Measurement of the tt̅ production cross-section and lepton differential distributions in eμ dilepton events from pp collisions at √s = 13 TeV with the ATLAS detector

The inclusive top quark pair (tt̅) production cross-section σ_tt̅ has been measured in proton–proton collisions at √s = 13 TeV, using 36.1 fb⁻¹ of data collected in 2015–2016 by the ATLAS experiment at the LHC. Using events with an opposite-charge eμ pair and b-tagged jets, the cross-section is measured to be:

σ_tt̅ = 826.4 ± 3.6 (stat)  ± 11.5 (syst) ± 15.7 (lumi)  ± 1.9 (beam) pb,

where the uncertainties reflect the limited size of the data sample, 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 theoretical QCD calculations at next-to-next-to-leading order. It is used to determine the top quark pole mass via the dependence of the predicted cross-section on m^pole_t, giving m^pole_t =173.1^+2.0_−2.1 GeV. It is also combined with measurements at √s = 7 TeV and √s = 8 TeV to derive ratios and double ratios of tt̅ and Z cross-sections at different energies. The same event sample is used to measure absolute and normalised differential cross-sections as functions of single-lepton and dilepton kinematic variables, and the results are compared with predictions from various Monte Carlo event generators.