Dilepton production and off-shell transport dynamics at SIS energies

Understand

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.