Measurement of the helicity fractions of W bosons from top quark decays using fully reconstructed t(t)over-bar events with CDF II

We present a measurement of the fractions F(0) 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 (t) over bar 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 cos theta(*) distribution in the data is determined by full kinematic reconstruction of the t (t) over bar candidates. We find F(0)=0.85(-0.22)(+0.15)(stat)+/- 0.06(syst) and F(+)=0.05(-0.05)(+0.11)(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(+)< 0.26 at the 95% confidence level.