Energy and Centrality Dependence of Rapidity Densities at RHIC

The energy and centrality dependence of the charged multiplicity per participant nucleon, $dN_{ch}/d\eta/< N_{part}>$, is shown to be able to differentiate clearly between saturation and fixed scale ($p_0$) pQCD models of initial entropy production in high-energy heavy-ion collisions. The energy dependence is shown to test the nuclear enhancement of the mini-jet component of the initial conditions, while the centrality dependence provides a key test of whether gluon saturation is reached at RHIC energies ($\sqrt{s}\sim 50-200$ AGeV). Models such as HIJING with $p_0$ fixed from $p+p(\bar{p})$ data predict that the rapidity density increases with the number of binary interactions, while the EKRT saturation model predicts that the rapidity density per participant is essentially independent of centrality.