# Anisotropic flow of thermal photons at RHIC and LHC

## TL;DR

This study calculates the anisotropic flow of thermal photons in heavy-ion collisions at RHIC and LHC energies, revealing centrality dependence, underestimation by models, and correlations with initial spatial anisotropies.

## Contribution

It provides detailed event-by-event hydrodynamic calculations of thermal photon flow and explores the impact of initial state fluctuations at RHIC and LHC energies.

## Key findings

- Photon elliptic flow increases towards peripheral collisions.
- Triangular flow shows weak centrality dependence.
- Thermal photon production is lower in Cu+Cu than in Au+Au collisions.

## Abstract

We calculate elliptic and triangular flow parameters of thermal photons using an event-by-event hydrodynamic model with fluctuating initial conditions at 200A GeV Au+Au collisions at RHIC and at 2.76A TeV Pb+Pb collisions at the LHC for three different centrality bins. The photon elliptic flow shows strong centrality dependence where $v_2(p_T)$ increases towards peripheral collisions both at RHIC and at the LHC energies. However, the triangular flow parameter does not show significant dependence to the collision centrality. The elliptic as well as the triangular flow parameters found to underestimate the PHENIX data at RHIC by a large margin for all three centrality bins. We calculate $p_T$ spectrum and anisotropic flow of thermal photons from 200A GeV Cu+Cu collisions at RHIC for 0--20\% centrality bin and compare with the results with those from Au+Au collisions. The production of thermal photon is found to decrease significantly for Cu+Cu collisions compared to Au+Au collisions. However, the effect of initial state fluctuation is found to be more pronounced for anisotropic flow resulting in larger $v_2$ and $v_3$ for Cu+Cu collisions. We study the correlation between the anisotropic flow parameters and the corresponding initial spatial anisotropies from their event by event distributions at RHIC and at the LHC energies. The linear correlation between $v_2$ and $\epsilon_2$ is found be stronger compared to the correlation between $v_3$ and $\epsilon_3$. In addition, the correlation coefficient is found to be larger at LHC than at RHIC.