The Chiral Restoration Transition of QCD and Low Mass Dileptons

TL;DR

This paper reviews recent advances in understanding the chiral phase transition in QCD through vector-meson spectral functions, highlighting model-independent approaches, theoretical predictions, and experimental dilepton data in heavy-ion collisions.

Contribution

It synthesizes various theoretical methods and experimental results to elucidate the in-medium modifications of vector mesons near the QCD phase transition.

Findings

Broadening of the rho spectral function near the phase boundary

Degeneration of dilepton rates with quark-antiquark annihilation in QGP

Quantitative agreement with CERN-SPS dilepton data

Abstract

Recent developments in the evaluation of vector-meson spectral functions in hot and dense matter are discussed with emphasis on connections to the chiral phase transition in QCD. Model independent approaches including chiral low-density expansions, lattice QCD, chiral and QCD sum rules are put into context with model predictions for in-medium vector-spectral function utilizing effective Lagrangians. Hadronic many-body calculations predict a strong broadening (and little mass shift) of the $\rho$ spectral function which rapidly increases close to the expected phase boundary of hadronic and quark-gluon matter. Pertinent dilepton rates appear to degenerate with perturbative quark-antiquark annihilation in the Quark-Gluon Plasma, suggestive for chiral symmetry restoration. Applications to low-mass dilepton spectra in heavy-ion collisions result in quantitative agreement with recent high-quality data at the CERN-SPS. Thermal radiation from temperatures around $T_c$ consistently reproduces the experimental dilepton excess observed at masses above 1 GeV as well. The interpretation of dilepton sources at high transverse momentum appears to be more involved.