  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 nucleon-nucleon
collision, with a larger effect at lower incident energy, and we compare this
result with previous observations by other fixed-target experiments. An attempt
to disentangle the different contributions to the observed suppression has been
carried out by studying the dependence of nuclear effects on x_2, the fraction
of nucleon momentum carried by the interacting parton in the target nucleus.

Nuclear Experiment