J/Ψ suppression and the quark-gluon plasma

All measured Feynman xF distributions of the ratio, R, of J/Ψ production in nuclei relative to production on protons fall off with xF. They show that absorption of charmonium cannot be the only source of J/Ψ suppression and that energy loss of the constituents of the incident proton prior to the J/Ψ production, because of the exponential s dependence of the charmonium cross section, should not be neglected. Including the effects of initial state energy loss we find that the latest measured Pb–Pb J/Ψ cross sections do not provide any evidence for deconfinement.