Dilepton production at intermediate-energy heavy-ion collisions

Dilepton production is studied in heavy-ion collisions at bombarding energies from 60 to 400 MeV/nucleon. The dynamical evolution of the nucleus-nucleus collisions is described by a transport equation of the Vlasov-Uehling-Uhlenbeck type including explicitly pion and Δ(1232) degrees of freedom and considering free on-shell production processes. We calculate the contribution of proton-neutron bremsstrahlung, ${\mathrm{\pi }}^0$ and Δ Dalitz decay, and ${\mathrm{\pi }}^{+}$ ${\mathrm{\pi }}^{\mathrm{-}}$ annihilation. At 60 MeV/nucleon bombarding energy proton-neutron bremsstrahlung dominates while around 100 MeV/nucleon most of the ${\mathit{e}}^{+}$ ${\mathit{e}}^{\mathrm{-}}$ cross section arises from ${\mathrm{\pi }}^0$ decay. At 400 MeV/nucleon, however, the Δ Dalitz decay is the most important dilepton source for invariant masses M>140 MeV thus offfering the possibility to study in-medium properties of this resonance.