The Double Radiative Decays Btoγγ in the Heavy Quark Limit

We analyze the double radiative B-meson decays B_s -> gamma gamma and B_d -> gamma gamma in QCD factorization based on the heavy-quark limit m_b >> Lambda_QCD. We systematically discuss the various contributions to these exclusive processes. The dominant effect arises from the magnetic-moment type transition b -> s(d) gamma where an additional photon is emitted from the light quark (one-particle reducible diagram). The contributions from one-particle irreducible diagrams are power suppressed. We argue that they are still calculable within QCD factorization. They are used to compute the CP-asymmetries in B -> gamma gamma and to estimate so-called long-distance contributions in B and D -> gamma gamma. Numerical results are presented for branching ratios and CP asymmetries.