Dielectron production in Ar + KCl collisions at 1.76AGeV

Abstract

We present results on dielectron production in 40 Ar + KCl collisions at 1.76A GeV. For the first time ω 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 ω multiplicity has been extracted and compared to the yields of other particles, in particular of the φ 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 nontrivial 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 Dalitz decays.

Virtual photon polarization and dilepton anisotropy in relativistic nucleus–nucleus collisions

“…In this context, we note that the large transverse polarization obtained by the HADES Collaboration [10] in Ar-KCl at 1.76 AGeV is not consistent with the annihilation processes in local thermal equilibrium considered here. The observed anisotropy may be due to non-equilibrium effects or dominated by another process, like, e.g., the ∆ Dalitz decay.…”

“…In a first measurement of the dilepton angular anisotropy, the NA60 Collaboration found that the anisotropy coefficients in 158 AGeV In-In collisions are consistent with zero [9], while the HADES Collaboration [10] finds a substantial transverse polarization in Ar-KCl at 1.76 AGeV.…”

Strangeness in nuclei and neutron stars

“…[168]. Right panel: φ/ω ratio measured in different colliding systems as a function of the φ excess energy [169].…”

Coherent lepton pair production in hadronic heavy ion collisions

“…Dileptons has been suggested as "penetrating probes" for the hot and dense medium created in heavy-ion collisions [2], because they are produced during the whole evolution and not subject to the violent strong interactions in the medium. Various dilepton measurements have been performed since the early days of heavy-ion collisions [3][4][5][6][7][8][9][10][11][12][13][14]. Of particular interest, a clear enhancement in the low (M ll < M φ ) and intermediate mass (M φ < M ll < M J/ψ ) region [15] has been observed when compared to known hadronic source.…”