J/$\psi$ production in proton-nucleus collisions at 158 and 400 GeV

The NA60 experiment has studied J/psi production in p-A collisions at 158 and 400 GeV, at the CERN SPS. Nuclear effects on the J/psi yield have been estimated from the A-dependence of the production cross section ratios sigma_{J/psi}^{A}/sigma_{J/psi}^{Be} (A=Al, Cu, In, W, Pb, U). We observe a significant nuclear suppression of the J/psi yield per N-N collision, with a larger effect at lower incident energy, and we compare this result with previous observations by fixed-target experiments. By studying, at the two energies, the dependence of nuclear effects on x_2, the fraction of nucleon momentum carried by the interacting parton from the target nucleus, we conclude that a combination of nuclear shadowing and break-up by target nucleons cannot be the only source of nuclear suppression of the J/psi.