Probing resonance matter with virtual photons

In the energy domain of 1 − 2 GeV per nucleon, HADES has measured rare penetrating probes (e⁺e⁻) 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. An experimentally constrained N+N reference spectrum was established. Our results demonstrate that the gross features of di-electron spectra in C+C collisions can be explained as a superposition of independent N+N collisions. On the other hand, 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, which we attribute to radiation from resonance matter. Moreover, the combined measurement of di-electrons and strangeness in Ar+KCl collisions has provided further intriguing results which are also discussed.