Combination of the W boson polarization measurements in top quark decays using ATLAS and CMS data at sqrt(s) = 8 TeV

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 121 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 (F0), left-handed (FL), or right-handed (FR) polarizations. The resulting combined measurements of the polarization fractions are F0 = 0.693 \ub1 0.014 and FL = 0.315 \ub1 0.011. The fraction FR is calculated from the unitarity constraint to be FR = 120.008 \ub1 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 F0 (FL) with respect to the most precise single measurement. A limit on anomalous right-handed vector (VR), and left- and right-handed tensor (gL, gR) tWb couplings is set while fixing all others to their standard model values. The allowed regions are [ 120.11, 0.16] for VR, [ 120.08, 0.05] for gL, and [ 120.04, 0.02] for gR, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived