Dielectron production in Ar+KCl collisions at 1.76A GeV
  We present results on dielectron production in Ar+KCl collisions at 1.76A GeV. For the first time $\omega$ mesons could be reconstructed in a heavy-ion reaction at a bombarding energy which is well below the production threshold in free nucleon-nucleon collisions. The omega multiplicity has been extracted and compared to the yields of other particles, in particular of the phi meson. At intermediate e+e- invariant masses, we find a strong enhancement of the pair yield over a reference spectrum from elementary nucleon-nucleon reactions suggesting the onset of non-trivial effects of the nuclear medium. Transverse-mass spectra and angular distributions have been reconstructed in three invariant mass bins. In the former unexpectedly large slopes are found for high-mass pairs. The latter, in particular the helicity-angle distributions, are largely consistent with expectations for a pair cocktail dominated at intermediate masses by delta Dalitz decays. 
G. Agakishiev, A. Balanda, D. Belver, A. Belyaev, A. Blanco, M.   B\"ohmer, J. L. Boyard, P. Cabanelas, E. Castro, S. Cherne...
nucl-ex