The production of $e^{+}{e}^{-}$ pairs for $m_{e^{+}{e}^{-}}<0.3\mathrm{GeV}/c^2$ and $1<p_T<5\mathrm{GeV}/c$ is measured in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{s_{NN}}=200\mathrm{GeV}$. An enhanced yield above hadronic sources is observed. Treating the excess as photon internal conversions, the invariant yield of direct photons is deduced. In central $\mathrm{Au}+\mathrm{Au}$ collisions, the excess of the direct photon yield over $p+p$ is exponential in transverse momentum, with an inverse slope $T=221\pm {19}^{\mathrm{stat}}\pm {19}^{\mathrm{syst}}\mathrm{MeV}$. Hydrodynamical models with initial temperatures ranging from $T_{\mathrm{init}}\sim 300–600\mathrm{MeV}$ at times of $\sim 0.6–0.15\mathrm{fm}/c$ after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at $\sim 170\mathrm{MeV}$.

• Received 25 April 2008

DOI:https://doi.org/10.1103/PhysRevLett.104.132301