Enhanced production of direct photons in Au+Au collisions at sqrt(sNN)=200 GeV and implications for the initial temperature
Abstract
The production of low mass e+e- pairs for me+e- < 300 MeV/c2 and 1 < pT <5 GeV/c is measured in p+p and Au+Au collisions at sqrt(sNN)=200 GeV. Enhanced yield above hadronic sources is observed. Treating the excess as internal conversions, the invariant yield of direct photons is deduced. In central Au+Au collisions, the excess of direct photon yield over p+p is exponential in transverse momentum, with inverse slope T = 221 +/- 19 (stat) +/- 19 (syst) MeV. Hydrodynamical models with initial temperatures ranging from 300--600 MeV at times of ~ 0.6 - 0.15 fm/c after the collision are in qualitative agreement with the data. Lattice QCD predicts a phase transition to quark gluon plasma at ~ 170 MeV.
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