Production and evolution path of dileptons at HADES energies
  Dilepton production in intermediate energy nucleus-nucleus collisions as well as in elementary proton-proton reactions is analysed within the UrQMD transport model. For C+C collisions at 1 AGeV and 2 AGeV the resulting invariant mass spectra are compared to recent HADES data. We find that the experimental spectrum for C+C at 2 AGeV is slightly overestimated by the theoretical calculations in the region around the vector meson peak, but fairly described in the low mass region, where the data is satisfactorily saturated by the Dalitz decay of the $\eta$ meson and the $\Delta$ resonance. At 1 AGeV an underestimation of the experimental data is found, pointing that at lower energies the low mass region is not fully saturated by standardly parametrized $\Delta$ Dalitz decays alone. Furthermore, predictions for dilepton spectra for $pp$ reactions at 1.25 GeV, 2.2 GeV and 3.5 GeV and Ar+KCl reactions at 1.75 AGeV are presented. The study is complemented by a detailed investigation of the role of absorption of the parent particles on the corresponding dilepton yields in the regime which has so far been probed by HADES. 
K. Schmidt, E. Santini, S. Vogel, C. Sturm, M. Bleicher and H.   Stoecker