Enhancement of the $\Lambda_{b}$ decay rate

The enhancement \Delta \Gamma (\Lambda_b) of the \Lambda_b decay rate due to four-fermion processes of weak scattering and Pauli interference is calculated within the quark model. An estimate of the relative bu wave function at zero separation, |\Psi(0)|^2_{bu}, is obtained in terms of the \Sigma_b^* - \Sigma_b hyperfine splitting, the B^* - B hyperfine splitting, and the B meson decay constant f_B. For M(\Sigma_b^*) - M(\Sigma_b) = 56 \pm 16 MeV, M(B^*) - M(B) = 46 MeV, and f_B = 190 \pm 40 MeV, we find \Delta \Gamma(\Lambda_b) = (0.025 \pm 0.013) ps^{-1}, to be compared with the observed enhancement \Gamma(\Lambda_b) - \Gamma(B^0) = 0.20 \pm 0.05 ps^{-1}. Even such a meager enhancement entails a value of |\Psi(0)|^2_{bu} considerably larger than the corresponding value of |\Psi(0)|^2_{cd} in the \Lambda_c baryon.