Enhanced production of direct photons in Au+Au collisions at √sNN=200GeV and implications for the initial temperature

Abstract

The production of e+e- pairs for me+e -<0.3GeV/c2 and 1<pT<5GeV/c is measured in p+p and Au+Au collisions at √sNN=200GeV. 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 Au+Au collisions, the excess of the direct photon yield over p+p is exponential in transverse momentum, with an inverse slope T=221±19stat±19systMeV. Hydrodynamical models with initial temperatures ranging from Tinit∼300-600MeV at times of ∼0.6-0.15fm/c after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at ∼170MeV. © 2010 The American Physical Society.