Effective Lagrangian approach to weak radiative decays of heavy hadrons

Motivated by the observation of the decay B¯→K¯^*γ by the CLEO Collaboration, we have systematically analyzed the two-body weak radiative decays of bottom and charmed hadrons. There exist two types of weak radiative decays: One proceeds through the short-distance b→sγ transition and the other occurs through W exchange accompanied by a photon emission. Effective Lagrangians are derived for the W-exchange bremsstrahlung processes at the quark level and then applied to various weak electromagnetic decays of heavy hadrons. Predictions for the branching ratios of B¯⁰→D^*0γ, Λ_b⁰→Σ_c⁰γ, Ξ_b⁰→Ξ_c⁰γ, and Ξ_b⁰→Ξ’_b⁰γ are given. In particular, we find scrB(B¯⁰→D^*0γ)≊0.9×10^-6. Order of magnitude estimates for the weak radiative decays of charmed hadrons, D⁰→K¯^*0γ, Λ_c⁺→Σ⁺γ, and Ξ_c⁰→Ξ⁰γ, are also presented. Within this approach, the decay asymmetry for antitriplet to antitriplet heavy baryon weak radiative transitions is uniquely predicted by heavy quark symmetry. The electromagnetic penguin contribution to Λ_b⁰→Λγ is estimated by two different methods and its branching ratio is found to be of the order of 1×10^-5. We conclude that weak radiative decays of bottom hadrons are dominated by the short-distance b→sγ mechanism.