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

Ratios of top-quark pair to Z-boson cross sections measured from protonproton collisions at the LHC centre-of-mass energies of √ s = 13 TeV, 8 TeV, and 7 TeV are presented by the ATLAS Collaboration. Single ratios, at a given √ s for the two processes and at different √ s for each process, as well as double ratios of the two processes at different √ s, 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 → ` +` − where ` = e, µ at √ s = 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 √ s 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 lightquark sea for x < 0.02.