Hadron production in nuclear collisions at RHIC and high density QCD

We analyze the first results on charged particle multiplicity at RHIC in the conventional eikonal approach and in the framework of high density QCD. We extract the fraction $F$ of the hadron multiplicity originating from ``hard’’ (i.e. proportional to the number of binary collisions) processes; we find a strong growth of this fraction with energy: $F(\sqrt{s}=56 {\rm GeV}) \simeq 22%$,while$F(\sqrt{s}=130 {\rm GeV}) \simeq 37 %$. This indicates a rapid increase in the density of the produced particles. We outline the predictions of high density QCD for the centrality, energy, and atomic number dependence of hadron production. Surprisingly, the predictions of the conventional eikonal approach and of high density QCD for centrality dependence of hadron multiplicity at $\sqrt{s}=130\mathrm{G}\mathrm{e}\mathrm{V}$ appear very similar.