Abstract
The inclusive top quark pair (tbar{t}) production cross-section sigma _{tbar{t}} has been measured in proton–proton collisions at sqrt{s}=13,text {TeV}, using 36.1 fb^{-1} of data collected in 2015–2016 by the ATLAS experiment at the LHC. Using events with an opposite-charge emu pair and b-tagged jets, the cross-section is measured to be: σtt¯=826.4±3.6(stat)±11.5(syst)±15.7(lumi)±1.9(beam)pb,\\documentclass[12pt]{minimal}\t\t\t\t\\usepackage{amsmath}\t\t\t\t\\usepackage{wasysym}\t\t\t\t\\usepackage{amsfonts}\t\t\t\t\\usepackage{amssymb}\t\t\t\t\\usepackage{amsbsy}\t\t\t\t\\usepackage{mathrsfs}\t\t\t\t\\usepackage{upgreek}\t\t\t\t\\setlength{\\oddsidemargin}{-69pt}\t\t\t\t\\begin{document}$$\\begin{aligned} \\sigma _{t\\bar{t}} = 826.4 \\pm 3.6\\,\\mathrm {(stat)}\\ \\pm 11.5\\,\\mathrm {(syst)}\\ \\pm 15.7\\,\\mathrm {(lumi)}\\ \\pm 1.9\\,\\mathrm {(beam)}\\,\\mathrm {pb}, \\end{aligned}$$\\end{document}where the uncertainties reflect the limited size of the data sample, experimental and theoretical systematic effects, the integrated luminosity, and the LHC beam energy, giving a total uncertainty of 2.4%. The result is consistent with theoretical QCD calculations at next-to-next-to-leading order. It is used to determine the top quark pole mass via the dependence of the predicted cross-section on m_t^{mathrm{pole}}, giving m_t^{mathrm{pole}}=173.1^{+2.0}_{-2.1},text {GeV}. It is also combined with measurements at sqrt{s}=7,text {TeV} and sqrt{s}=8,text {TeV} to derive ratios and double ratios of tbar{t} and Z cross-sections at different energies. The same event sample is used to measure absolute and normalised differential cross-sections as functions of single-lepton and dilepton kinematic variables, and the results are compared with predictions from various Monte Carlo event generators.
Highlights
Parton distribution function uncertainties were evaluated by reweighting the baseline Powheg + Pythia8 ttsample using generator weights associated with each of the 100 variations provided by the NNPDF3.0 authors [44], and calculating the RMS of the changes induced in eμ, Geμ and Gieμ
The normalisation of the W t background was varied by 5.3%, corresponding to the parton distribution function (PDF) and QCD scale uncertainties on the approximate NNLO cross-section prediction discussed in Sect
2.124−+00..003256 llaetpetdonbeutnwceeerntatihnetie√ssw=ere13coTneVsertvtaatnivdeZlymtreeaasteudreamseunntsc.oTrrheelargest uncertainties in the double ratio are associated with ttmodelling, and these were treated in the same way as for the updated ttcross-sectio√n ratios discussed above, including the 0.46% increase of the s = 13 TeV σttvalue corresponding to the use of a Powheg + Pythia6 CT10 nominal ttsimulation sample
Summary
The ATLAS detector [30,31,32] at the LHC covers nearly the entire solid angle around the collision point. The background with two real prompt leptons is dominated by the associated production of a W boson and single top quark, W t This process was simulated using Powheg v1 [54] with the CT10 NLO PDF set [18], interfaced to Pythia (v6.428) [55] with the P2012 tune [56]. Production of ttin association with a leptonically decaying W , Z or Higgs boson gives a negligible contribution to the opposite-sign eμ samples compared to inclusive ttproduction, but is significant in the same-sign control samples used to assess the background from misidentified leptons These processes were simulated using aMC@NLO + Pythia (tt + W/Z ) or Powheg + Pythia (tt + H ) [61]. Other backgrounds, including processes with two misidentified leptons, are negligible after the event selections used in the analysis
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