Dielectron production in Ar+KCl collisions at 1.76A GeV

TL;DR

This study reports the first reconstruction of omega mesons in heavy-ion collisions below their free nucleon-nucleon production threshold, revealing medium effects and enhancements in dielectron yields at intermediate masses.

Contribution

It provides new experimental data on dielectron production and omega meson reconstruction at sub-threshold energies in heavy-ion collisions, highlighting medium modifications.

Findings

First observation of omega mesons in sub-threshold heavy-ion collisions.

Strong enhancement of dielectron pairs at intermediate invariant masses.

Large transverse-mass slopes for high-mass pairs.

Abstract

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.