Low mass dilepton radiation at RHIC

TL;DR

This paper models low mass dilepton emission in heavy-ion collisions using hydrodynamics and chiral symmetry constraints, comparing results with PHENIX data to understand the hadronic phase contributions.

Contribution

It introduces a detailed calculation of dilepton rates constrained by chiral symmetry and compares chemical equilibrium and off-equilibrium scenarios with experimental data.

Findings

Off-equilibrium hadronic gas partially explains low mass enhancement.

Significant discrepancy remains between model and data.

Dilepton rates are linked to vacuum correlators from experimental measurements.

Abstract

In this work we discuss the emission of low mass dilepton radiation from a hydrodynamic evolution model of Au-Au collisions and make comparisons with recent PHENIX measurements. The dilepton emission rates from the hadronic phase are treated at finite temperature and baryon density and are completely constrained by broken chiral symmetry in a density expansion. The rates are expressed in terms of vacuum correlators which are measured in $e^+e^-$ annihilation, $\tau$ decays and photo-reactions on nucleons and nuclei. We consider two possibilities for the hadronic phase: A chemical equilibrated an off equilibrium hadronic gas. We find that while chemical off-equilibrium helps explain part of the low mass (0.15 $\leq$ M GeV $\leq$ 0.7) enhancement seen in the data there is still a large discrepancy.