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, 8 TeV, and 7 TeV 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 toverline{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 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 → e+e− and the Z → μ+μ− 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.
Highlights
The experimental precision of such measurements has reached the few-percent level in the case of the total ttproduction cross section, σtttot, and the sub-percent level for the Z-boson production cross section with subsequent Z → + − decay within the fiducial region defined by the detector acceptance, σZfid
The three parton distribution functions (PDF) sets used in the PDF4LHC prescription [76], CT14, NNPDF3.0, and MMHT14, predict the largest ratios
The HERA-based HERAPDF2.0 and ATLAS-epWZ12 sets are in the middle
Summary
Precision measurements of top-quark-pair [1,2,3,4,5,6,7] and Z-boson [8,9,10,11,12,13,14,15] production by the ATLAS [16] and CMS [17] collaborations at the CERN Large Hadron Collider (LHC) [18]provide important tests of the Standard Model (SM). The experimental precision of such measurements has reached the few-percent level in the case of the total ttproduction cross section, σtttot, and the sub-percent level for the Z-boson production cross section with subsequent Z → + − decay within the fiducial region defined by the detector acceptance, σZfid. This experimental precision is complemented by an accurate determination of the proton-proton, pp, collision luminosity, which has reached a precision of approximately. The entry “(lepton)” in “Muon (lepton) trigger” refers to the tttrigger for the 7 and 8 TeV data set which quotes a single uncertainty for the combined effects of the uncertainties in the electron and muon triggers and so there is a corresponding entry “–” for the electron trigger for the 7 and 8 TeV ttdata set
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