Factorization and Effective Theories

We prove factorization in the decay of a B meson into a D* + jet using the Large Energy Effective Theory. The proof is non perturbative, does not require any gauge fixing and is exact in the limit of a very narrow jet. On the other hand, it is shown that this theory is unable to consistently describe completely exclusive processes such as for example a B which decays into a D* + pion, due to an oversimplification of transverse momentum dynamics. Therefore we present a variant of the Large Energy Effective Theory, i.e. a new effective theory for massless particles which properly takes into account transverse degrees of freedom and is the natural framework to study exclusive non-leptonic decays.