Measurement of the Helicity of W Bosons 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 pb$^{-1}$ of $\bar{p}p$ 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. We find $F_0=0.74^{+0.22}_{-0.34}$, and $F_+<0.27$ at the 95% confidence level. These measurements are in agreement with the standard model predictions.