The first measurement of energy produced transverse to the beam direction at the Relativistic Heavy-Ion Collider at Brookhaven National Laboratory is presented. The midrapidity transverse energy density per participating nucleon rises steadily with the number of participants, closely paralleling the rise in charged-particle density, such that $〈E_T〉/〈N_{\mathrm{ch}}〉$ remains relatively constant as a function of centrality. The energy density calculated via Bjorken’s prescription for the $2\%$ most central $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{\mathrm{NN}}}=130\mathrm{}\mathrm{GeV}$ is at least ${\epsilon }_{\mathrm{Bj}}=4.6\mathrm{}\mathrm{GeV}/{\mathrm{fm}}^3$, which is a factor of 1.6 larger than found at $\sqrt{s_{\mathrm{NN}}}=17.2\mathrm{}\mathrm{GeV}$ ( $\mathrm{Pb}+\mathrm{Pb}$ at CERN).

• Received 18 April 2001

DOI:https://doi.org/10.1103/PhysRevLett.87.052301