In-Medium Vector Mesons, Dileptons and Chiral Restoration

TL;DR

This paper reviews how medium modifications of the electromagnetic spectral function, especially the broadening of the rho meson, relate to experimental dilepton spectra and the quest for chiral symmetry restoration in hot and dense matter.

Contribution

It provides a comprehensive review of medium modifications of vector mesons and their implications for dilepton spectra and chiral symmetry restoration, integrating experimental data and theoretical insights.

Findings

Strong rho meson broadening matches dilepton spectra in nuclear and heavy-ion collisions.

The large dilepton excess at RHIC is not explained by current models.

Connections to lattice QCD may advance understanding of chiral symmetry restoration.

Abstract

Medium modifications of the electromagnetic spectral function in hadronic and quark-gluon matter are reviewed. A strong broadening of the $\rho$ meson, which dominates the spectral function in the low-mass regime, is quantitatively consistent with dilepton excess spectra measured in photoproduction off cold nuclei (CLAS/JLab) and in fixed-target ultrarelativistic heavy-ion collisions (NA45,NA60/CERN-SPS). The large excess observed by PHENIX at RHIC remains unexplained to date, but is most likely not due to emission from the Quark-Gluon Plasma. Connections to thermal lattice QCD promise progress in the search for chiral symmetry restoration.