arXiv2010
Exclusive processes in hard electroproduction are one of the best place for understanding QCD in view of its factorization properties. In particular, in the limit of asymptotic gamma* p center of mass energy, they give an access to the perturbative Regge properties of QCD. The HERA experiment recently provided precise data for rho 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 rho) matrix elements. Such a factorization is up to now only proven for a longitudinaly polarized rho. Within the kt-factorization approach (valid at large gamma* p invariant mass), we evaluate the impact factor of the transition gamma* -> rhoT 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, and clarifies the equivalence of two proposed parametrizations of the rhoT distribution amplitudes, illustrated here by the gamma* -> rhoT impact factor.