Sensitivity of electromagnetic spectra to equation of state and initial energy density in the Pb+Pb collisions at SPS

We study Pb + Pb collisions at 158A GeV/c using a hydrodynamical approach. We test different equations of state (EoSs) and different initial condition s and show that there are more than one initial state for each EoS which re produce the observed hadronic spectra, We also find that different equation s of state favor different freeze-out temperature. Simultaneously we calcul ate the thermal dilepton and photon spectra for each EoS and initial state, We compare the dilepton mass spectrum to data measured by the CERES collab oration and find that the differences in spectra obtained using different E oSs and initial states are not resolvable within the current experimental r esolution. However, at invariant masses over 2 GeV the difference in the yi eld due to various initial states is close to an order of magnitude, We als o study the rapidity distribution of lepton pairs and find that for masses around 800 MeV the shape of the distribution depends strongly on the EoS, ( C) 1999 Elsevier Science B.V.