$J/\psi$ at small-$x$

Abstract

It has been suggested that the suppression of $J/\psi$ production in heavy nuclei is a signature of the formation of quark-gluon plasma. We here show that this phenomenon can be understood in terms of conventional physics, {\it i.e.} {\it i)} perturbative QCD, {\it ii)} the parton recombination implementation of shadowing in the initial state, and {\it iii)} final state interactions with the hadronic debris of the nuclear target. Unlike previous calculations we include both the direct $J/\psi$ production and its production via radiative $\chi$ decays ($\chi_J \to J/\psi + \gamma$). We are able to reproduce the experimental data including their small-$x$ behavior. We emphasize the importance of studying the $x_2$-dependence of the ratio $\sigma(b A)/\sigma(b N)$, where $b$ designates the beam and $x_2$ is the momentum fraction of the parton from the nuclear target.