We present a self-consistent method for treating nonperturbative effects in inclusive nonleptonic and semileptonic decays of heavy flavour hadrons. These effects give rise to powerlike corrections ∝ 1 mQ n with n ≥ 2. The leading correction to the semileptonic branching ratio occurs for n = 2. It is expressed in terms of the vector-pseudoscalar mass splitting: δBRsl BRsl≅BRnl· 6 [ (Mv 2-Mp 2) mQ 2](c+ 2-c- 2 ) 2Nc and yields a reduction of BRsl. This nonperturbative correction contributes to the nonleptonic width with a sign opposite to that of the perturbative terms that are non-leading in 1 Nc. In beauty decays the former reduces the latter by 20% whereas in charm decays they more or less cancel. This leads to a reduction of BRsl by no more than 10% in beauty decays and by a factor of roughly two in charm decays. Phenomenological models of heavy flavour decays are unable to mimic these leading corrections by a specific choice of quark masses or by invoking Fermi motion.
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N.U. and A.V. gratefully acknowledge illuminating discussions on the subject of this paper with M.A. Shifman and M.B. Voloshin. N.U. thanks also V.M. Braun, M. Neubert and M.B. Wise for informative discussions on EHQT. He has benefitted from the inspiring atmosphere at the Theoretical Physics Institute of the University of Minnesota; together with I.B. he is grateful to the members of the FNAL Theory Division for their kind hospitality and in particular to Joanne Cohn for her great tolerance and patience. This work was supported in part by the National Science Foundation under grant number PHY 89-09929.