Diffractive exclusive photon production in DIS at DESY HERA

We demonstrate that perturbative QCD allows one to calculate the absolute cross section of diffractive exclusive production of photons at large (Formula presented) at DESY HERA, while the aligned jet model allows one to estimate the cross section for intermediate (Formula presented) Furthermore, we find that the imaginary part of the amplitude for the production of real photons is larger than the imaginary part of the corresponding DIS amplitude by about a factor of (Formula presented) leading to the prediction of a significant counting rate for the current generation of experiments at HERA. We also find a large azimuthal angle asymmetry in (Formula presented) scattering for HERA kinematics which allows one to directly measure the real part of the DVCS amplitude and hence the nondiagonal parton distributions.