Normalization of QCD corrections in top quark decay
  We discuss the effects of QCD corrections to the on-shell decay $t \to b W$. We resolve the scale ambiguity using the Brodsky-Lepage-Mackenzie scheme, and find that the appropriate coupling constant is $\alpha_{s}^{ \overline{MS}} (0.122m_{t})$. The largest long distance contribution comes from the definition of the on-shell mass of the top quark. We note that QCD corrections to the electroweak $\rho$ parameter are extremely small when the $\rho$ parameter is expressed in terms of the top quark width. 
Brian H. Smith and M.B. Voloshin