Exclusive/inclusive ratio of semileptonic Lambda(b) decays - art. no. 074008

We present theoretical evidence that the exclusive/inclusive ratio of semil eptonic Lambda(b) decays exceeds that of semileptonic B decays where the ex perimental exclusive/inclusive ratio amounts to about 66%. We start from th e observation that the spectator quark model provides a lower bound on the leading order Isgur-Wise function of the Lambda(b) --> Lambda(c) transition in terms of the corresponding B --> D, D* mesonic Isgur-Wise function. Usi ng experimental data for the B --> D,D* mesonic Isgur-Wise functions this b ound is established. Applying a Bethe-Salpeter model including spectator qu ark interactions and a QCD sum rule estimate of the Lambda(b) --> Lambda(c) transition form factor which satisfy the spectator quark model bound we pr edict the exclusive/inclusive ratio of semileptonic Lambda(b) decay rates t o he in a range between 0.81 and 0.92. We also provide an upper bound on th e baryonic Isgur-Wise function which is determined from the requirement tha t the exclusive rate should not exceed the inclusive rate.