Energy and Centrality Dependence of Rapidity Densities at RHIC

The energy and centrality dependence of the charged multiplicity per participant nucleon is shown to be able to differentiate between final-state saturation and fixed scale 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. HIJING model predicts that the rapidity density per participant increases with centrality, while the saturation model prediction is essentially independent of centrality.