Dilepton Production in Heavy-Ion Collisions

TL;DR

This paper reviews electromagnetic radiation from hot fireballs in heavy-ion collisions, emphasizing how medium effects relate to chiral symmetry restoration and how dilepton data supports resonance melting near the pseudo-critical temperature.

Contribution

It integrates lattice QCD calculations, sum rules, and experimental data to provide a comprehensive interpretation of dilepton production and medium effects in heavy-ion collisions.

Findings

Most radiation originates near the pseudo-critical temperature.

Resonance melting is confirmed as the main mechanism in this regime.

Recent photon spectra support the chiral restoration picture.

Abstract

The properties of electromagnetic radiation from hot fireballs as created in ultra-relativistic heavy-ion collisions are reviewed. We first outline how the medium effects in the electromagnetic spectral function, which governs thermal production rates, relate to the (partial) restoration of chiral symmetry. In particular, we show how chiral and QCD sum rules, together with constraints from lattice QCD, can render these relations quantitative. Turning to dilepton data, we elaborate on updates in the space-time evolution and quark-gluon plasma emission rates from lattice-QCD calculations. With a now available excitation function in dilepton spectra from the RHIC beam-energy scan connecting down to SPS energies, we argue that a consistent interpretation of dilepton data emerges. Combining well-constrained space-time evolutions with state-of-the-art emission rates identifies most of the radiation to emanate from around the pseudo-critical temperature, and thus confirms resonance melting as the prevalent mechanism in this regime, compatible with chiral restoration. Recent measurements of a relatively soft slope and large elliptic flow in direct-photon spectra at RHIC and LHC lend further support to this picture.