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 (p) over barp 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 -> Wb ->center dot nu b (where center dot=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.