Dilepton production and off-shell transport dynamics at SIS energies

Dilepton production in nucleus-nucleus collisions at 1-2 A GeV as well as in elementary pp and pd reactions is studied within the microscopic HSD transport approach which includes the off-shell dynamics of vector mesons explicitly. The study addresses additionally the production of $\pi^0$ and $\eta$ mesons since their Dalitz decays provide a sizeable contribution to the dilepton invariant mass spectra up to about 0.5 GeV. Our transport results agree with the TAPS experimental data on $\pi$ and $\eta$ multiplicities in C+C collisions from 0.8 to 2 A GeV. We find that the 'DLS-puzzle' - which addresses an underestimation of the $e^+e^-$ yield in the mass range from 0.2 to 0.5 GeV in C+C and Ca+Ca collisions - may be solved when incorporating a stronger bremsstrahlung contribution in line with recent OBE calculations. Moreover, the HSD results with 'enhanced' bremsstrahlung cross sections agree very well with the HADES experimental data for the dilepton mass spectra for C+C at 1 and 2 A GeV, especially when including a collisional broadening in the vector-meson spectral functions. Detailed predictions for dilepton spectra from pp and pn/pd reactions at 1.25 GeV, 2.2 GeV and 3.5 GeV are presented which will allow to verify/falsify the larger bremsstrahlung contributions from the experimental side in the near future.