TTP97-31 Calculation of the semileptonic decay width of the $\Lambda_b$ baryon

We extend the approach based on the light-cone expansion and the heavy quark effective theory to the inclusive semileptonic decay of an unpolarized $b$-flavored hadron. It is applied to calculate the semileptonic decay width of the $\Lambda_b$ baryon and its ratio to that of the $B$ meson. We obtain $\Gamma_{\rm SL}(\Lambda_b)= 51\pm 9 \ |V_{cb}|^2\mbox{ps}^{-1}$ and $\Gamma_{\rm SL}(\Lambda_b)/\Gamma_{\rm SL}(B)= 1.052\pm 0.006$. From the latter, the semileptonic branching fraction for $\Lambda_b$ is predicted to be $(8.7\pm 0.7)\%$.