Date of this Version
We measure the branching fraction of the top quark to longitudinally and right-handed polarized W bosons, F0 and F+, using approximately 200 pb-1 of p̅p 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 → ℓνb (where ℓ = ℯ or μ), and the transverse momentum of the charged lepton. Constrained fits yield F0 = 0.7̅4̅-0.34+0.22, and F+ < 0.2̅7̅ at the 95% confidence level. These measurements are consistent with the standard model predictions.