Dilepton production in heavy-ion collisions at intermediate energies

We present a unified description of the vector meson and dilepton production in elementary and in heavy ion reactions. The production of vector mesons (ρ,ω) is described via the excitation of nuclear resonances R. The theoretical framework is an extended vector meson dominance model (eVMD). The treatment of the resonance decays R↦NV with arbitrary spin is covariant and kinematically complete. The eVMD thereby includes excited vector meson states in the transition form factors. This ensures correct asymptotics and provides a unified description of photonic and mesonic decays. The resonance model is successfully applied to the ω production in p-p reactions. The same model is applied to the dilepton production in elementary reactions (p-p,p-d). Corresponding data are well reproduced. However, when the model is applied to heavy ion reactions in the BEVALAC/SIS energy range, the experimental dilepton spectra measured by the DLS Collaboration are significantly underestimated at small invariant masses. As a possible solution of this problem, the destruction of quantum interference in a dense medium is discussed. A decoherent emission through vector meson decays enhances the corresponding dilepton yield in heavy ion reactions. In the vicinity of the ρ/ω peak, the reproduction of the data requires further a substantial collisional broadening of the ρ and, in particular, of the ω meson.