Combination of the ATLAS and CMS measurements of the W-boson polarization in top-quark decays

Abstract

We present the combination of measurements of the W-boson polarization in top-quark decays performed by the ATLAS and CMS Collaborations. The measure- ments are based on proton-proton collision data corresponding to integrated luminosities ranging from 35 pb-1 to 2.2 fb-1 produced at the LHC at a center-of-mass energy of √s = 7 TeV. The results are quoted as helicity fractions, i.e. the fractions of events which contain W bosons with longitudinal and left-handed polarization. The combined helicity fractions are F0 = 0.626 0.034 (stat.) 0.048 (syst.) , FL= 0.359 0.021 (stat.) 0.028 (syst.), with a total correlation of -0.86. The fraction of W bosons with right-handed polarization is calculated assuming the sum of all fractions to be unity: FR =0.0150.034, where the uncertainty includes the statistical and systematic uncertainties.

Exclusion limits on anomalous Wtb couplings are derived from these results: firstly, assuming VL = 1 and VR = 0, and assuming that the imaginary part of all couplings is 0, limits on the anomalous couplings gL, gR are derived. Then, leaving Re(gR) as a free parameter while fixing VL = 1, VR = gL = 0, our results yield Re(gR) = −0.10 0.06 (stat.) +0.07 −0.08(syst.).

The results are in agreement with predictions from NNLO QCD.

TOP-12-025 Results: figures and tables

Figure Caption
summary.png Fig 1: overview of the four measurements included in the combination as well as the results of the combination. The inner and outer error bars correspond to the statistical and the total uncertainty, respectively. The green solid line indicates the predictions of NNLO QCD calculations. Version in pdf
summary_2d.png Fig 2: the combined values of FL and F0 as well as the 68% CL interval. Also shown are the helicity fractions predicted by NNLO QCD calculations including the theoretical uncertainty. The black line indicates the boundary of the unitarity constraint that the helicity fractions sum up to unity. Version in pdf
hglgrFullRange.png Fig 3: allowed regions for the Wtb anomalous couplings using the combined helicity fractions and assuming VL = 1 and VR = 0, at 68% and 95% CL. The region of Re(gR) around 0.8 is highly disfavoured by the single-top cross section measurements by the Tevatron and LHC. Version in pdf

Table Caption
CorrelF0F0.png Correlation coefficients ρ(F0,F0) and ρ(FL,FL) assumed for each measurement and uncertainty. Version in pdf
CorrelF0FL.png Correlation coefficients ρ(F0,FL) assumed for each measurement and uncertainty. Version in pdf
TableComb.png Uncertainties on the ATLAS and CMS combined FL and F0. Version in pdf
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Topic revision: r2 - 2013-03-14 - RobertoTenchini
 
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