Measurement of the W polarisation in top decays from t-tbar production in lepton+jets events at the LHC

Abstract

We report on the measurement of the W-boson helicity fractions in semileptonic decays of ttbar events, using a sample of events from proton-proton collisions at a centre-of-mass energy of 7 TeV, collected in 2011 with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 5.0 fb-1. The longitudinal, left and right helicity fractions, denoted as F0, FL, FR, are measured using cos(θ*) and |coshad(θ*)| variables, in the leptonic and hadronic branches respectively, in top decay modes to muons and electrons. The measured fractions are consistent with the standard model predictions, and were used to probe the existence of anomalous Wtb couplings. Exclusion limits on the real components of the anomalous couplings gL, gR are derived.

Results

Our best measurement corresponds to the combination of muon and electron channels using the leptonic branch, and yields F0=0.682 +- 0.030 (stat.) +- 0.033 (syst.), FL=0.310+- 0.022 (stat.) +- 0.022 (syst.) and FR= 0.008 +- 0.012 (stat.) +-0.014 (syst.), consistent with the standard model predictions. If the constraint FR=0 is imposed, combining both leptonic branches in the muon and electron channels we obtain F0 = 0.685 +- 0.017 (stat.) +- 0.021 (syst.) or, equivalently (since FL=1-F0 for FR=0) FL=0.315 +- 0.017 (stat.) +- 0.021 (syst.).

Assuming the standard model predictions for the other anomalous couplings describing the Wtb vertex, VL=1, VR=gL=0, we measure gR= -0.008 +- 0.024 (stat.)+0.029- 0.030 (syst.). In terms of the effective dimension-six Lagrangian we obtain Re(C33uW)/Λ2 = -0.088 +- 0.280 (stat.) + 0.339- 0.352 (syst.) TeV-2, where C33uW is the effective operator coefficient and Λ the scale of new physics.

Tables

Tables of systematic uncertainties

Table1.jpg Summary of the systematic uncertainties for the analysis using only the leptonic branch of the event, for the 3D fit, fitting F0,FL, and the ttbar normalization Ftt (columns 2-3 for muon+jets analysis, 5-6 for electron+jets analysis, and 8-9 for the combina- tion of both decay modes); and the 2D fit, fitting F0 and Ftt only (column 4 for muon+jets analysis, 7 for electron+jets analysis, and 10 for the combination of both decay modes). The numbers given correspond to the absolute uncertainty with respect to the central analysis: ΔF = (Fcentral-Fcheck). Get pdf version
Table2.jpg Systematic uncertainties for the 2D fits using the hadronic branch of the tt system, and for the muon channel, electron channel, as well as the combination of both decay channels. The numbers given correspond to the absolute uncertainty with respect to the central analysis: ΔF = (Fcentral-Fcheck). Get pdf version

Tables of helicity results

Table3.jpg Measurements of the W-boson helicity fractions from the cos(θ) (leptonic branch) and abs(coshadθ) (hadronic branch) distributions. The columns show the fit type, the decay channel, and the measurement of each helicity parameter, together with the statistical and systematic uncertainties. For the 3D fits, the last column presents the statistical correlation between F0 and FL. Get pdf version
Table4.jpg The combined helicity fractions and their uncertainties, including the type of fit performed, the channels (l = e,muon combination) and branches of the tt system ("l" for leptonic, and "h" for hadronic) used in the combination, as well as the total correlation between F0 and FL. Get pdf version

Figures

Basic kinematic distributions after event selection: leptons

Figure Caption
TwoTagspatMuPt_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_patElePtELE.gif Transverse momentum distribution for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt signal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Get pdf version (a) and Get pdf version (b)
TwoTagspatMuEta_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_patEleEtaELE.gif Pseudorapidity distribution for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt signal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Get pdf version (a) and Get pdf version (b)
TwoTagslepNuPt_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_lepNuPtELE.gif Transverse momentum distribution of the reconstructed neutrino in the leptonic branch for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt signal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Get pdf version (a) and Get pdf version (b)

Basic kinematic distributions after event selection: jets

TwoTagslepBPt_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_lepBPtELE.gif Transverse momentum distribution of the b-jet in the leptonic branch for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt signal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Get pdf version (a) and Get pdf version (b)
TwoTagshadBPt_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_hadBPtELE.gif Transverse momentum distribution of the b-jet in the hadronic branch for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt signal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Get pdf version (a) and Get pdf version (b)
TwoTagshadqqPt_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_hadqqPtELE.gif Transverse momentum distribution of the W decay products for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt signal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Get pdf version (a) and Get pdf version (b)

Helicity angle distribution

TwoTagsCosThetaStar_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_CosThetaStarELE.gif cos(θ*) distribution for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt signal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Systematic uncertainties are shown as hatched histograms. Get pdf version (a) and Get pdf version (b)
TwoTagsHadCosThetaStar_fix0_t15_W10MUON.gif   fix0_t15_W10TwoTags_HadCosThetaStarELE.gif Abs(coshad(θ*)) distribution for (a) muons and (b) electrons. Data are displayed as solid points, simulated tt sig- nal distributions as red histograms, and the contribution from other background processes as coloured histograms. At the bottom, the ratio between prediction and data is displayed. Systematic uncertainties are shown as hatched histograms.Get pdf version (a) and Get pdf version (b)

Summary plots

money_JBLUE_3D_Preliminary_11Jul.png Summary of W helicity measurements in the muon and electron channels using the leptonic branch. Get pdf version
money_JBLUE_2D_Preliminary_11Jul.png Summary of W helicity measurements in the muon and electron channels using the leptonic and hadronic branches and setting FR=0. Get pdf version

Limits on gL, gR

hglgr.gif Limits on the real components of the anomalous couplings gL, gR at 68 and 95% CL, for VL=1 and VR=0. Get pdf version
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