SEMILEPTONIC AND EXCLUSIVE RARE B-DECAYS

The exclusive rare decay B --> Kgamma takes place in a region of maxi mum recoil, q2 = 0, posing a problem for nonrelativistic quark models which are usually thought to be most reliable at zero recoil. The Baue r-Stech-Wirbel (BSW) model, formulated in the infinite-momentum-frame (IMF) formalism, is designed to work at q2 = 0. We show in this model that the ratio relating the decay B --> Kgamma and the q2-spectrum of the semileptonic decay B --> rhoenuBAR, becomes independent of the wa ve function in the SU(3) flavor symmetry limit. We show that this feat ure is also true in relativistic quark models formulated in the IMF or light-cone formalism, if the b quark is infinitely heavy. In fact, th ese relativistic models, which have a different spin structure from th e BSW case, reduce to the BSW model in the heavy b-quark limit. A dire ct measurement of the q2-spectrum of the semileptonic decay can theref ore provide accurate information for the exclusive rare decay.