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

Abstract

Measurements of top-quark pair to Z-boson cross-section ratios at √ s = 13, 8, 7 TeV with the ATLAS detectorThe ATLAS Collaboration 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 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-tonext-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. c 2017 CERN for the benefit of the ATLAS Collaboration. ContentsA Predictions involving Z-boson total cross sections 30 B Acceptance factors and results in the Z-boson total phase space 31 C Tables of results 31 Events containing a Z-boson candidate are chosen by requiring exactly two selected leptons of the same flavour but of opposite charge with an invariant mass of 66 < m < 116 GeV. A total of 1,367,026 candidates and 1,735,197 candidates pass all requirements in the electron and muon channels, respectively. 9

Heavy-flavor parton distributions without heavy-flavor matching prescriptions

“…This means that it may not be easy to use single top LHC data to discriminate between these two approaches. An alternative may be the measurement of the ratio σ(tt)/σ(Z) which, as can be seen in figure 11, is currently known with accuracy of about 2.8% [67]. Future improvements in this ratio may become the leading candidate for distinguishing the predictions based on these two approaches.…”

DYTurbo: fast predictions for Drell–Yan processes

“…Preliminary versions of the DYTurbo program were used by the ATLAS Collaboration in Refs. [6,78,79]. The DYTurbo program is publicly available [80].…”

Constraints on the Parton Density Functions of the Proton by Measurements with the ATLAS Detector

“…A new measurement [21] quantifies the ratios of the ttbar cross section with respect to the the Z cross section at the different beam energies, 7, 8 and 13 TeV. Shown in Figure 4, a spread of the predictions is observed which suggests potential for a tighter constraint the parton distributions using these data.…”

“…-27 - Figure 4: The Ratio of the tt to Z cross sections at 7 and 8 TeV collision energies (left) and the ratio of the gluon distributions profiled epWZ12 excluding or including the top and Z data (right) [21].…”