Charmonium production and absorption in proton-nucleus collisions

The $J/\psi$, $\psi'$ and $\chi_c$ yields are expected to be considerably suppressed if a deconfined medium is formed in high-energy heavy-ion collisions. However, already in $pA$ collisions the charmonium production cross sections scale less than linearly with the number of binary $NN$ collisions. This ``normal nuclear absorption'' must be accounted for before signals of the QCD medium can be identified in the $AA$ measurements. We compare the $J/\psi$ and $\psi'$ production in fixed-target $pA$ interactions ($200 < E_{\rm lab} < 920$~GeV) and in d+Au collisions at RHIC ($\sqrt{s}=200$~GeV) with Glauber calculations using several sets of \emph{nuclear} parton densities. We find a significant energy dependence of the mid-rapidity charmonium ``absorption cross sections'', indicating stronger nuclear absorption than previously assumed at $E_{\rm lab} = 158$~GeV, the CERN SPS lead beam energy. We also show that the absorption depends on the charmonium rapidity, even close to mid-rapidity. These new findings indicate stronger nuclear absorption than previously estimated from the SPS heavy-ion data.