  We prove rigorously factorization in the seminclusive decay $B\to D^{(*)} +
jet$ using the large energy effective theory. It is also shown that this
effective theory is unable to consistently describe completely exclusive
processes, such as the decay $B\to D^{(*)}+\pi(\rho)$, and therefore also
related properties such as factorization. This is due to an oversimplification
of transverse momentum dynamics. 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 momentum dynamics and is therefore the
natural framework to study exclusive non-leptonic decays. Finally, it is shown
that the collinear instability of the large energy effective theory disappears
when seminclusive observables are considered.
