Adomeit, S.; Bender, M.; Biebel, O.; Bock, C.; Chow, B. K. B.
(2017):
Measurements of topquark pair to Zboson crosssection ratios at root s=13, 8, 7 TeV with the ATLAS detector.
In: Journal of High Energy Physics, No. 2, 117: pp. 154

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Abstract
Ratios of topquark pair to Zboson cross sections measured from protonprotoncollisions at the LHC centreofmass energies of root S = 13 TeV, 8 TeV, and 7 TeV are presented by the ATLAS Collaboration. Single ratios, at a given root S for the two processes and at different root S , for each process, as well as double ratios of the two processes at different root S , are evaluated. The ratios are constructed using previously published ATLAS measurements of the t (t) over bar and Zboson production cross sections, corrected to a common phase space where required, and a new analysis of Z > l(+)l() where l = e, mu at root 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 > mu (+) mu () channels for each value. The results are compared to calculations performed at nexttonexttoleadingorder accuracy using recent sets of parton distribution functions. The data demonstrate significant power to constrain the gluon distribution function for the Bjorkenx values near 0.1 and the lightquark sea for x < 0.02.