  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.
