Low mass dilepton production at ultrarelativistic energies

Dilepton production in pp   and Au+AuAu+Au nucleus–nucleus collisions at s=200GeV as well as in In+InIn+In and Pb+AuPb+Au at 158AGeV is studied within the microscopic HSD transport approach. A comparison to the data from the PHENIX Collaboration at RHIC shows that standard in-medium effects of the ρ,ωρ,ω vector mesons—compatible with the NA60 data for In+InIn+In at 158AGeV and the CERES data for Pb+AuPb+Au at 158AGeV—do not explain the large enhancement observed in the invariant mass regime from 0.2 to 0.5 GeV in Au+AuAu+Au collisions at s=200 GeV relative to pp collisions.