Single Photon Production in 200 A•GeV Sulphur on Gold Collisions WA80 Collaboration

The investigation of highly excited and compressed nuclear matter created in relativistic heavy ion collisions requires probes sensitive to the different stages of the development of the system. Among the characteristic signals listed in table 1, hadrons mainly probe the late stage of the reaction and their final spectra are influenced by their last scatterings. Electromagnetic probes, on the other hand, escape from the reaction zone without rescattering and probe the very early stage of the reaction, where temperatures and densities are highest. Electromagnetic probes are therefore particularly useful to study the possible formation of a Quark-Gluon Plasma, which is expected only at extreme values of temperature and density. The disadvantage of electromagnetic probes is that the signals are usually small and have to be disentangled from a tremendous physical background requiring a very high experimental precision and detailed treatment of the various background sources. In the case of photons this background consists mainly of photons from γ decaying hadrons like π0 and η.