Dilepton production studied with the HADES spectrometer

Abstract

With the HADES spectrometer at GSI we have studied dilepton production in various collision systems from elementary N_s16N, over p_s16A, up to the medium-heavy Ar_s16KCl system. We have confirmed the puzzling results of the former DLS collaboration at the B evalac. While we have traced the origin of the excess pair yield in C_s16C collisions to elementary p_s16p and n_s16p processes, we find a significant contribution from the dense phase of the collision in larger Ar_s16KCl system. From recently obtained e_s16e- pair spec tra in p_s16p and p_s16Nb interactions at 3.5 GeV kinetic beam energy the inclusive production cross sections for neutral pions, eta , omega and rho mesons are extracted for the first time at this beam energy. Furthermore, the production mechanisms of the vecto r mesons, which are not known at these energies, are investigated. The direct comparison of p_s16p and p_s16Nb data allows us to investigate in-medium mass modifications of vector mesons at nuclear ground state density.On the other hand, exclusive production of omega and eta mesons in p_s16p reactions at 3.5 GeV were studied via their pi_s16pi-pi0 decay channel. Production cross sections and angular distributions for both states were determined.