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 (F0), left-handed (FL), or right-handed (FR) polarizations. The resulting combined measurements of the polarization fractions are F0 = 0.693 ± 0.014 and FL = 0.315 ± 0.011. The fraction FR is calculated from the unitarity constraint to be FR = −0.008 ± 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 [−0.11, 0.16] for VR, [−0.08, 0.05] for gL, and [−0.04, 0.02] for gR, at 95% confidence level. Limits on the corresponding Wilson coefficients are also derived.
Highlights
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
This paper describes the combination of the W boson polarization fractions measured √
Summary
Three measurements of the W boson polarization in the top quark decay from top quark pair production events in the +jets channel and one from events with a single top quark signature are the four input measurements in this combination. The measurements based on tt production events were performed by the ATLAS [5] and CMS [6] experiments, where the latter was separated in electron and muon channels. The measurements were based on pp collision data at s = 8 TeV, corresponding to integrated luminosities of 20.2 and 19.7 fb−1 for the ATLAS and CMS experiments, respectively. All measurements were based on fits where the polarization fractions were adjusted to describe the observed cos θ∗ distributions of the semileptonically decaying top quark, taking into account the SM predictions for the backgrounds. These measurements are summarized in the rest of the section. Detailed descriptions of the ATLAS and CMS detectors can be found elsewhere [8, 9]
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