Abstract:
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 anti-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 --> Wb --> l nu b (where l = 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.