Longitudinal, transverse-plus and transverse-minus W-bosons in unpolarized top quark decays at O(alpha_s)

We consider the O(alpha_s) radiative corrections to the decay of an unpolarized top quark into a bottom quark and a W-gauge boson where the helicities of the W are specified as longitudinal, transverse-plus and transverse-minus. The O(alpha_s) radiative corrections lower the normalized longitudinal rate Gamma_L/Gamma by 1.06% and increase the the normalized transverse-minus rate Gamma_-/Gamma by 2.17%. We find that the normalized transverse-plus rate Gamma_+/Gamma, which vanishes at the Born term level for m_b->0, receives radiative correction contributions at the sub-percent level. We discuss m_b!=0 effects for the Born term and the alpha_s-contributions but find these to be small. Our results are discussed in the light of recent measurements of the helicity content of the W in top quark decays by the CDF Collaboration.