Charged-Particle Multiplicity near Midrapidity in Central $Au+Au$ Collisions at $\sqrt{{\mathit{s}}_{\mathit{NN}}}=56\mathrm{}$ and $130\mathrm{GeV}$

We present the first measurement of pseudorapidity densities of primary charged particles near midrapidity in $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=56\mathrm{}$ and $130\mathrm{GeV}$. For the most central collisions, we find the charged-particle pseudorapidity density to be $\mathrm{dN}/d\eta {|}_{|\eta |<1\mathrm{}}=408\mathrm{}\pm 12\left(\mathrm{stat}\right)\pm 30\left(\mathrm{syst}\right)$ at $56\mathrm{GeV}$ and $555\pm 12\left(\mathrm{stat}\right)\pm 35\left(\mathrm{syst}\right)$ at $130\mathrm{GeV}$, values that are higher than any previously observed in nuclear collisions. Compared to proton-antiproton collisions, our data show an increase in the pseudorapidity density per participant by more than $40\%$ at the higher energy.