Instruct: Identify the main and secondary category of Arxiv papers based on the titles and abstracts
Query: 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, 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.

2
The categories of this paper are: math
11
The categories of this paper are: hep-lat
31
The categories of this paper are: ao-sci
19
The categories of this paper are: eess
6
The categories of this paper are: astro-ph
21
The categories of this paper are: adap-org
33
The categories of this paper are: plasm-ph
34
The categories of this paper are: chem-ph
26
The categories of this paper are: bayes-an
24
The categories of this paper are: dg-ga
20
The categories of this paper are: acc-phys
0
The categories of this paper are: hep-ph
14
The categories of this paper are: math-ph
4
The categories of this paper are: cond-mat
28
The categories of this paper are: alg-geom
16
The categories of this paper are: stat