Anisotropic flow of photons in relativistic heavy ion collisions

Abstract

Electromagnetic radiations are one of the potential probes to study the initial state of the hot and dense quark-gluon plasma (QGP) produced during the collision of heavy nuclei at relativistic energies. Photons are emitted throughout the lifetime of the evolving system and carry undistorted information from the production point to the detector. The observation of large anisotropic flow of charged particles provides a strong confirmation of QGP formation and collective behaviour of the produced matter in these collisions. However, the theoretical model calculations which explain the hadronic spectra and anisotropic flow successfully, underpredict the experimental data of elliptic as well as triangular flow of photons by a large margin. This discrepancy between data and theory results is known as direct photon puzzle. In this article, we review the anisotropic flow of photons calculated using hydrodynamical model framework for different collision systems and beam energies. In addition, we propose some new ideas which can be valuable for understanding direct photon puzzle.