Thermal photon $v_3$ at LHC from fluctuating initial conditions

TL;DR

This paper calculates the triangular flow ($v_3$) of thermal photons in Pb-Pb collisions at LHC using event-by-event hydrodynamics, revealing significant $v_3$ driven by initial state fluctuations and radial flow.

Contribution

It introduces a detailed calculation of thermal photon $v_3$ with fluctuating initial conditions, highlighting the role of initial density deformations and radial flow.

Findings

Thermal photon $v_3$ is positive and significant.

Photon $v_3$ as a function of $p_T$ resembles $v_2$.

Radial flow from fluctuations drives large $v_3$.

Abstract

We calculate the triangular flow parameter $v_3$ of thermal photons for 0--40\% central collisions of Pb nuclei at LHC using an event-by-event hydrodynamic model with fluctuating initial conditions. Thermal photon $v_3$ with respect to the the participant plane angle is found to be positive and significant compared to the elliptic flow parameter $v_2$ of thermal photons. In addition, photon $v_3$ as a function of $p_T$ shows similar qualitative nature to photon $v_2$ in the region $1< p_T <6$ GeV/$c$. We argue that while $v_3$ originates from $\epsilon_3$ deformations of the initial state density distribution, fast buildup of radial flow due to fluctuations is the main driving mechanism for the observed large value.