Collision geometry scaling ofAu+Aupseudorapidity density fromsNN=19.6to200GeV

The centrality dependence of the midrapidity charged particle multiplicity in $\mathrm{Au}+\mathrm{Au}$ heavy-ion collisions at $\sqrt{s_{NN}}=19.6$ and $200\mathrm{GeV}$ is presented. Within a simple model, the fraction of hard (scaling with number of binary collisions) to soft (scaling with number of participant pairs) interactions is consistent with a value of $x=0.13\pm 0.01\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right)$ at both energies. The experimental results at both energies, scaled by inelastic $p\left(\overline{p}\right)+p$ collision data, agree within systematic errors. The ratio of the data was found not to depend on centrality over the studied range and yields a simple linear scale factor of $R_{200∕19.6}=2.03\pm 0.02\left(\mathrm{stat}\right)\pm 0.05\left(\mathrm{syst}\right)$.