Centrality Dependence of the Charged Particle Multiplicity near Mid-Rapidity in Au+Au Collisions at sqrt(s_NN) = 130 and 200 GeV

The PHOBOS experiment has measured the charged particle multiplicity at mid-rapidity in Au+Au collisions at sqrt(s_NN) = 200 GeV as a function of the collision centrality. Results on dN/deta(eta<1) divided by the number of participating nucleon pairs, <Npart/2>, are presented as a function of <Npart>. As was found from similar data at sqrt(s_NN) = 130 GeV, the data can be equally well described by parton saturation models and two-component fits which include contributions that scale as Npart and the number of binary collisions, Ncoll. We compare the data at the two energies by means of the ratio R(200/130) of the charged particle multiplicity for the two different energies as a function of <Npart>. For events with <Npart>>100$, we find that this ratio is consistent with a constant value of 1.14+-0.01(stat.)+-0.05(syst.).