Onset of perturbative color opacity at small x and Υ coherent photoproduction off heavy nuclei at LHC

We study photon-induced coherent production of Υ in ultraperipheral heavy ion collisions at the LHC and demonstrate that the counting rates will be sufficient to measure nuclear shadowing of generalized gluon distributions. This will establish the transition from the regime of color transparency to the regime of perturbative color opacity in an unambiguous way. We argue that such measurements will provide the possibility to investigate the interaction of ultra-small color dipoles with nuclei in QCD at large energies, which is beyond the reach of electron-nucleon (nucleus) colliders, and which will unambiguously discriminate between the leading twist and higher twist scenarios of gluon nuclear shadowing.