QCD factorization beyond leading twist in exclusive processes: ρT -meson production

Exclusive processes in hard electroproduction is one of the best place for understanding the factorization properties of QCD. The HERA experiment recently provided precise data for ρ electroproduction, including all spin density matrix elements. From QCD, it is expected that such a process should factorize between a hard (calculable) coefficient function, and hadronic (p and ρ) matrix elements. Such a factorization is up to now only proven for a longitudinaly polarized ρ . Within the kt-factorization approach (valid at large sγ∗ p), we evaluate the impact factor of the transition γ∗ → ρT taking into account the twist 3 contributions. We show that a gauge invariant expression is obtained with the help of QCD equations of motion. More generally, relying on these equations and on the invariance under rotation on the light-cone of the factorized amplitude, the non-perturbative Distribution Amplitudes can be reduced to a minimal set. This opens the way to a consistent treatment of factorization for exclusive processes with a transversally polarized vector meson. We prove the equivalence of two proposed parametrizations of the ρT distribution amplitudes.