Electromagnetic probes of the QGP

TL;DR

This paper studies electromagnetic signals like photons and dileptons from the quark-gluon plasma in heavy-ion collisions, analyzing their sources, flow, and in-medium effects to understand QCD matter properties.

Contribution

It introduces a detailed analysis of electromagnetic radiation in heavy-ion collisions using the PHSD model, highlighting the origins of photon flow and in-medium effects on dileptons.

Findings

Partonic sources significantly contribute to high-mass dileptons.

Different centrality dependence helps explain the photon v2 puzzle.

In-medium effects modify low-mass dilepton spectra.

Abstract

We investigate the properties of the QCD matter across the deconfinement phase transition in the scope of the parton-hadron string dynamics (PHSD) transport approach. We present here in particular the results on the electromagnetic radiation, i.e. photon and dilepton production, in relativistic heavy-ion collisions. By comparing our calculations for the heavy-ion collisions to the available data, we determine the relative importance of the various production sources and address the possible origin of the observed strong elliptic flow $v_2$ of direct photons. We argue that the different centrality dependence of the hadronic and partonic sources for direct photon production in nucleus-nucleus collisions can be employed to shed some more light on the origin of the photon $v_2$ "puzzle". While the dilepton spectra at low invariant mass show in-medium effects like an enhancement from multiple baryonic resonance formation or a collisional broadening of the vector meson spectral functions, the dilepton yield at high invariant masses (above 1.1 GeV) is dominated by QGP contributions for central heavy-ion collisions at ultra-relativistic energies. This allows to have an independent view on the parton dynamics via their electromagnetic massive radiation.