Exclusive-Inclusive Ratio of Semileptonic Lambda_b-Decays

We present theoretical evidence that the exclusive/inclusive ratio of semileptonic $\Lambda_b$-decays exceeds that of semileptonic B-decays where the experimental exclusive/inclusive ratio amounts to about 66%. We start from the observation that the spectator quark model provides a lower bound on the leading order Isgur-Wise function of the $\Lambda_b \to \Lambda_c$ transition in terms of the corresponding $B \to D,D^*$ mesonic Isgur-Wise function. Using experimental data for the $B \to D,D^*$ mesonic Isgur-Wise functions this bound is established. Applying a Bethe-Salpeter model including spectator quark interactions and a QCD sum rule estimate of the $\Lambda_b \to \Lambda_c$ transition form factor which satisfy the spectator quark model bound we predict the exclusive/inclusive ratio of semileptonic $\Lambda_b$ decay rates to lie in a range between 0.81 and 0.92. We also provide an upper bound on the baryonic Isgur-Wise function which is determined from the requirement that the exclusive rate should not exceed the inclusive rate.