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

Abstract

Ratios of top-quark pair to $Z$-boson cross sections measured from proton--proton collisions at the LHC centre-of-mass energies of $\sqrt{s}=13$TeV, 8TeV, and 7TeV are presented by the ATLAS Collaboration. Single ratios, at a given $\sqrt{s}$ for the two processes and at different $\sqrt{s}$ for each process, as well as double ratios of the two processes at different $\sqrt{s}$, are evaluated. The ratios are constructed using previously published ATLAS measurements of the $t\overline{t}$ and $Z$-boson production cross sections, corrected to a common phase space where required, and a new analysis of $Z\to {\ell }^{+}{\ell }^{-}$ where $\ell =e,\mu$ at $\sqrt{s}=13$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 $Z\to e^{+}{e}^{-}$ and the $Z\to {\mu }^{+}{\mu }^{-}$ channels for each $\sqrt{s}$ 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 $x<0.02$.

55 pages in total, author list starting page 34, 9 figures, 18 tables, submitted to J. High Energy Phys. All figures are available at http://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/STDM-2016-02/

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