HEAVY-QUARK DISTRIBUTION FUNCTION IN QCD AND THE AC(2)M(2) MODEL

We show the phenomenological AC2M2 ansatz to be consistent with QCD through order 1/m(b) in the description of B --> lnu(l)BAR + X(u) and B --> gamma X(s) transitions, including their energy spectra and differential distributions. This suggests a concrete realization of the QCD distribution function, which we call the ''Roman'' function. Yet the ACM2 description of the end-point domain in B --> lnu(l)BAR + X(c) is, formally speaking, incompatible with QCD: a different distribution function enters the description of b --> c decays as compared to the transitions to massless quarks. The intrinsic limitation of the AC2M2 model could reveal itself in different values of the effective b quark mass from fits of the LAMBDAb and B decays.