Enhanced Production of Direct Photons in Au+Au Collisions at sNN=200 GeV and Implications for the Initial Temperature

Le résumé fourni par la source

The production of ${e}^{+}{e}^{\ensuremath{-}}$ pairs for ${m}_{{e}^{+}{e}^{\ensuremath{-}}}<0.3\text{ }\text{ }\mathrm{GeV}/{c}^{2}$ and $1<{p}_{T}<5\text{ }\text{ }\mathrm{GeV}/c$ is measured in $p+p$ and $\mathrm{Au}+\mathrm{Au}$ collisions at $\sqrt{{s}_{NN}}=200\text{ }\text{ }\mathrm{GeV}$. 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 $T=221\ifmmode\pm\else\textpm\fi{}{19}^{\mathrm{stat}}\ifmmode\pm\else\textpm\fi{}{19}^{\mathrm{syst}}\text{ }\text{ }\mathrm{MeV}$. Hydrodynamical models with initial temperatures ranging from ${T}_{\mathrm{init}}\ensuremath{\sim}300--600\text{ }\text{ }\mathrm{MeV}$ at times of $\ensuremath{\sim}0.6--0.15\text{ }\text{ }\mathrm{fm}/c$ after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at $\ensuremath{\sim}170\text{ }\text{ }\mathrm{MeV}$.

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