DVCS amplitude with kinematical twist-3 terms

We compute the amplitude of deeply virtual Compton scattering (DVCS) using the calculus of QCD string operators in coordinate representation. To restore the electromagnetic gauge invariance (transversality) of the twist-2 amplitude we include the operators of twist-3 which appear as total derivatives of twist-2 operators. Our results are equivalent to a Wandzura–Wilczek approximation for twist-3 skewed parton distributions. We find that, for the tensor amplitude Tμν, this approximation gives a finite result for the longitudinal polarization of the virtual photon and a divergent result in case of transverse polarization. However, the divergent part has zero projection onto the polarization vector of the final real photon.