Charmonium suppression from purely geometrical effects

The extend to which geometrical effects contribute to the production and suppression of the J/ψ and qq 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 anti-shadowing, respectively. Due to those effects, the parton distributions in nuclei deviate from the naive extrapolation from the free nucleon result; fA 6= AfN . The strength of the shadowing/anti-shadowing 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/ψ’s has attracted great attention and are often interpreted as a signature of a quark gluon plasma. This publication will present a fresh look on hard QCD effects for the charmonium production level. It is shown that the apparent suppression of J/ψ’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 soon available. The consequences of shadowing effects for the xF distribution of J/ψ’s at √ s = 20 GeV, √ s = 200 GeV and √ s = 6 TeV are calculated for some relevant combinations of nuclei, as well as the pT distribution of minijets at midrapidity for Nf = 4 in the final state. Work supported by BMBF, DFG, GSI