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

Abstract

The semileptonic decays $\bar{B}\to D(D^{\ast })\pi \ell \bar{\nu }$ 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 $B^{-}\to D^{+}{\pi }^{-}{e}^{-}{\bar{\nu }}_e$ 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 ${\Lambda }_b^0\to {\Lambda }^0\gamma$ receives the electromagnetic penguin $b\to s\gamma$ contribution and has a branching ratio of order $2\times 10^{-5}$. The radiative decays $\bar{B}\to D^{\ast }\gamma ,{\Lambda }_b^0\to {\Sigma }_c^0\gamma ,{\Xi }_b^0\to {\Xi }_c^0\gamma$ and ${\Xi }_b^0\to {{\Xi }^{\prime }}_c^0\gamma$ 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.

Latex, 6 pages, ITP-SB-93-41