Abstract:
We present a measurement of the fractions F0 and F+ of longitudinally polarized and right-handed W 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 318 pb-1. We select t[overline t] candidate events with one lepton, at least four jets, and missing transverse energy. Our helicity measurement uses the decay angle theta*, 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 costheta* distribution in the data is determined by full kinematic reconstruction of the t[overline t] candidates. We find F0=0.85-0.22+0.15(stat)$\pm$0.06(syst) and F+=0.05-0.05+0.11(stat)$\pm$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+
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