Photons and neutral mesons from hot hadronic matter

Results from the experimental program with light ion beams and heavy target nuclei at the CERN SPS could demonstrate the occurrence of an unprecedented state of high density in hadronic matter. The thermal nature of the hadronic system has been investigated by analyzing spectra and production ratios of hadrons which reveal a large degree of rescattering of primary and secondary hadrons.Thermal photons from elementary quark-gluon interactions are considered a promising signal for the occurrence of a phase transition to the quark-gluon plasma. The predictions for thermal photons from elementary parton interactions are discussed and compared to the thermal emission rate of photons from a hot hadronic gas. Recent results from the photon spectrometers in heavy ion experiments are presented. Production cross sections of pi0 and eta mesons are determined and the projectile and target mass dependence is discussed. An upper limit for the single photon yield was determined for central O+Au reactions. Recent S+Au reactions exhibit an excess of photons over the yield expected from hadronic decays. The spectral shape of the expected single photon signal is discussed which might reveal the temperature of hot matter and indicate a phase transition.