Measurement of the W boson helicity in top quark decays at D0

Abstract

We present a measurement of the fraction f(+) of right-handed W bosons produced in top quark decays, based on a candidate sample of tt-bar events in the ℓ+jets and dilepton decay channels corresponding to an integrated luminosity of 370 pb(−1) collected by the D0 detector at the Fermilab Tevatron pp-bar Collider at s√=1.96 TeV. We reconstruct the decay angle θ∗ for each lepton. By comparing the cosθ∗ distribution from the data with that for the expected background and signal for various values of f(+) (where we assume that the fraction of longitudinally-polarized W bosons has the standard model value of 0.70), we find f(+)=0.056±0.080(stat)±0.057(syst) (f(+)<0.23 at 95% C.L.), consistent with the standard model prediction of f(+)=3.6×10(−4).