Enhanced Production of Direct Photons in Au plus Au Collisions at root s(NN)=200 GeV and Implications for the Initial Temperature
2010
The production of e(+)e(-) pairs for m(e+e-) < 0.3 GeV/c(2) and 1< p(T) < 5 GeV/c is measured in p + p and Au + Au collisions at root s(NN) = 200 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 Au + Au collisions, the excess of the direct photon yield over p + p is exponential in transverse momentum, with an inverse slope T = 221 +/- 19(stat) +/- 19(syst) MeV. Hydrodynamical models with initial temperatures ranging from T-
initsimilar to 300-600 MeV at times of similar to 0.6-0.15 fm/c after the collision are in qualitative agreement with the data.
Lattice QCDpredicts a phase transition to
quark gluon plasmaat similar to 170 MeV.
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