Inclusive charmless semileptonic decay of \Lambda_b and Br(b ---> X_u l \nu_l)

Abstract

We study the inclusive charmless semileptonic decay of $\Lambda_b$, $\Lambda_b \to X_u l \nu_l$, in the framework of heavy quark expansion and find that the rate is substantially enhanced by spectator effect. We obtain $Br(b \to X_u l \nu_l)$ to be $1.13 \times 10^{-3}$ for $|V_{ub}| = 3.3 \times 10^{-3}$. We discuss our result in light of the recent LEP measurements of the charmless semileptonic branching ratio where such enhancement was not taken into account in extracting $|V_{ub}|$.