Ratio of photon anisotropic flow in relativistic heavy ion collisions

TL;DR

This study proposes using the ratio of photon anisotropic flow coefficients, such as v2/v3, as a robust observable to better understand thermal photon production and initial conditions in relativistic heavy ion collisions, reducing uncertainties from non-thermal effects.

Contribution

It introduces the photon flow ratio v2/v3 as a new observable to minimize non-thermal uncertainties and constrain initial collision conditions.

Findings

The v2/v3 ratio remains stable for pT > 2 GeV but increases at lower pT.

The ratio is larger in peripheral collisions compared to central ones.

It is sensitive to initial formation time and freeze-out temperature.

Abstract

The $p_T$ dependent elliptic and triangular flow parameters of direct photons are known to be dominated by thermal radiations. The non-thermal 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. We show that the ratio of photon $v_n$ can be a potential observable in this regard by minimizing the uncertainties arising due to the non-thermal contributions. We calculate the $v_2/v_3$ of photons as a function of $p_T$ from heavy ion collisions at RHIC and compare the results with available experimental data. The ratio does not change significantly $p_T$ in the region $p_T>2$ GeV. However, it rises towards smaller $p_T$ ($< 2$ GeV) values. The ratio is found to be larger for peripheral collisions than for central collisions. In addition, it is found to be sensitive to the initial formation time and the final freeze-out temperature at different $p_T$ regions unlike 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 constraining the initial conditions.