Chiral Symmetry Restoration and Dileptons in Relativistic Heavy-Ion Collisions

TL;DR

This paper reviews the theoretical understanding of low-mass dilepton signals in relativistic heavy-ion collisions, focusing on chiral symmetry restoration effects and their implications for medium modifications of hadronic properties.

Contribution

It provides a comprehensive review of medium effects on dilepton production, emphasizing the importance of vector and axial-vector correlations and suggesting a quark-hadron duality near the phase boundary.

Findings

Medium effects significantly alter dilepton signals.

Parity partners of hadronic correlators become identical upon chiral symmetry restoration.

A quark-hadron duality emerges near the phase boundary.

Abstract

The current theoretical status in the analysis and interpretation of low-mass dilepton measurements in (ultra-) relativistic heavy-ion experiments is reviewed. Special emphasis is put on potential signals of (partial) restoration of dynamically broken chiral symmetry in a hot and dense hadronic medium. It follows from chiral symmetry alone that parity partners of hadronic correlation functions must become identical when the symmetry is restored. The assessment of medium effects in the vector channel, which governs the dilepton production, thus necessitates a simultaneous treatment of the vector and axialvector degrees of freedom. While significant progress in this respect has been made some open questions remain in establishing a rigorous link in the mass region below 1 GeV. From the present calculations a suggestive 'quark-hadron duality' emerges near the phase boundary. It implies substantial medium effects in the dilepton signal from the hadronic phase which smoothly matches a perturbative description within the plasma phase.