Probing the in-medium properties of hadrons with di-electrons

Thomas Eberl

The di-lepton spectrometer HADES at SIS/GSI is designed to study electron-positron pairs emitted in collisions of heavy ions at 1-2 GeV/nucleon as well as in proton and pion induced reactions on protons and nuclei. Major goals are to investigate the properties of hadrons in the vacuum and in nuclear matter in particular to search for in-medium mass modifications of light vector mesons. In C+C collisions at a bombarding energy of 2 GeV/nucleon almost 20000 e+e pairs have been reconstructed in the invariant mass range up to 1 GeV/c2. To determine the electron reconstruction efficiency the eta meson has been identified in the electromagnetic decay channel eta -> g e+e as well as in the hadronic decay channel eta -> pi+pi-pi0 in p+p collisions at 2.2 GeV incident energy. Di-electron distributions obtained in C+C reactions at 1 and 2 GeV/nucleon, Ar+KCl at 1.78GeV/u and in p+p collisions will be shown and compared to transport model calculations.