EXPERIMENTAL EVIDENCE FOR CHANGES OF THE VECTOR MESON PROPERTIES IN HEAVY-ION COLLISIONSa

The enhanced dilepton production observed by experiments at the CERN SPS is reviewed with emphasis on recent CERES Pb{Au data. The enhancement is strongest between the 2-pion threshold and the rho/omega resonance, and grows stronger than linear with centrality, which supports the expectation that pion annihilation of thermal origin is the dominant source. Theoretical calculations seem to require medium modiicationsas an essential ingredient to describe the low-mass dilepton excess by shape and magnitude, either by a dropping in-medium mass of the rho meson as a precursorof chiral symmetryrestoration,or by collisionalbroad-ening. The medium modiications manifest in the invariant mass spectra appear to be strongly localized at transverse momentum below a few hundred MeV/c. This suggests an explanation why low-mass spectra measured by NA38/50 at larger pt look so diierent.