USE AND MISUSE OF QCD SUM-RULES IN HEAVY-TO-LIGHT TRANSITIONS - THE DECAY B-]RHO-E-NU REEXAMINED

The existing calculations of the form factors describing the decay B - -> rho e nu from QCD sum rules have yielded conflicting results at sma ll values of the invariant mass squared of the lepton pair. We demonst rate that the disagreement originates from the failure of the shea-dis tance expansion to describe the rho meson distribution amplitude in th e region where almost the whole momentum is carried by one of the cons tituents. This limits the applicability of QCD sum rules based on the short-distance expansion of a three-point correlation function to heav y-to-light transitions and calls for an expansion around the light con e, as realized in the light-cone sum rule approach. We derive and upda te light-cone sum rules for all the semileptonic form factors, using r ecent results on the rho meson distribution amplitudes. The results ar e presented in detail together with a careful analysis of the uncertai nties, including estimates of higher-twist effects, and compared to la ttice calculations and recent CLEO measurements. We also derive a set of ''improved'' three-point sum rules, in which some of the problems o f the short-distance expansion are avoided and whose results agree to good accuracy with those from light-cone sum rules.