  We study photon-induced coherent production of Upsilon in ultraperipheral
heavy ion collisions at 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 are
beyond the reach of the electron-nucleon (nucleus) colliders, and will
unambiguously discriminate between the leading twist and higher twist scenarios
of gluon nuclear shadowing.
