Predictions for Semileptonic Decays $\bar{B}\to D(D^{\ast })\pi \mathrm{\ell }\bar{\nu }$ and Weak Radiative Decays of Bottom Hadrons

The semileptonic decays $\bar{B}\to D(D^{\ast })\pi \mathrm{\ell }\bar{\nu }$ are analyzed by heavy quark and chiral symmetries. The branching ratio is of order of a few percent for $\text{dpi}$, but it is much smaller for $\text{dstar}$, of order ${10}^{-4}-{10}^{-5}$. The decay mode $B^{-}\to D^{+}{\pi }^{-}{e}^{-}{\bar{\nu }}_e$ provides a nice place for extracting the $D^{\ast }D\pi$ 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 $\Lambda_b^0\to \Lambda^0\gamma$receivestheelectromagneticpenguin$b\to s\gamma$contributionandhasabranchingratiooforder$2\times 10^{-5}$. The radiative decays $\bar{B}\to D^*\gamma,~ \Lambda_b^0\to\Sigma_c^0\gamma,~\Xi_b^0\to \Xi_c^0\gamma$and$\Xi_b^0\to{\Xi’}_c^0\gamma$occurthrough$W$ 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.