First Measurement of Vector Boson Production Associated with Top-Antitop Pairs at 7 TeV (TOP-12-014)

Abstract

A measurement of vector boson production associated with top-antitop pairs in proton-proton collisions at √s = 7 TeV is presented. The results are based on data corresponding to an integrated luminosity of 4.98 fb-1, recorded in 2011 by the CMS detector at the CERN LHC. The measurement is performed in two independent channels: a trilepton analysis, which searches exclusively for ttbar+Z events, and a same-sign dilepton analysis, which searches inclusively for ttbar+V (V = W,Z) events. Combining the individual significances from all seven decay channels, a ttbar+V signal is established at a combined significance of 4.67 standard deviations from the background hypothesis. The combined ttbar+V cross section is measured to be sigma_ttV = 0.51 +0.15 -0.13 (stat) +0.05 -0.04 (syst) pb, which is found to be somewhat larger but compatible with the next-to-leading order calculation. In the trilepton channel, a direct measurement of the ttbar+V cross section sigma_ttZ = 0.30 +0.14 -0.11 (stat) +0.04 -0.02 (syst) pb is obtained with a significance of 3.66 standard deviations from the background hypothesis.

Trilepton

Figures Caption
click on it to get it Fig. 1 (left): Dilepton invariant mass distribution in events passing preselection requirements for cross-flavor dilepton events.
click on it to get it Fig. 1 (right): Dilepton invariant mass distribution in events passing preselection requirements for same-flavor dilepton events, after the third lepton requirement.
click on it to get it Fig. 2: Event yields after preselection plus third lepton requirements, separated in lepton flavor combinations. Only events compatible with a Z boson decay (70 < mll < 110 GeV) are considered. The MC backgrounds are rescaled with the data-driven techniques described in the text, and the uncertainty on the background yield is superimposed with a grey diagonal hash.
click on it to get it Fig. 3 (top left): Data-MC comparison for events passing the preselection, third lepton and loose dilepton invariant mass (70 < mll < 110 GeV) requirements for loose b-jet multiplicity.
click on it to get it Fig. 3 (top right): Data-MC comparison for events passing the preselection, third lepton and loose dilepton invariant mass (70 < mll < 110 GeV) requirements for medium b-jet multiplicity
click on it to get it Fig. 3 (bottom left): Data-MC comparison for events passing the preselection, third lepton and loose dilepton invariant mass (70 < mll < 110 GeV) requirements for Z transverse momentum.
click on it to get it Fig. 3 (bottom right): Data-MC comparison for events passing the preselection, third lepton and loose dilepton invariant mass (70 < mll < 110 GeV) requirements for HT.
click on it to get it Fig. 4: Event yields after optimized selection requirements, separated in lepton flavor com- binations. The total yield is shown in the rightmost bin. The expected contribution of the signal and background processes is shown, and the uncertainty on the background yield is superim- posed with a grey diagonal hash.

Dilepton

Figures Caption
click on it to get it Fig. 5 (top left): Comparison between data and the fully data-driven background estimation for events passing preselection requirements: transverse momentum of the leading lepton
click on it to get it Fig. 5 (top right): Comparison between data and the fully data-driven background estimation for events passing preselection requirements: transverse momentum of the subleading lepton.
click on it to get it Fig. 5 (bottom left): Comparison between data and the fully data-driven background estimation for events passing preselection requirements: medium b-jet multiplicity.
click on it to get it Fig. 5 (bottom right): Comparison between data and the fully data-driven background estimation for events passing preselection requirements: HT.
click on it to get it Fig. 6: Comparison of background prediction and observed number of events in the ee, emu and mumu channel as well as all channels combined.

Combination

Figures Caption
click on it to get it Table 1: Summary of measured cross sections in the trilepton and dilepton channels, compared to the NLO calculation, separately for the tt+Z, tt+W and tt+V processes. Measurements in bold- face are directly measured. When two separate uncertainties are quoted, the first refers to the statistical component, the second to the systematic one. No uncertainty is quoted on the NLO prediction for tt+V, as no error is currently available for the prediction of sigma_ttZ.
click on it to get it Figure 7: Measurement of the tt+V production cross section: the cross section from the trilepton channel (blue triangle), from the same sign dilepton channel (grey square) and the combination of the two measurements (black circle) are compared to the NLO calculation (red line). Internal error bars for the measurements represent the statistical component of the uncertainty.

Summary

-- BenjaminStieger - 21-Jun-2012

Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf ObsPred_HT_lin.pdf r1 manage 15.5 K 2012-06-21 - 14:10 BenjaminStieger Same-sign dilepton plots
PNGpng ObsPred_HT_lin.png r1 manage 96.3 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf ObsPred_NBJetsMed_lin.pdf r1 manage 14.3 K 2012-06-21 - 14:10 BenjaminStieger Same-sign dilepton plots
PNGpng ObsPred_NBJetsMed_lin.png r1 manage 83.7 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf ObsPred_PT1_lin.pdf r1 manage 16.3 K 2012-06-21 - 14:10 BenjaminStieger Same-sign dilepton plots
PNGpng ObsPred_PT1_lin.png r1 manage 114.3 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf ObsPred_PT2_lin.pdf r1 manage 16.2 K 2012-06-21 - 14:10 BenjaminStieger Same-sign dilepton plots
PNGpng ObsPred_PT2_lin.png r1 manage 114.4 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf ObsPred_TTbarWSel.pdf r1 manage 14.5 K 2012-06-21 - 14:10 BenjaminStieger Same-sign dilepton plots
PNGpng ObsPred_TTbarWSel.png r1 manage 76.5 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf channelYields_final.pdf r1 manage 6.1 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng channelYields_final.png r1 manage 10.5 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf channelYields_presel.pdf r1 manage 6.2 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng channelYields_presel.png r1 manage 11.3 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf ht.pdf r1 manage 7.0 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng ht.png r1 manage 27.4 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf mZll_OF_prepresel_scaled.pdf r1 manage 11.0 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng mZll_OF_prepresel_scaled.png r1 manage 37.3 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf mZll_presel_scaled.pdf r1 manage 10.4 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng mZll_presel_scaled.png r1 manage 33.0 K 2012-06-21 - 14:45 BenjaminStieger png versions
PDFpdf measurement_vs_NLO.pdf r1 manage 4.9 K 2012-06-21 - 14:11 BenjaminStieger Combination plot
PNGpng measurement_vs_NLO.png r1 manage 10.3 K 2012-06-21 - 14:46 BenjaminStieger png versions
PDFpdf nBJets_loose.pdf r1 manage 5.5 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng nBJets_loose.png r1 manage 23.8 K 2012-06-21 - 14:46 BenjaminStieger png versions
PDFpdf nBJets_medium.pdf r1 manage 5.2 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng nBJets_medium.png r1 manage 11.9 K 2012-06-21 - 14:46 BenjaminStieger png versions
PDFpdf ptZll.pdf r1 manage 6.8 K 2012-06-21 - 14:11 BenjaminStieger Trilepton plots
PNGpng ptZll.png r1 manage 28.1 K 2012-06-21 - 14:46 BenjaminStieger png versions
PDFpdf table1.pdf r3 r2 r1 manage 134.3 K 2012-07-02 - 11:31 BenjaminStieger  
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Topic revision: r6 - 2013-08-21 - FrancescoPandolfi
 
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