Centrality dependence of the charged particle multiplicity near midrapidity inAu+Aucollisions atsNN=130and 200 GeV

The PHOBOS experiment has measured the charged particle multiplicity at midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=200\mathrm{}\hspace{0.5em}\mathrm{GeV}$ as a function of the collision centrality. Results on ${\mathrm{dN}}_{\mathrm{ch}}/d\eta {|}_{|\eta |<1\mathrm{}}$ divided by the number of participating nucleon pairs $〈N_{\mathrm{part}}〉/2\mathrm{}$ are presented as a function of $〈N_{\mathrm{part}}〉.$ As was found from similar data at $\sqrt{s_{\mathrm{NN}}}=130\mathrm{}\hspace{0.5em}\mathrm{GeV},$ the data can be equally well described by parton saturation models and two-component fits, which include contributions that scale as $N_{\mathrm{part}}$ and the number of binary collisions $N_{\mathrm{coll}}.$ 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 $〈N_{\mathrm{part}}〉.$ For events with $〈N_{\mathrm{part}}〉>100,$ we find that this ratio is consistent with a constant value of $1.14\pm 0.01\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right).\mathrm{}$