Intermediate-mass dilepton production in heavy-ion collisions at 200A GeV

Through the analysis of HELIOS-3 data obtained at the CERN 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 to 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 as compared to the background comes mainly from the secondary processes which are germane to heavy-ion collisions. Furthermore, we find $\pi a_1\to l{\bar l}$ to be the most important process in this mass region, as was found by thermal rate calculations. We emphasize the constraints on the elementary cross sections by the experimental data from $e^+e^-$ annihilation.