Predictions for Semileptonic Decays $\bar{B}\to D(D^*)π\ell\barν$ and Weak Radiative Decays of Bottom Hadrons

Abstract

The semileptonic decays $\bar{B}\to D(D^*)π\ell\barν$ are analyzed by heavy quark and chiral symmetries. The branching ratio is of order of a few percent for $\dpi$, but it is much smaller for $\dstar$, of order $10^{-4}-10^{-5}$. The decay mode $B^-\to D^+π^-e^-\barν_e$ provides a nice place for extracting the $D^*Dπ$ coupling constant from the semileptonic decays of a $\bar{B}$ meson. Two types of weak radiative decays of $\bar{B}$ mesons and bottom baryons are studied. The decay $Λ_b^0\to Λ^0γ$ receives the electromagnetic penguin $b\to sγ$ contribution and has a branching ratio of order $2\times 10^{-5}$. The radiative decays $\bar{B}\to D^*γ,~ Λ_b^0\toΣ_c^0γ,~Ξ_b^0\to Ξ_c^0γ$ and $Ξ_b^0\to{Ξ'}_c^0γ$ occur through $W$ exchange accompanied with a photon emission and their branching ratios are estimated to be much smaller. We conclude that the weak radiative decays of bottom hadrons are dominated by the short-distance electromagnetic penguin mechanism.