Measurement of theWboson helicity in top quark decays at D0

We present a measurement of the fraction $f_{+}$ of right-handed $W$ bosons produced in top quark decays, based on a candidate sample of $t\overline{t}$ events in the $\ell +\mathrm{\text{jets}}$ and dilepton decay channels corresponding to an integrated luminosity of $370{\mathrm{pb}}^{-1}$ collected by the D0 detector at the Fermilab Tevatron $p\overline{p}$ Collider at $\sqrt{s}=1.96\mathrm{TeV}$. We reconstruct the decay angle ${\theta }^{*}$ for each lepton. By comparing the $\cos {\theta }^{*}$ 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 $f_{+}=0.056\pm 0.080(\mathrm{stat})\pm 0.057(\mathrm{syst})$ ($f_{+}<0.23$ at 95% C.L.), consistent with the standard model prediction of $f_{+}=3.6\times {10}^{-4}$.