Elliptic flow of thermal photons in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV

TL;DR

This paper predicts the behavior of elliptic flow of thermal photons in gold-gold collisions at 200 GeV using 3D hydrodynamics, highlighting its dependence on transverse momentum, centrality, and rapidity.

Contribution

It introduces a detailed 3D hydrodynamic model to predict the elliptic flow of thermal photons, emphasizing the rapidity dependence as a tool to probe initial conditions.

Findings

Elliptic flow v2 increases then decreases with pt, peaking around 2 GeV/c.

v2 is maximal at about 50% centrality.

Rapidity dependence of v2 is sensitive to initial energy density distribution.

Abstract

The transverse momentum (pt) dependence, the centrality dependence and the rapidity dependence of the elliptic flow of thermal photons in Au+Au collisions at $\sqrt{s_{NN}}=200$ GeV are predicted, based on a three-dimensional ideal hydrodynamic description of the hot and dense matter. The elliptic flow parameter $v_{2}$, i.e. the second Fourier coefficient of azimuthal distribution, of thermal photons, first increases with $\pt$ and then decreases for $\pt>$ 2 GeV/$c$, due to the weak transverse flow at the early stage. The $\pt$-integrated $v_{2}$ first increases with centrality, reaches a maximum at about 50% centrality, and decreases. The rapidity dependence of the elliptic flow $v_{2}(y)$ of direct photons (mainly thermal photons) is very sensitive to the initial energy density distribution along longitudinal direction, which provides a useful tool to extract the realistic initial condition from measurements.