DIRECT PRODUCTION OF LOW MASS DILEPTONS AND PHOTONS PROBING NUCLEAR MATTER AT THE CERN SPS

摘要

Electromagnetic radiation has been widely discussed as one of the most sensitive signals to probe the dynamics of nuclear interactions. Recently first data has become available from experiments at CERN with S-beam of 200 GeV/nucleon. While, in spite of the large effort, no clear evidence of direct photon production is found, new and interesting results are obtained for dileptons. Three experiments -CERES, HELIOS-3 and NA38 - report an enhanced production of dileptons in S-induced reactions, with a yield far beyond a mere superposition of p-p oc p-nucleus collisions. The new signal sets in above masses ~ 250 MeV/c3 and extends over a wide range up to the mass of the J/1/1. The properties of the excess below 1 GeV/c2 suggests that TTTr-annihilation is the dominant source. Moreover the spectral shape of the mass distribution might indicate that the hot system of interacting hadrons created during the collision is close to chiral symmetry restoration.