Charmonia production in 450 GeV/c proton induced reactions

Absolute J/ψ and ψ′ production cross sections have been measured at the CERN SPS, with 450 GeV/c protons incident on a set of C, Al, Cu and W targets. Complementing these values with the results obtained by experiment NA51, which used the same beam and detector with H and D targets, we establish a coherent picture of charmonia production in proton-induced reactions at SPS energies. In particular, we show that the scaling of the J/ψ cross section with the mass number of the target, A, is well described as Aα, with αψ=0.919±0.015. The ratio between the J/ψ and ψ′ yields, in our kinematical window, is found to be independent of A, with αψ′−αψ=0.014±0.011.