A new class of enhanced nonperturbative corrections to the inclusive $\overline{B}\to X_s\gamma$ decay rate is identified, which contribute first at order $\Lambda /m_b$ in the heavy-quark expansion and cannot be described using a local operator product expansion. Instead, these effects are described in terms of hadronic matrix elements of nonlocal operators with component fields separated by lightlike distances. They contribute to the high-energy part of the photon-energy spectrum but do not reduce to local operators when an integral over energy is taken to obtain the total inclusive decay rate. The dominant corrections depend on the flavor of the $B$-meson spectator quark and are described by trilocal four-quark operators. Their contribution is estimated using the vacuum insertion approximation. The corresponding uncertainty in the total decay rate is found to be at the few percent level. This new effect accounts for the leading contribution to the rate difference between $B^{-}$ and ${\overline{B}}^0$ mesons.

• Received 23 September 2006

DOI:https://doi.org/10.1103/PhysRevD.75.114005