Radiation of single photons from Pb+Pb collisions at relativistic energies and the quark-hadron phase transition

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Abstract
The production of single photons in Pb+Pb collisions at relativistic energies as measured by the WA98 experiment is analyzed. A quark-gluon plasma is assumed to be formed initially, which expands, cools, hadronizes, and undergoes freeze-out. A rich hadronic equation of state is used and the transverse expansion of the interacting system is taken into account. The recent estimates of photon production in quark matter (at the two-loop level) along with the dominant reactions in the hadronic matter leading to photons are used. About 50% of the single photons are seen to have a thermal origin. An addition of the thermal and prompt photons is seen to provide a very good description of the data. Most of the thermal photons having large transverse momenta arise from the quark matter, which contributes dominantly through the mechanism of annihilation of quarks with scattering, and which in turn is possible only in a hot and dense plasma of quarks and gluons. The results are thus compatible with the formation of quark-gluon plasma and the existence of this mechanism of the production of single photons.