TOP-13-008

Measurement of the W-boson helicity in top decays from tt¯ production in lepton+jets events at the LHC at √ s=8 TeV

Abstract

We report on the measurement of the W-boson helicity fractions in decays of tt¯ in the muon+jets channel, using a sample of events from proton-proton collisions at a centre-of-mass energy of 8 TeV, collected in 2012 with the CMS detector at the LHC. The sample corresponds to an integrated luminosity of 19.6 fb-1. The longitudinal, left and right helicity fractions, denoted as F0, FL, FR, are measured using the cos(θ*) distribution, being θ* the angle between the direction of the W-boson in the leptonic branch and the three-momentum of the muon, the latter boosted to the W-boson rest frame. The measured fractions are F0=0.659 ± 0.015 (stat.) ± 0.023 (syst.), FL=0.350 ± 0.010 (stat.) ± 0.024 (syst.), and FR=-0.009 ± 0.006 (stat.) ± 0.020 (syst.), consistent with the standard model expectations.

Figures and tables

FiguresCaption
Uncert.png Data distributions of the cosine of the helicity angles helicity in the leptonic branch, compared to expectations from the SM, using the measured helicity fractions. 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. Statistical uncertainties are shown as error bars, and systematic uncertainties as hatched areas.

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Uncert.png Data distributions of the cosine of the helicity angles helicity in the hadronic branch, compared to expectations from the SM, using the measured helicity fractions. 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. Statistical uncertainties are shown as error bars, and systematic uncertainties as hatched areas.

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TablesCaption
Uncert.png Summary of the systematic uncertainties for the analysis.

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Yields.png Number of events expected from signal and background processes, together with the observed number of events in data. The second column represents the number of selected events, and the third column the fraction of those containing a reconstructed top pair via a Kinematic Fit that has converged. The difference between the predicted ("total MC") and observed ("Data") yields is covered by uncertainties which are sufficiently large to make them compatible.

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Topic attachments
I Attachment History Action Size Date Who Comment
PDFpdf CosThetaStar_MUON_measfrac.pdf r1 manage 24.1 K 2013-09-17 - 13:40 MartijnMulders  
PNGpng CosThetaStar_MUON_measfrac.png r1 manage 27.9 K 2013-09-17 - 13:40 MartijnMulders  
PDFpdf HadCosThetaStar_MUON_measfrac.pdf r1 manage 24.3 K 2013-09-17 - 13:40 MartijnMulders  
PNGpng HadCosThetaStar_MUON_measfrac.png r1 manage 27.8 K 2013-09-17 - 13:40 MartijnMulders  
PNGpng Uncert.png r1 manage 92.9 K 2013-09-17 - 13:40 MartijnMulders  
PDFpdf UncertCrop.pdf r1 manage 79.7 K 2013-09-17 - 13:40 MartijnMulders  
PDFpdf Yields.pdf r1 manage 51.2 K 2013-09-17 - 13:39 MartijnMulders  
PNGpng Yields.png r1 manage 59.6 K 2013-09-17 - 13:40 MartijnMulders  
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Topic revision: r3 - 2014-03-26 - AndreasMeyer
 
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