Intermediate-Mass Dilepton Production in Heavy-Ion Collisions at200AGeV

Through the analysis of HELIOS-3 data obtained at the CERN Super Proton Synchrotron (SPS), we demonstrate the importance of secondary processes for dilepton production in heavy-ion collisions in the intermediate invariant mass region. We find that while the dilepton spectra between 1 and 2.5 GeV from proton-induced reactions can be attributed to 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 secondary processes. Furthermore, we find $\pi a_1\to l\overline{l}$ to be the most important process in this mass region, as was found by thermal rate calculations.