Enhancement of Low-Mass Dileptons in Heavy Ion Collisions

Abstract

Using a relativistic transport model for the expansion stage of S+Au collisions at 200 GeV/nucleon, we show that the recently observed enhancement of lowmass dileptons by the CERES collaboration can be explained by the decrease of vector meson masses in hot and dense hadronic matter.

ρ→ππ decay in nuclear medium

“…In this context, the properties of vector mesons (especially of the ρ) in a hadronic environment become of particular interest, since the Vector Meson Dominance Model is commonly used to make the estimates of the dilepton yields from vector-meson decays. There are numerous analysis of the effect in the literature, both in hydrodynamic approaches [42,70,71,72,73], and in transport theories [13,74,75,76,77,78,79,80]. The dilepton-rate formula [81,82,83,84,85] from ρ meson decays can be written in a manifestly Lorentz covariant way as follows:…”

“…An indirect indication for the modification of vector mesons has been provided by the dilepton production measurements in relativistic heavy-ion collisions (CERES [40], HELIOS [41]). The dilepton productions has been theoretically studied with medium-modified vector mesons [42,43,44,45,46,47], which helps to explain the low-mass enhancement of the dilepton yields.…”

Momentum dependence of the pion cloud for ϱ-mesons in nuclear matter

“…The final result for the pion selfenergy thus becomes 10) and the in-medium pion propagator is obtained as…”

Multiphase transport model for relativistic heavy ion collisions

“…Resonances such as ρ and are formed from pion-pion and pion-nucleon scattering, respectively, with cross sections given by the standard Breit-Wigner form, and they also decay according to their respective widths. In all calculations presented in this study, the masses and widths of resonances are taken to be their values in the vacuum; i.e., effects due to possible modifications in dense hadronic matter [80] are neglected. Also, we turned off the potentials in the AMPT model because their effects are much less important than scatterings in high-energy heavy ion collisions such as at SPS and RHIC.…”