  Fully reconstructed top-antitop-events are used to determine the fractions of
right-handed and longitudinally polarized W bosons produced in top-quark
decays. The helicity fractions are sensitive to the couplings and the Dirac
structure of the Wtb vertex. This paper 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 proton-antiproton collisions collected by
the CDF II detector operating at the Fermilab Tevatron.
