FACTORIZATION AND EFFECTIVE THEORIES

We prove factorization in the decay B --> D() + jet using the Large E nergy Effective Theory. The proof is non perturbative, does not requir e any gauge fixing and is exact in the limit of a very narrow jet. On the other band, it is shown that the Large Energy Effective Theory is unable to consistently describe completely exclusive processes such as for example B --> D() + pi due to an oversimplification of transvers e momentum dynamics. Therefore we present a variant of the Large Energ y Effective Theory, i.e. a new effective theory for massless particles which properly takes into account transverse degrees of freedom and i s the natural framework to study exclusive non-leptonic decays. (C) 19 98 Published by Elsevier Science B.V. All rights reserved.