QCD factorization of exclusive processes beyond leading twist: gamma*T -> rhoT impact factor with twist three accuracy

Understand

We describe a consistent approach to factorization of scattering amplitudes for exclusive processes beyond the leading twist approximation.

• The method involves the Taylor expansion of the scattering amplitude in the momentum space around the dominant light-cone direction and thus naturally introduces an appropriate set of non-perturbative correlators which encode effects not only of the lowest but also of the higher Fock states of the produced particle.
• The reduction of original set of correlators to a set of independent ones is achieved with the help of equations of motion and invariance of the scattering amplitude under rotation on the light-cone.
• We compare the proposed method with the covariant method formulated in the coordinate space, based on the operator product expansion.