  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.
