Coherent Photoproduction from Nuclei

Abstract

We argue that study of the cross section of coherent photo(electro) production of vector mesons off nuclear targets provides an effective method to probe the leading twist hard QCD regimes of color transparency and perturbative color opacity as well as the onset of black body limit (BBL) in the soft and hard QCD interactions. In the case of intermediate energies we use the Generalized Vector Dominance Model to take into account coherence effects for two distinctive limits - the soft interactions for production of $ρ$ and $ρ'$-mesons and the color transparency regime for production of charmonium states. We demonstrate that GVDM describes very well $ρ$-meson coherent photoproduction at $6 \leq E_γ \leq 10$ GeV and predict an oscillating energy dependence for the coherent charmonium production. In the limit of small $x$ we find that hard QCD leads to onset of the perturbative color opacity even for production of very small onium states, like $Υ$. The advantages of the process of coherent dijet photoproduction and hard diffractive processes in general for probing the onset of BBL and measuring the light-cone wave function of the photon in a hard scattering regime where decomposition over twists becomes inapplicable are explained. We apply this analysis to the study of the photon induced coherent processes in Ultra Peripheral Collisions of ions at LHC and demonstrate that the counting rates will be sufficient to study the physics of color opacity and color transparency at the energies beyond the reach of the electron-nucleon(nucleus) colliders.