Effective temperatures of the QGP from thermal photon and dilepton production

Abstract

Thermal electromagnetic radiation is emitted by the quark-gluon plasma (QGP) throughout its space-time evolution, with production rates that depend characteristically on the temperature. We study this temperature using thermal photons and dileptons using the Trajectum heavy ion code, which is constrained by Bayesian analysis. In addition we present the elliptic flow of both the thermal photons and thermal dileptons including systematic uncertainties corresponding to the model parameter uncertainty. We give a comprehensive overview of the resulting effective temperatures \(T_{\textrm{eff}},\) obtained from thermal photon transverse momentum and thermal dilepton invariant mass distributions, as well as the dependence of \(T_{\textrm{eff}}\) on various selection criteria of these probes. We conclude that the \(T_{\textrm{eff}}\) obtained from thermal photons is mostly insensitive to the temperature of the QGP with a value of \(T_{\textrm{eff}} \sim 250\)–300 MeV depending on their transverse momentum, almost independent of collision centrality. Thermal dileptons are much better probes of the QGP temperature as they do not suffer from a blue shift as their invariant mass is used, allowing for a more precise constraint of the QGP temperature during different stages of the evolution of the system. By applying selection criteria on the dilepton transverse momentum and the invariant mass we are able to extract fluid temperatures on average times ranging from late emission \((\langle \tau \rangle = 5.6~\text {fm}/c)\) to very early emissions \((\langle \tau \rangle < 1.0~\text {fm}/c).\) Furthermore, we show how these selection criteria can be used to map the elliptic flow of the system all throughout its evolution.