DILEPTON AND OR PHOTON PRODUCTION IN HEAVY-ION COLLISIONS AND THE QCDPHASE-TRANSITION/

We study the electromagnetic production from highly excited hadronic m atter created in heavy ion collisions. The rates include the usual low est order processes in quark-gluon plasma plus the usual reactions in the hadronic phase, related with rho,a(1) mesons. The space-time integ ration is done using a hydrodynamical model. Conventional (q) over bar q (pi(+)pi(-)) 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 obs erved effect, provided it shifts into the region of 0.4-0.5 GeV near t he 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) --> pi e(+)e(-) populates the l ow mass region seen in the same experiment. The results for direct pho ton production are below the current WA80 experimental bounds, for all variants considered.