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

We consider the O(α_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(α_s) radiative corrections lower the normalized longitudinal rate Γ_L/Γ by 1.06% and increase the normalized transverse-minus rate Γ_-/Γ by 2.17%. We find that the normalized transverse-plus rate Γ₊/Γ, 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 α_s-contributions, but find these to be small. Our results are discussed in light of recent measurements of the helicity content of the W in top quark decays by the CDF Collaboration.