Measurements of top-pair to Z-boson cross section ratios at center-of-mass energies of √(s) = 13, 8, 7 TeV with the ATLAS detector and evaluation of measured cross sections constraints on PDFs and strong coupling constant

Ratios of top-quark pair to Z-boson production (Z → l⁺l^- where l = e, μ) cross sections at center-of-mass energies of √(s) = 13, 8, 7 TeV are measured using data recorded by the ATLAS experiment at the LHC. Data at √(s) = 13 TeV for both analyses are collected in 2015 with total integrated luminosity of 3.2 fb^-1. Previously published Z-boson and top-quark pair cross sections measured at √(s) = 8 and 7 TeV are corrected to phase space of measurements at √(s) = 13 TeV and used for the ratios calculation. Single cross section ratios at a given √(s) for the two processes and same process at different √(s), as well as double ratios of two processes at different √(s), are evaluated. The ratio results are compared to the predictions calculated at next-to-next-to-leading order for different parton distribution functions (PDFs). The correlated cross section measurements are used to obtain constraints on PDFs and the strong coupling constant α_s.