The Double Radiative Decays B → γγ in the Heavy Quark Limit

We analyze the double radiative B-meson decays Bs → γγ and Bd → γγ in QCD factorization based on the heavy-quark limit mb ≫ ΛQCD. We systematically discuss the various contributions to these exclusive processes. The dominant effect arises from the magnetic-moment type transition b → s(d)γ 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-asymmetry in B → γγ and to estimate so-called long-distance contributions in B and D → γγ. Numerical results are presented for branching ratios and CP asymmetries.