QCD corrections to the charged-Higgs-boson decay of a heavy top quark

It is shown that up to an overall scale the lowest-order QCD corrections to $t\to H^{+}b$ and to $t\to W^{+}b$ are the same in the heavy-top-quark limit. Asymptotically, they are given by $-\left(\frac{4{\alpha }_s}{3\pi }\right)[\frac{{\pi }^2}{3}-\frac{5}{4}]$, resulting in a reduction in the decay rate by about 9%, rather than 6% reported previously in the literature. This is verified explicitly by an analytic calculation. The application of the equivalence theorem to this process is also discussed.