The production of intermediate mass dileptons in ultrarelativistic nuclear collisions at SPS energies is studied. The acceptance and detector resolution inherent to measurements by the NA50 experimental collaboration are accurately modeled. The measured centrality dependence of the intermediate mass lepton pair excess is also addressed.

The production of single photons in P b + P b collisions at the CERN SPS as measured by the WA98 experiment is analysed. A very good description of the data is obtained if a quark gluon plasma is assumed to be formed initially, which expands, cools, hadronizes, and undergoes freeze-out. A rich hadronic equation of state is used and the transverse expansion of the interacting system is taken into account. The recent estimates of photon production in quark-matter (at two loop level) along with the dominant reactions in the hadronic matter leading to photons are used. About 50% of the single photons are seen to have a thermal origin. An addition of the thermal and prompt photons is seen to provide a very good description of the data. Most of the thermal photons having large transverse momenta arise from the quark-matter, which contributes dominantly through the mechanism of annihilation of quarks with scattering, and which in turn is possible only in a hot and dense plasma of quarks and gluons. The results thus confirm the formation of quark gluon plasma and the existence of this mechanism of the production of single photons.PACS numbers: 12.38Mh The search for quark-gluon plasma, which filled the early universe microseconds after the big bang and which may be present in the core of neutron stars, is one of the most notable collective efforts of the present day nuclear physics community. Its discovery will provide an important confirmation of the predictions of the statistical Quantum Chromodynamics (QCD) based on lattice calculations. It has been recognised for a long time [1] that electromagnetic radiations from relativistic heavy ion collisions in these experiments would be a definitive signature of the formation of a hot and dense plasma of quarks and gluons, consequent to a quark-hadron phase transition [1]. Once other signs of the quark-hadron transition, e.g., an enhanced production of strangeness, a suppression of J/ψ production, radiation of dileptons, etc., started to emerge [2], it was imperative that the more direct, yet much more difficult to isolate, signature of the hot and dense quark-gluon plasma, the single photons were identified. The WA98 experiment [3] has now reported observation of single photons in central P b + P b collisions at the CERN SPS.In the present work we show that these data are very well described if we assume that a quark-gluon plasma was formed in the collision.In order to put our findings in a proper perspective, let us recall that the publication of the upper limit of the production of single photons in S+Au collisions at CERN SPS [4] by the WA80 experiment was preceded and followed by several papers [5,6] exploring their connection to the quark-hadron phase transition. An early work, by the present authors [5], reported that the data were consistent with a scenario where a quark gluon plasma was formed at an initial time τ 0 ∼ 1 fm/c, which expanded and cooled, got into a mixed phase of quarks, gluons, and hadrons, and ultimately underwent a freezeout from a state of hadr...

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