Thermal photons as a quark-gluon plasma thermometer revisited

TL;DR

This paper revisits how thermal photon spectra in heavy-ion collisions relate to the quark-gluon plasma temperature, emphasizing the influence of radial flow and the need for additional evidence to distinguish emission stages.

Contribution

It analyzes the relationship between effective photon temperature and true medium temperature, highlighting the impact of radial flow and proposing tools to differentiate emission phases.

Findings

Most photons originate near the phase transition temperature T_c.

Radial flow significantly enhances the effective photon temperature.

Large effective temperatures do not necessarily indicate emission from regions above T_c.

Abstract

Photons are a penetrating probe of the hot medium formed in heavy-ion collisions, but they are emitted from all collision stages. At photon energies below 2-3 GeV, the measured photon spectra are approximately exponential and can be characterized by their inverse logarithmic slope, often called "effective temperature" $T_\mathrm{eff}$. Modelling the evolution of the radiating medium hydrodynamically, we analyze the factors controlling the value of $T_\mathrm{eff}$ and how it is related to the evolving true temperature $T$ of the fireball. We find that at RHIC and LHC energies most photons are emitted from fireball regions with $T{\,\sim\,}T_\mathrm{c}$ near the quark-hadron phase transition, but that their effective temperature is significantly enhanced by strong radial flow. Although a very hot, high pressure early collision stage is required for generating this radial flow, we demonstrate that the experimentally measured large effective photon temperatures $T_\mathrm{eff}{\,>\,}T_\mathrm{c}$, taken alone, do not prove that any electromagnetic radiation was actually emitted from regions with true temperatures well above $T_\mathrm{c}$. We explore tools that can help to provide additional evidence for the relative weight of photon emission from the early quark-gluon and late hadronic phases. We find that the recently measured centrality dependence of the total thermal photon yield requires a larger contribution from late emission than presently encoded in our hydrodynamic model.