Effective Lagrangian Approach to Weak Radiative Decays of Heavy Hadrons

Abstract

Motivated by the observation of the decay $\bar{B}\to \bar{K}^*γ$ by CLEO, 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 $b\to sγ$ 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 $\bar{B}^0\to D^{*0} γ,~Λ_b^0\toΣ_c^0γ,~Ξ_b^0\to Ξ_c^0γ$ and $Ξ_b^0\to\xip_c^0γ$ are given. In particular, we found ${\cal B}(\bar{B}^0 \to D^{*0}γ)\approx 0.9\times 10^{-6}$. Order of magnitude estimates for the weak radiative decays of charmed hadrons: $~D^0\to \bar{K}^{*0}γ,~Λ_c^+\toΣ^+γ$ and $Ξ_c^0\toΞ^0γ$ 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 $Λ_b^0\toΛγ$ is estimated by two different methods and its branching ratio is found to be of order $1\times 10^{-5}$. We conclude that weak radiative decays of bottom hadrons are dominated by the short-distance $b\to sγ$ mechanism.