Charmonium suppression from shadowing effects

The extent to which geometrical effects contribute to the production and suppression of the $J/\psi$ and $q\overline{q}$ minijet pairs in general is investigated for high-energy heavy-ion collisions at SPS, RHIC, and LHC energies. For the energy range under investigation, the geometrical effects referred to are shadowing and antishadowing, respectively. Due to those effects, the parton distributions in nuclei deviate from the naive extrapolation from the free nucleon result; $f_A\ne {\mathrm{Af}}_N.$ The strength of the shadowing/antishadowing effect increases with the mass number. Therefore it is interesting to see the difference between cross sections for, e.g., $S+U$ vs Pb+Pb at SPS. The recent NA50 results for the survival probability of produced $J/\psi$’s has attracted great attention and are often interpreted as a signature of a quark gluon plasma. This paper will present a fresh look on hard QCD effects for the charmonium production level. It is shown that the apparent suppression of $J/\psi$’s must also be linked to the production process. Due to the uncertainty in the shadowing of gluons the suppression of charmonium states might not give reliable information on a created plasma phase at the collider energies now available. The consequences of shadowing effects for the $x_F$ distribution of $J/\psi$’s at $\sqrt{s}=20\mathrm{}\mathrm{GeV},$ $\sqrt{s}=200\mathrm{}\mathrm{GeV},$ and $\sqrt{s}=6\mathrm{}\mathrm{TeV}$ are calculated for some relevant combinations of nuclei, as well as the $p_T$ distribution of minijets at midrapidity for $N_f=4\mathrm{}$ in the final state.