Combination of the W boson polarization measurements in top quark decays using ATLAS and CMS data at $\sqrt{s} =$ 8 TeV
Atlas Collaborations
Abstract
The combination of measurements of the W boson polarization in top quark decays performed by the ATLAS and CMS Collaborations is presented. The measurements are based on proton-proton collision data produced at the LHC at a centre-of-mass energy of 8 TeV, and corresponding to an integrated luminosity of about 20 fb$^{-1}$ for each experiment. The measurements used events containing one lepton and having different jet multiplicities in the final state. The results are quoted as fractions of W bosons with longitudinal ($F_0$), left-handed ($F_\\mathrm{L}$), or right-handed ($F_\\mathrm{R}$) polarizations. The resulting combined measurements of the polarization fractions are $F_0 =$ 0.693 $\\pm$ 0.014 and $F_\\mathrm{L} =$ 0.315 $\\pm$ 0.011. The fraction $F_\\mathrm{R}$ is calculated from the unitarity constraint to be $F_\\mathrm{R} = -$0.008 $\\pm$ 0.007. These results are in agreement with the standard model predictions at next-to-next-to-leading order in perturbative quantum chromodynamics and represent an improvement in precision of 25 (29)% for $F_0$ ($F_\\mathrm{L}$) with respect to the most precise single measurement. A limit on anomalous right-handed vector ($V_{\\text{R}}$), and left- and right-handed tensor ($g_{\\text{L}},g_{\\text{R}}$) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [$-$0.11, 0.16] for $V_{\\text{R}}$, [$-$0.08, 0.05] for $g_{\\text{L}}$, and [$-$0.04, 0.02] for $g_{\\text{R}}$, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived.