Measurement of the W boson polarization in top decay at CDF at sqrt[s] = 1.8-TeV

The polarization of the W boson in t{yields}Wb decay is unambiguously predicted by the standard model of electroweak interactions and is a powerful test of our understanding of the tbW vertex. We measure this polarization from the invariant mass of the b quark from t{yields}Wb and the lepton from W{yields}l{nu} whose momenta measure the W decay angle and direction of motion, respectively. In this paper we present a measurement of the decay rate (f{sub V+A}) of the W produced from the decay of the top quark in the hypothesis of V+A structure of the tWb vertex. We find no evidence for the nonstandard V+A vertex and set a limit on f{sub V+A} < 0.80 at 95% confidence level. By combining this result with a complementary observable in the same data, we assign a limit on f{sub V+A} < 0.61 at 95% CL. This corresponds to a constraint on the right-handed helicity component of the W polarization of f{sub +}<0.18 at 95% CL. This limit is the first significant direct constraint on f{sub V+A} in top decay.