Thermal Dileptons as Fireball Thermometer and Chronometer

TL;DR

This paper uses thermal dilepton radiation data from high-energy heavy-ion collisions to estimate fireball temperatures and lifetimes, providing a potential method to identify phase transitions in QCD matter.

Contribution

It combines hadronic many-body theory and lattice-QCD based equations of state to quantitatively describe dilepton spectra and systematically extract fireball properties.

Findings

Quantitative description of dilepton spectra at SPS and RHIC.

Predictions of fireball lifetime and temperature excitation functions.

Potential to identify QCD phase transition signatures.

Abstract

Thermal dilepton radiation from the hot fireballs created in high-energy heavy-ion collisions provides unique insights into the properties of the produced medium. We first show how the predictions of hadronic many-body theory for a melting $\rho$ meson, coupled with QGP emission utilizing a modern lattice-QCD based equation of state, yield a quantitative description of dilepton spectra in heavy-ion collisions at the SPS and the RHIC beam energy scan program. We utilize these results to systematically extract the excess yields and their invariant-mass spectral slopes to predict the excitation function of fireball lifetimes and (early) temperatures, respectively. We thereby demonstrate that future measurements of these quantities can yield unprecedented information on basic fireball properties. Specifically, our predictions quantify the relation between the measured and maximal fireball temperatures, and the proportionality of excess yields and total lifetime. This information can serve as a "caloric" curve to search for a first-order QCD phase transition, and to detect non-monotonous lifetime variations possibly related to critical phenomena.