Abstract: Dilepton production in elementary and HI collisions with HADES.

HADES is a secondary generation experiment operated at GSI Darmstadt with the main goal to study dielectron production in proton, pion and heavy ion induced reactions. The first part of the HADES mission is to reinvestigate the puzzling pair excess measured by the DLS collaboration in C+C and Ca+Ca collisions at 1 AGeV. For this purpose dedicated measurements with the C+C system at 1 and 2 AGeV were peroformed. Pair excess above a cocktail of free hadronic decays has benn extracted and compared to the one measured by DLS. Furthermore, the excess will be confronted with predictions of various model calculations. These calculations suffer from incomplete knowledge of some elementary processes, most importantly Dalitz decay of baryonic resonances and NN bremsstrahlung. In order to shed more light on these processes p-p and d-p collisions have been investigated. Preliminary results from these experiments will be presented and discussed.