We demonstrate that perturbative QCD allows one to calculate the absolute cross section of diffractive exclusive production of photons at large $Q^2$ at DESY HERA, while the aligned jet model allows one to estimate the cross section for intermediate $Q^2\sim 2\hspace{0.5em}{\mathrm{GeV}}^2.$ 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 $2,$ 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 $\mathrm{ep}$ scattering for HERA kinematics which allows one to directly measure the real part of the DVCS amplitude and hence the nondiagonal parton distributions.

• Received 30 January 1998

DOI:https://doi.org/10.1103/PhysRevD.58.114001