Performance of the ATLAS Trigger System in 2015

This article presents measurements of tt̅ differential cross-sections in a fiducial phase-space region, using an integrated luminosity of 3.2 fb⁻¹ of proton–proton data at a centre-of-mass energy of √s̅ = 13 TeV recorded by the ATLAS experiment at the LHC in 2015. Differential cross-sections are measured as a function of the transverse momentum and absolute rapidity of the top quark, and of the transverse momentum, absolute rapidity and invariant mass of the tt̅ system. The tt̅ events are selected by requiring one electron and one muon of opposite electric charge, and at least two jets, one of which must be tagged as containing a b-hadron. The measured differential cross-sections are compared to predictions of next-to-leading order generators matched to parton showers and the measurements are found to be consistent with all models within the experimental uncertainties with the exception of the Powheg-Box+ Herwig++ predictions, which differ significantly from the data in both the transverse momentum of the top quark and the mass of the tt̅ system.