Through the analysis of HELIOS-3 data on dilepton spectra, we demonstrate the importance of secondary processes for the dilepton production in heavy-ion collisions in the intermediate mass region. We find that, while the dilepton spectra in proton-induced reactions can be nicely explained by the decay of primary vector mesons, charmed hadrons, and initial Drell-Yan processes, the strong enhancement seen in the heavy-ion data comes mainly from the secondary meson-meson interactions which are unique to heavy-ion collisions.
The experimental measurement and theoretical investigation of dilepton production in heavy-ion collisions constitutes one of the most active and exciting fields in physics. Because of their relatively weak final-state interactions with the hadronic environment, dileptons are considered ideal probes of the early stage of heavy-ion collisions, where quark-gluon-plasma (QGP) formation and chiral symmetry restoration are expected.
Recent observation of the enhancement of low-mass dileptons in central heavy-ion collisions by the CERES [1,2] and the HELIOS-3 [3] collaborations has generated a great deal of theoretical activities. The results from many groups with standard scenarios (i.e., using vacuum meson properties) are in remarkable agreement with each other, but in significant disagreement with the data: the experimental spectra in the mass region from 0.3-0.6 GeV are substantially underestimated [2]. This has led to the suggestion of various medium effects that might be responsible for the observed enhancement [4,5].
Another piece of interesting experimental data that has not received much theoretical attention is the dilepton spectra in the intermediate-mass region between 1 and 2.5 GeV. Both the HELIOS-3 and NA38/NA50 collaborations have observed a significant enhancement of dilepton yield in this mass region in central S+W and S+U collisions as compared to that in the proton-induced reactions [3,6]. The intermediate-mass dilepton spectra are particularly useful for the search of the QGP. It was originally suggested that in this mass region, the electromagnetic radiation from the QGP phase might shine over that from the hadronic phase. However, to extract from the measured dilepton spectra any information about the phase transition and the properties of the QGP, it is essential that the