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
π0 and
η 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
π and
η multiplicities in
13d4528cfc17e967f0a02e1861922"> collisions from 0.8 to 2
A GeV. We find that the ‘DLS-puzzle’—which stands for a theoretical underestimation of the
e+e− yield in the mass range from 0.2 to 0.5 GeV in
d02fd07"> and
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
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.