Measurements of top-quark pair to Z-boson cross-section ratios at vs =13, 8, 7 TeV with the ATLAS detector

Ratios of top-quark pair to Z-boson cross sections measured from proton-proton collisions at the LHC centre-of-mass energies of sv=13TeV, 8TeV, and 7TeV are presented by the ATLAS Collaboration. Single ratios, at a given vs for the two processes and at different vs for each process, as well as double ratios of the two processes at different vs , are evaluated. The ratios are constructed using previously published ATLAS measurements of the tt¯ and Z-boson production cross sections, corrected to a common phase space where required, and a new analysis of Z ? l+l- where l = e, µ at sv=13 TeV performed with data collected in 2015 with an integrated luminosity of 3.2 fb-1. Correlations of systematic uncertainties are taken into account when evaluating the uncertainties in the ratios. The correlation model is also used to evaluate the combined cross section of the Z ? e+e- and the Z ? µ+µ- channels for each vs value. The results are compared to calculations performed at next-to-next-to-leading-order accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorken-x values near 0.1 and the light-quark sea for x < 0.02.