Measurement of W-boson helicity fractions in top-quark decays using cos theta

Abstract Fully reconstructed t t ¯ → W + b W − b ¯ → l ν q q ¯ ′ b b ¯ events are used to determine the fractions of right-handed ( f + ) and longitudinally polarized ( f 0 ) W bosons produced in top-quark decays. The helicity fractions are sensitive to the couplings and the Dirac structure of the Wtb vertex. This Letter reports measurements of the W -boson helicity fractions from two different methods using data corresponding to an integrated luminosity of 1.9 fb − 1 of p p ¯ collisions at a center-of-mass energy of 1.96 TeV collected by the CDF II detector operating at the Fermilab Tevatron. Combining the results from the two methods, we find f 0 = 0.62 ± 0.10 ( stat ) ± 0.05 ( syst ) under the assumption that f + = 0 , and f + = − 0.04 ± 0.04 ( stat ) ± 0.03 ( syst ) with f 0 fixed to the theoretically expected value of 0.70. Model-independent fits are also performed and simultaneously determine f 0 = 0.66 ± 0.16 ( stat ) ± 0.05 ( syst ) and f + = − 0.03 ± 0.06 ( stat ) ± 0.03 ( syst ) . All these results are consistent with standard model expectations.