PAS TOP-14-016 (ATLAS-CONF-2014-054)
Combination of ATLAS and CMS top quark pair cross section measurements in the eμ final state using proton-proton collisions at 8 TeV

Abstract

A combination of ATLAS and CMS measurements of the top quark pair production cross section (sigma_tt) using proton-proton collisions at a centre-of-mass energy of 8 TeV is presented. The measurements make use of top quark pair events in the dilepton decay channel with exactly one electron and one muon in the final state. The integrated luminosities of the two measurements are 20.3 and 5.3 /fb for ATLAS and CMS, respectively. The combined cross section is determined to be sigma_tt = 241.5 1.4 (stat.) 5.7 (syst.) 6.2 (lumi.) pb = 241.5 8.5 pb for a top quark mass reference value of 172.5 GeV. The total uncertainty of the combined result is 3.5%. An uncertainty on the LHC beam energy of 0.66% translates into an additional uncertainty of 4.2 pb on sigma_tt. The combined cross section depends on the assumed top quark mass value m_t with a slope of 1/sigma_tt dsigma_tt/dm_t = -0.46%/GeV around the reference value. The result is in good agreement with the standard model prediction calculated in perturbative QCD to next-to-next-to-leading order.

Input measurements

Public document

CMS-PAS TOP-14-016: CDS entry

Result

  • Combination: sigma_tt = 241.5 1.4 (stat.) 5.7 (syst.) 6.2 (lumi.) pb = 241.5 8.5 pb for a top quark mass reference value of 172.5 GeV
  • Relative precision: 3.5%
  • Correlation between the measurements: 24%
  • Weights: 73% (ATLAS), 27% (CMS)
  • Chi2 probability: 82%
  • Dependence of the combined cross section on the top quark mass: 1/sigma_tt dsigma_tt/dm_t = -0.46%/GeV around the reference value
  • Uncertainty due to the calibration of the beam energy (not included in the combination or in the figures below): 4.2 pb

Figures

Figure Caption
tt_combi_lhc8_sep14.png Input sigma_tt measurements by the ATLAS and CMS collaborations and the result of the LHC combination. The band corresponds to the NNLO+NNLL QCD calculation with Top++2.0 of sigma_tt = 252.9 +6.4 -8.6 (scale) 11.7 (PDF+alpha_S) pb for a top quark mass of 172.5 GeV. The uncertainty due to the LHC beam energy is 4.2 pb and it is not included in the total uncertainty on the measurements or the combination. This uncertainty has to be added in quadrature to the total uncertainty for the comparison of the measurements with theoretical predictions.
PDF PNG
tt_combi_lhc8_sep14_wa.png Input sigma_tt measurements by the ATLAS and CMS collaborations and the result of the LHC combination. The band corresponds to the NNLO+NNLL QCD calculation with Top++2.0 of sigma_tt = 246.7 +6.2 -8.4 (scale) 11.4 (PDF+alpha_S) pb for a top quark mass of 173.34 GeV. The experimental results are corrected to the world average top quark mass value using the measured sigma_tt dependence on the assumed top quark mass. The uncertainty due to the LHC beam energy is 4.2 pb and it is not included in the total uncertainty on the measurements or the combination. This uncertainty has to be added in quadrature to the total uncertainty for the comparison of the measurements with theoretical predictions.
PDF PNG

Tables

Figure Caption
table-1.png Table of uncertainties in the tt cross section used in the BLUE combination. Cross sections and uncertainties are in pb. For the ATLAS measurement, the contribution from pileup effects is already included in the JES and in the Lepton identification uncertainties. The contribution from Radiation is also included in the latter. For the CMS measurement, the contribution from Jet identification is negligible. The contribution from bJES is already included in the FlavourJES uncertainty.
table-2.png Change of the central value and the uncertainty in pb depending on the size of the correlation of the luminosity uncertainty. The differences with respect to the combined central cross section value and to the total uncertainty are reported.
table-3.png Change of the central value and the uncertainty in pb depending on the assumed correlation for Scale and Generator and parton shower uncertainties. The differences with respect to the combined central cross section value and to the total uncertainty are reported.
table-4.png Change of the central value and the uncertainty in pb depending on the assumed correlation of the multiple JES components. The differences with respect to the combined central cross section value and to the total uncertainty are reported.

-- MariaAldaya - 28 Sep 2014

Topic attachments
I Attachment History Action Size Date Who Comment
PNGpng table-1.png r1 manage 79.9 K 2014-10-10 - 23:07 MariaAldaya  
PNGpng table-2.png r1 manage 6.7 K 2014-10-10 - 23:07 MariaAldaya  
PNGpng table-3.png r1 manage 14.3 K 2014-10-10 - 23:07 MariaAldaya  
PNGpng table-4.png r1 manage 25.0 K 2014-10-10 - 23:07 MariaAldaya  
PDFpdf tt_combi_lhc8_sep14.pdf r1 manage 5.7 K 2014-09-28 - 18:50 MariaAldaya  
PNGpng tt_combi_lhc8_sep14.png r1 manage 29.2 K 2014-09-28 - 18:50 MariaAldaya  
PDFpdf tt_combi_lhc8_sep14_wa.pdf r1 manage 5.8 K 2014-09-28 - 18:50 MariaAldaya  
PNGpng tt_combi_lhc8_sep14_wa.png r1 manage 29.4 K 2014-09-28 - 18:50 MariaAldaya  
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Topic revision: r9 - 2014-10-11 - AndreasMeyer
 
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