Parton saturation and Npart scaling of semi-hard processes in QCD

We argue that the suppression of high p(t) hadrons discovered recently in heavy ion collisions at RHIC may be a consequence of saturation in the color glass condensate. We qualitatively and semi-quantitatively describe the data, in particular, the dependence upon the number of nucleon participants. We show that if parton saturation sets in at sufficiently small energy, then in nucleus-nucleus collisions at RHIC and LHC energies the cross sections of semi-hard processes should scale approximately with the number of participants, N-part. Our results provide a possible explanation of both the absence of apparent jet quenching at SPS energies and its presence at RHIC. Under the same assumption we predict that in semi-central and central p(A) (dA) collisions at collider energies the dependence of semi-hard processes on the number of participating nucleons of the nucleus will change to similar to(N-part(A))(1/2). The forthcoming data on dA collisions will provide a crucial test of this description. (C) 2003 Published by Elsevier Science B.V.