\mathrm{pole}$, giving a result of $m_{t}^\mathrm{pole}=173.1^{+2.0}_{-2.1}$ GeV. It is also combined with previous measurements at $\sqrt{s}=7$ TeV and $\sqrt{s}=8$ TeV to derive ratios and double ratios of $t\bar{t}$ and $Z$ cross-sections at different energies. The same event sample is also used to measure absolute and normalised differential cross-sections as functions of single lepton and dilepton kinematic variables, and the results compared to the predictions of various Monte Carlo event generators.