Single Photons from Relativistic Heavy Ion Collisions and Quark-Hadron Phase Transition

The present status of theoretical expectations of studies of single photons from relativistic heavy ion collisions is discussed. It is argued that the upper limit of single photon radiation from S+Au collisions at CERN SPS obtained by the WA80 collaboration perhaps rules out any reasonable description of the collision process which does not involve a phase transition to quark gluon plasma. Predictions for single photons from the quark-matter likely to be created in collision of two lead nuclei at RHIC and LHC energies are given with a proper accounting of chemical equilibration and transverse expansion. Finally, it is pointed out that, contrary to the popular belief of a quadrilateral dependence of electromagnetic radiations ($N_\gamma$) from such collisions on the number of charged particles ($N_{ch})$, we may only have $N_\gamma \propto N_{ch}^{1.2}$.