Production and propagation of heavy-quark pairs in nuclei

We study the phenomenology of nuclear charmonium and bottomonium production by high-energy protons incident on nuclear targets. We combine concepts of color (singlet) trans-parency with the proposition that colored states can propagate in the medium over a limited space-time range (color opacity). It is shown that color transparency alone is incapable of explaining the significant depletion of quarkonium production rates in nuclei. We suggest that, under quite modest assumptions, the propagation of color-octet quark-antiquark pairs in intermediate states can provide a successful description of the observed A-dependence.