Probing resonance matter with virtual photons

TL;DR

This paper investigates di-electron production in various nuclear collisions at 1-2 GeV per nucleon, revealing that heavier systems exhibit excess radiation likely from resonance matter, advancing understanding of nuclear matter under extreme conditions.

Contribution

It provides the first reconstruction of electron pairs from quasi-free n+p reactions and establishes an NN reference spectrum, enabling comparison with heavier collision systems.

Findings

Di-electron spectra in C+C can be explained by independent NN collisions.

An excess of di-electron yield is observed in Ar+KCl, indicating resonance matter radiation.

Combined measurements reveal intriguing correlations between di-electrons and strangeness.

Abstract

In the energy domain of 1-2 GeV per nucleon, HADES has measured rare penetrating probes (e+e-) in C+C, Ar+KCl, d+p, p+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 NN 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 NN collisions. On the other hand, a direct comparison of the NN 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.