Medium effects on low mass dileptons from relativistic nuclear collisions

Abstract

We study the dilepton spectra measured by the CERES Collaboration in Pb+Au collisions at various values of centrality/charge multiplicity with particular emphasis on characterizing the medium effects of hadrons. The full in-medium spectral function of the vector mesons calculated from quantum hadrodynamics has been used along with other scenarios of medium modifications in the presence of baryons. We show that the CERES acceptance cuts considerably reduce baryonic effects on the spectra of dileptons. The sensitivity on different versions of vector meson dominance has been contrasted. Medium effects have also been incorporated in the equation of state used in the hydrodynamic evolution of the fireball. We observe that a thermal source with initial temperature ∼200 MeV can reproduce the observed enhancement in the low mass region of the dilepton spectra. We also estimate the number of π–π collisions near the ρ-peak of the dilepton spectra and argue that thermal equilibrium may have been achieved in the system, justifying the use of a hydrodynamical model to describe the spacetime evolution. The dilepton spectra in Au+Au collisions at RHIC energies are evaluated with and without medium effects on hadrons.