Direct photon spectrum and elliptic flow produced from Pb+Pb collisions at $\sqrt{s_{NN}}=2.76$ TeV at the CERN Large Hadron Collider within an integrated hydrokinetic model

TL;DR

This study investigates photon production and elliptic flow in Pb+Pb collisions at 2.76 TeV using the integrated hydrokinetic model, highlighting the effects of different emission sources and medium evolution on observable anisotropies.

Contribution

It introduces a comprehensive model including various photon sources and medium evolution approaches, providing detailed predictions for photon spectra and flow in heavy-ion collisions.

Findings

Thermal photon anisotropy is suppressed by prompt photons, especially in central collisions.

Model results are consistent with experimental data within error bars, with some underestimation in central events.

Including additional photon radiation from deconfined matter could improve agreement with observations.

Abstract

The photon transverse momentum spectrum and its anisotropy from Pb+Pb collisions at the CERN Large Hadron Collider energy $\sqrt {s_{NN}}=2.76$ TeV are investigated within the integrated hydrokinetic model (iHKM). Photon production is accumulated from the different processes at the various stages of relativistic heavy ion collisions: from the primary hard photons of very early stage of parton collisions to the thermal photons from equilibrated quark-gluon and hadron gas stages. Along the way a hadronic medium evolution is treated in two distinct, in a sense opposite, approaches: chemically equilibrated and chemically frozen system expansion. Studying the centrality dependence of the results obtained allows us to conclude that a relatively strong transverse momentum anisotropy of thermal radiation is suppressed by prompt photon emission which is an isotropic. We find out that this effect is getting stronger as centrality increases because of the simultaneous increase in the relative contribution of prompt photons in the soft part of the spectra. The substantial results obtained in iHKM with nonzero viscosity ($\eta/s=0.08$) for photon spectra and $v_2$ coefficients are mostly within the error bars of experimental data, but there is some systematic underestimation of both observables for the near central events. We claim that a situation could be significantly improved if an additional photon radiation that accompanies the presence of a deconfined environment is included. Since a matter of a space-time layer where hadronization takes place is actively involved in anisotropic transverse flow, both positive contributions to the spectra and $v_2$ are considerable, albeit such an argument needs further research and elaboration.