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

Abstract

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 pA (dA) collisions at collider energies the dependence of semi--hard processes on the number of participating nucleons of the nucleus will change to \sim (N_{part}^A)^{1/2}. The forthcoming data on dA collisions will provide a crucial test of this description.

Figures (4)

• Figure 1: The simplest Mueller diagram for the high $p_t$ jet inclusive production.
• Figure 2: The $p_t$-spectrum of charged hadrons in central ($0-10\%$ centrality cut) $Au-Au$ collisions at $\sqrt{s} = 130$ GeV. The data is from phenixpt.
• Figure 3: Centrality dependence of hadron yields per participant pair in $Au-Au$ collisions at $\sqrt{s}=130$ and $200$ GeV in different $p_t$ bins; the yields are normalized to the yield in peripheral collisions. Upper solid lines show the behavior expected in perturbative QCD.
• Figure 4: Same as in Fig.\ref{['cent']} in the $p_t$ bins of $6$ and $9$ GeV.