  We present O(alpha_s^2) corrections to the decay t-->bW in the limit of a
very large top quark mass, m_t >> 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(1-0.8alpha_s(m_t)-1.7alpha_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.

High Energy Physics - Phenomenology