Low mass dilepton production at ultrarelativistic energies

Dilepton production in $pp$ and $Au+Au$ nucleus-nucleus collisions at $\sqrt{s}$ = 200 GeV as well as in $In+In$ and $Pb+Au$ at 158 A$\cdot$GeV 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 $\rho, \omega$ vector mesons - compatible with the NA60 data for $In+In$ at 158 A$\cdot$GeV and the CERES data for $Pb+Au$ at 158 A$\cdot$GeV - do not explain the large enhancement observed in the invariant mass regime from 0.2 to 0.5 GeV in $Au+Au$ collisions at $\sqrt{s}$ = 200 GeV relative to $pp$ collisions.