Dilepton and/or photon production in heavy ion collisions and the QCD phase transition

We study the electromagnetic production from highly excited hadronic matter created in heavy ion collisions. The rates include the usual lowest order processes in quark-gluon plasma plus the usual reactions in the hadronic phase, related with $\rho {,a}_1$ mesons. The space-time integration is done using a hydrodynamical model. Conventional $\overline{q}q$ $\left({\pi }^{+}{\pi }^{-}\right)$ annihilation in quark-gluon plasma and hadronic phase cannot explain the observed dilepton spectrum, especially that by the CERES experiment at CERN. A decreased $\rho$ mass can account for the observed effect, provided it shifts into the region of 0.4–0.5 GeV near the phase transition. In order to test this hypothesis one should also look at the chiral partner of $\rho$, the axial $a_1$ meson: its mass must then behave similarly. Its decay $a_1\to \pi e^{+}{e}^{-}$ populates the low mass region seen in the same experiment. The results for direct photon production are below the current WA80 experimental bounds, for all variants considered.