Effective Lagrangian approach to weak radiative decays of heavy hadrons

Motivated by the observation of the decay B¯→K${\mathrm{¯}}^{\mathrm{*}}$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${\mathrm{¯}}^0$→${\mathit{D}}^{\mathrm{*}0}$γ, ${\mathrm{\Lambda }}_{\mathit{b}}^0$→${\mathrm{\Sigma }}_{\mathit{c}}^0$γ, ${\mathrm{\Xi }}_{\mathit{b}}^0$→${\mathrm{\Xi }}_{\mathit{c}}^0$γ, and ${\mathrm{\Xi }}_{\mathit{b}}^0$→Ξ${\mathcal{’}}_{\mathit{b}}^0$γ are given. In particular, we find scrB(B${\mathrm{¯}}^0$→${\mathit{D}}^{\mathrm{*}0}$γ)≊0.9×${10}^{\mathrm{-}6}$. Order of magnitude estimates for the weak radiative decays of charmed hadrons, ${\mathit{D}}^0$→K${\mathrm{¯}}^{\mathrm{*}0}$γ, ${\mathrm{\Lambda }}_{\mathit{c}}^{+}$→${\mathrm{\Sigma }}^{+}$γ, and ${\mathrm{\Xi }}_{\mathit{c}}^0$→${\mathrm{\Xi }}^0$γ, 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 ${\mathrm{\Lambda }}_{\mathit{b}}^0$→Λγ is estimated by two different methods and its branching ratio is found to be of the order of 1×${10}^{\mathrm{-}5}$. We conclude that weak radiative decays of bottom hadrons are dominated by the short-distance b→sγ mechanism.