Ratio of photon anisotropic flow in relativistic heavy ion collisions

Abstract

The elliptic and triangular flow parameters of direct photons are known to be dominated by thermal radiations. The nonthermal contributions dilute the photon anisotropic flow by adding extra weight factor in the ${v}_{n}$ calculation. The discrepancy between experimental photon anisotropic flow data and results from theoretical model calculations is not well understood even after significant developments in the model calculations as well as in the experimental analysis methods. We show that the ratio of photon ${v}_{n}$ can be a potential observable in this regard by minimizing the nonthermal contributions. We calculate the ${v}_{2}/{v}_{3}$ of photons as a function of ${p}_{T}$ from heavy ion collisions at the BNL Relativistic Heavy Ion Collider (RHIC) and compare the results with available experimental data. The ratio is found to be larger for peripheral collisions than for central collisions. However, the ${p}_{T}$ dependent behavior of the ratio is found to be different than the individual flow parameters. The ratio is found to be sensitive to the initial conditions of the model calculation at different ${p}_{T}$ regions compared to the individual anisotropic flow parameters. We show that the photon ${v}_{1}/{v}_{2}$ and ${v}_{1}/{v}_{3}$ along with the ${v}_{2}/{v}_{3}$ results may help us constrain the initial conditions in heavy ion collisions.