Gluon shadowing in nuclei and the role of the $c \bar c g$ state in the coherent photoproduction of $J/\psi$ in nucleus-nucleus collisions

Coherent $J/\psi$ photoproduction in ultraperipheral nucleus--nucleus collisions is widely used as a proxy for small-$x$ nuclear gluon shadowing. In the color-dipole picture, multiple scatterings of the $c\bar c$ dipole in the nuclear medium generate a sizable suppression already at $x\sim 10^{-2}$. At higher energies, however, inelastic (Gribov) shadowing associated with higher Fock states becomes important. We summarize a dipole calculation in which the leading $c\bar c g$ state is included on top of Glauber--Gribov rescattering, and we show that it provides the additional suppression required by the LHC data at small~$x$.