Measurement of the W-boson Helicity Fractions in Top-quark Decays at Cdf

We present a measurement of the fractions F0 and F+ of longitudinally polarized and righthandedW bosons in top-quark decays using data collected with the CDF II detector. The data set used in the analysis corresponds to an integrated luminosity of approximately 955 pb. We select tt̄ candidate events with one lepton, at least four jets, and missing transverse energy. Our helicity measurement uses the decay angle θ∗, which is defined as the angle between the momentum of the charged lepton in the W boson rest-frame and the W momentum in the top-quark rest-frame. The cos θ∗ distribution in the data is determined by full kinematic reconstruction of the tt̄ candidates. We find F0 = 0.59 ± 0.12(stat) +0.07 −0.06(syst) and F+ = −0.03± 0.06(stat) −0.03(syst), which is consistent with the standard model prediction. We set an upper limit on the fraction of right-handed W bosons of F+ ≤ 0.10 at the 95% confidence level.