HYDRODYNAMICAL DESCRIPTION OF 200A GEV C S+AU COLLISIONS - HADRON ANDELECTROMAGNETIC SPECTRA/

We study relativistic S+Au collisions at 200A GeV/c using a hydrodynam ical approach. We test various equations of state (EOS's), which are u sed to describe the strongly interacting matter at densities attainabl e in the CERN-SPS heavy ion experiments. For each EOS, suitable initia l conditions can be determined to reproduce the experimental hadron sp ectra; this emphasizes the ambiguity between the initial conditions an d the EOS in such an approach. Simultaneously, we calculate the result ing thermal photon and dielectron spectra, and compare with experiment s. If one allows the excitation of resonance states with increasing te mperature, the electromagnetic signals from scenarios with and without phase transition are very similar and are not resolvable within the c urrent experimental resolution. Only EOS's with a few degrees of freed om up to very high temperatures can be ruled out presently. We deduce an upper bound of about 250 MeV for the initial temperature from the s ingle photon spectra of WA80. With regard to the CERES dilepton data, none of the EOS's considered, in conjunction with the standard leading order dilepton rates, succeed in reproducing the observed excess of d ileptons below the rho peak. Our work, however, suggests that an impro ved measurement of the photon and dilepton spectra has the potential t o strongly constrain the EOS.