Two-loop QCD corrections to top quark width

We present O(alpha(s)(2)) corrections to the decay t --> bW in the limit of a very large top quark mass, m(t) much greater than m(w). We find that the O(alpha(s)(2)) effects decrease the top quark decay width Gamma(t) by about 2%: Gamma(t) = Gamma(0) (1 - 0.8 alpha(s)(m(t)) - 1.7 alpha(s)(2)). The complete corrections are smaller by about 24% than their estimate based on the BLM effects O(beta(0)alpha(s)(2)). We explain how to compute a new type of diagrams which contribute to Gamma(t --> bW) at the O(alpha(s)(2)) level. (C) 1999 Elsevier Science B.V.