Measurement of the helicity ofWbosons in top-quark decays

We measure the branching fraction of the top quark to longitudinally and right-handed polarized $W$ bosons, $F_0$ and $F_{+}$, using approximately $200{\mathrm{pb}}^{-1}$ of $\overline{p}p$ collisions collected by the CDF II detector. We analyze two quantities sensitive to the $W$ helicity: the invariant mass of the charged lepton and the bottom-quark jet in the decay $t\to Wb\to \ell \nu b$ (where $\ell =e$ or $\mu$), and the transverse momentum of the charged lepton. Constrained fits yield $F_0={0.74}_{-0.34}^{+0.22}$, and $F_{+}<0.27$ at the 95% confidence level. These measurements are consistent with the standard model predictions.