Flavor-changing two-photon decay

We discuss a class of flavor-changing two-photon rare decays, such as b→sγγ. Both dimension-six and dimension-eight operators are shown to contribute to these decays. For a heavy top, contributions from dimension-six operators become sizable and even dominant. They can be simply related to a single-photon decay like b→sγ. Implications for ${\mathit{B}}_{\mathit{s}}$→γγ are discussed, especially with regard to the spin orientations of the photons.