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 anti-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 cos theta* distribution in the data is determined by full
kinematic reconstruction of the tt-bar candidates. We find F_0 =
0.85+0.15_-0.22(stat) +/- 0.06(syst) and F_+ = 0.05 0.11_-0.05 (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.
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Updated: Tuesday, 2007 March 20 13:59:01 CDT automatically from input from Carol Picciolo