DILEPTONS AND PHOTONS AT THE CERN SPS

CERES, HELIOS-3 and NA38 have observed a significant excess of lepton pairs in collisions of 200 GeV/nucleon S beams with heavy targets in c omparison to the expected yield from known hadronic sources. The exces s was confirmed by results obtained by CERES and NA50 experiments with the Pb beam of 160 GeV/nucleon. The enhancement is more pronounced in the low-mass region (m = 0.2 - 1.5 GeV/c(2)) measured close to mid-ra pidity and seems to hint at in-medium modifications of the vector meso ns and in particular at a decrease of the rho-meson mass interpreted a s a precursor of chiral symmetry restoration. On the other hand, there is no evidence of a signal in the search for direct photons and the v arious measurements, carried out by the CERES, HELIOS-2 and WA80 exper iments, allow to set an upper limit of the order of 10 %. This paper r eviews all experimental results on dileptons (electron and muon pairs) with invariant mass m < 3 GeV/c(2) and real photons obtained from the CERN SPS heavy-ion program with S and Pb beams, together with the cur rent attempts to understand them.