Dilepton production studied with the HADES spectrometer

With the HADES spectrometer at GSI we have studied dilepton production in various collision systems from elementary N+N, over p+A, up to the medium-heavy Ar+KCl system. We have confirmed the puzzling results of the former DLS collaboration at the Bevalac. While we have traced the origin of the excess pair yield in C+C collisions to elementary p+p and n+p processes, we find a significant contribution from the dense phase of the collision in larger Ar+KCl system. From recently obtained e⁺e^- pair spectra in p+p and p+Nb interactions at 3.5 GeV kinetic beam energy the inclusive production cross sections for neutral pions, η , ω and ρ mesons are extracted for the first time at this beam energy. Furthermore, the production mechanisms of the vector mesons, which are not known at these energies, are investigated. The direct comparison of p+p and p+Nb data allows us to investigate in-medium mass modifications of vector mesons at nuclear ground state density. On the other hand, exclusive production of ω and η mesons in p+p reactions at 3.5 GeV were studied via their π⁺π^-π⁰ decay channel. Production cross sections and angular distributions for both states were determined.