Abstract
The HADES experiment aims to explore strongly interacting baryon dominated matter at moderate temperatures using rare and penetrating probes. Special emphasis is put on the region of moderate beam energies where comparatively long-lived states of compressed matter are created. In the energy domain of 1–2 GeV per nucleon, HADES has measured hadrons and di-electron signals in C + C , Ar + KCl , p+p, d+p and p+ Nb collisions. For the first time the electron pairs were reconstructed from quasi-free n+p sub-reactions by detecting the proton spectator from the deuteron breakup. For the first time di-electrons radiated from cold nuclear matter in a kinematic regime, where strong medium effects are expected, have been detected. An experimentally constrained N + N reference spectrum was established. A direct comparison of the N + N reference spectrum with the e+e- invariant mass distribution measured in the heavier system Ar + KCl at 1.76 GeV/u shows an excess yield above the reference. The observation may be interpreted as the onset of an actual medium effect. We argue, that couplings between the ρ meson and the baryonic resonances are of fundamental importance in understanding dilepton spectra at SIS energies.
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