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 proton-proton 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 t t ¯ 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 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 light-quark sea for x < 0 . 02.