Abstract

The inclusive top quark pair (toverline{t}) production cross-section {sigma _{toverline{t}}} has been measured in proton–proton collisions at {sqrt{s}=7~mathrm{TeV}} and {sqrt{s}=8~mathrm{TeV}} with the ATLAS experiment at the LHC, using toverline{t} events with an opposite-charge emu pair in the final state. The measurement was performed with the 2011 7 TeV dataset corresponding to an integrated luminosity of 4.6 {mathrm{fb}^{-1}} and the 2012 8 TeV dataset of 20.3 {mathrm{fb}^{-1}}. The numbers of events with exactly one and exactly two b-tagged jets were counted and used to simultaneously determine {sigma _{toverline{t}}} and the efficiency to reconstruct and b-tag a jet from a top quark decay, thereby minimising the associated systematic uncertainties. The cross-section was measured to be: \\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$\\begin{aligned} {\\sigma _{t\\overline{t}}}&= 182.9\\pm 3.1\\pm 4.2\\pm 3.6\\pm 3.3~\\mathrm{pb}\\ ({\\sqrt{s}=7~\\mathrm{TeV}})\\quad \\mathrm{and} \\\\ {\\sigma _{t\\overline{t}}}&= 242.4\\pm 1.7\\pm 5.5\\pm 7.5\\pm 4.2~\\mathrm{pb}\\ ({\\sqrt{s}=8~\\mathrm{TeV}}), \\end{aligned}$$\\end{document}σtt¯=182.9±3.1±4.2±3.6±3.3pb(s=7TeV)andσtt¯=242.4±1.7±5.5±7.5±4.2pb(s=8TeV),where the four uncertainties arise from data statistics, experimental and theoretical systematic effects, knowledge of the integrated luminosity and of the LHC beam energy. The results are consistent with recent theoretical QCD calculations at next-to-next-to-leading order. Fiducial measurements corresponding to the experimental acceptance of the leptons are also reported, together with the ratio of cross-sections measured at the two centre-of-mass energies. The inclusive cross-section results were used to determine the top quark pole mass via the dependence of the theoretically predicted cross-section on {m}_{t}^mathrm{pole} giving a result of {m}_{t}^mathrm{pole} =172.9^{+2.5}_{-2.6} GeV. By looking for an excess of toverline{t} production with respect to the QCD prediction, the results were also used to place limits on the pair-production of supersymmetric top squarks {tilde{t}_{1}} with masses close to the top quark mass, decaying via {tilde{t}_{1}}rightarrow t{tilde{chi }_{1}^{0}} to predominantly right-handed top quarks and a light neutralino tilde{chi }_{1}^{0}, the lightest supersymmetric particle. Top squarks with masses between the top quark mass and 177 GeV are excluded at the 95 % confidence level.

Highlights

  • The top quark is the heaviest known fundamental particle, with a mass that is much larger than any of the other quarks, and close to the scale of electroweak symmetry breaking

  • This paper describes a measurement in the dileptonic eμ channel, tt → W +bW −b → e±μ∓ννbb, selecting events with an eμ pair with oppositesign electric charges,1 and one or two hadronic jets from the b quarks

  • Σtt = 242.4 ± 1.7 ± 5.5 ± 7.5 ± 4.2 pb ( s = 8 TeV), where the four uncertainties arise from data statistics, experimental and theoretical systematic effects related to the analysis, knowledge of the integrated luminosity and of the Large Hadron Collider (LHC)

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Summary

Introduction

The top quark is the heaviest known fundamental particle, with a mass (mt ) that is much larger than any of the other quarks, and close to the scale of electroweak symmetry breaking. At the LHC, top quarks are primarily produced in quark–antiquark pairs (tt), and the precise prediction of the corresponding inclusive cross-section (σtt ) is a substantial challenge for quantum chromodynamics (QCD) calculational techniques. Precise measurements of σtt are sensitive to the gluon parton distribution function (PDF), the top quark mass, and potential enhancements of the cross-section due to physics beyond the Standard Model. The rates of events with an eμ pair and one or two tagged b-jets were used to measure simultaneously the tt production cross-section and the combined probability to reconstruct and b-tag a jet from a top quark decay. Theoretical predictions for σtt are described, followed by the data and Monte Carlo (MC) simulation samples, the object and event selection, and the extraction of the tt cross-section in Sect.

Theoretical cross-section predictions
Data and simulated samples
Object and event selection
Extraction of the t t cross-section
Background estimation
Systematic uncertainties
Background modelling
Additional correlation studies
Results
Fiducial cross-sections
Top quark mass determination
Constraints on stop-pair production
Conclusions
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