Enhanced production of direct photons in Au+Au collisions at sqrt(s_NN)=200 GeV and implications for the initial temperature

TL;DR

This study measures low mass e+e- pairs in high-energy collisions, revealing an excess of direct photons that suggests initial temperatures consistent with quark-gluon plasma formation, supporting hydrodynamical models.

Contribution

It provides the first measurement of direct photon yields in Au+Au collisions at 200 GeV, linking photon spectra to initial temperatures of the created hot medium.

Findings

Enhanced direct photon yield in Au+Au collisions over p+p.

Inverse slope T = 221 +/- 19 MeV indicating high initial temperature.

Hydrodynamical models with initial temperatures 300-600 MeV agree with data.

Abstract

The production of low mass e+e- pairs for m_{e+e-} < 300 MeV/c^2 and 1 < p_T <5 GeV/c is measured in p+p and Au+Au collisions at sqrt(s_NN)=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.