Combination of inclusive top-quark pair production cross-section measurements using ATLAS and CMS data at $\sqrt{s}=7$ and 8 TeV

Abstract

A combination of measurements of the inclusive top-quark pair production cross-section performed by ATLAS and CMS in proton-proton collisions at centre-of-mass energies of 7 and 8 TeV at the LHC is presented. The cross-sections are obtained using top-quark pair decays with an opposite-charge electron-muon pair in the final state and with data corresponding to an integrated luminosity of about 5 fb${}^{-1}$ at $\sqrt{s}=7$ TeV and about 20 fb${}^{-1}$ at $\sqrt{s}=8$ TeV for each experiment. The combined cross-sections are determined to be $178.5\pm 4.7$ pb at $\sqrt{s}=7$ TeV and $243.3_{-5.9}^{+6.0}$ pb at $\sqrt{s}=8$ TeV with a correlation of 0.41, using a reference top-quark mass value of 172.5 GeV. The ratio of the combined cross-sections is determined to be $R_{8/7}=1.363\pm 0.032$. The combined measured cross-sections and their ratio agree well with theory calculations using several parton distribution function (PDF) sets. The values of the top-quark pole mass (with the strong coupling fixed at 0.118) and the strong coupling (with the top-quark pole mass fixed at 172.5 GeV) are extracted from the combined results by fitting a next-to-next-to-leading-order plus next-to-next-to-leading-log QCD prediction to the measurements. Using a version of the NNPDF3.1 PDF set containing no top-quark measurements, the results obtained are $m_t^{\text{pole}}=173.4_{-2.0}^{+1.8}$ GeV and $\hat{I}{\pm }_{\text{s}}(m_Z)=0.1170_{-0.0018}^{+0.0021}$.

59 pages in total, author lists starting at page 24, 4 figures, 6 tables. All figures including auxiliary figures are available at https://atlas.web.cern.ch/Atlas/GROUPS/PHYSICS/PAPERS/TOPQ-2018-39 and at https://cms-results.web.cern.ch/cms-results/public-results/publications/TOP-18-014/index.html