Dilepton production at intermediate-energy heavy-ion collisions.

Abstract

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 \ensuremath{\Delta}(1232) degrees of freedom and considering free on-shell production processes. We calculate the contribution of proton-neutron bremsstrahlung, ${\mathrm{\ensuremath{\pi}}}^{0}$ and \ensuremath{\Delta} Dalitz decay, and ${\mathrm{\ensuremath{\pi}}}^{+}$${\mathrm{\ensuremath{\pi}}}^{\mathrm{\ensuremath{-}}}$ annihilation. At 60 MeV/nucleon bombarding energy proton-neutron bremsstrahlung dominates while around 100 MeV/nucleon most of the ${\mathit{e}}^{+}$${\mathit{e}}^{\mathrm{\ensuremath{-}}}$ cross section arises from ${\mathrm{\ensuremath{\pi}}}^{0}$ decay. At 400 MeV/nucleon, however, the \ensuremath{\Delta} 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.