Large $O(m_c^{-2})$ nonperturbative correction to the inclusive rate of the decay $B \to X_s γ$

It is shown that the inclusive rate of the rare weak radiative decays $B \to X_s \gamma$ contains a nonperturbative correction, whose `short distance' scale is set by $m_c^{-1}$ rather than by $m_b^{-1}$. This correction is expressed through the chromomagnetic interaction of the b quark inside the B meson and the relative magnitude of the effect is determined by the ratio $<{\overline B}| \bar b \sigma \cdot G b |B> / m_c^2$. The correction is found to be negative and amounts (in the lowest order in $\alpha_s$) to approximately 20% reduction of the rate in comparison with the perturbative result. Possible further implications of the presence of the distance scale $m_c^{-1}$ in the operator product expansion for inclusive decay rates of b hadrons are briefly discussed.