Radiative energy loss and v2 spectra for viscous hydrodynamics

TL;DR

This paper studies how viscous corrections and radiative energy loss affect particle spectra and elliptic flow in heavy ion collisions, revealing species-dependent effects and challenging common assumptions about momentum dependence.

Contribution

It introduces a new understanding of the momentum dependence of viscous corrections, especially the radiative energy loss influence, and explains constituent quark scaling without coalescence.

Findings

Viscous corrections are largest at freezeout surface.

Radiative energy loss causes a $p_T^{3/2}$ dependence at high momentum.

Species-dependent equilibration rates explain quark scaling.

Abstract

This work investigates the first correction to the equilibrium phase space distribution and its effects on spectra and elliptic flow in heavy ion collisions. We show that the departure from equilibrium on the freezeout surface is the largest part of the viscous corrections to $v_2(p_T)$. However, the momentum dependence of the departure from equilibrium is not known {\it a priori}, and it is probably not proportional to $p_T^2$ as has been assumed in hydrodynamic simulations. At high momentum in weakly coupled plasmas it is determined by the rate of radiative energy loss and is proportional to $p_T^{3/2}$. The weaker $p_T$ dependence leads to straighter $v_2(p_T)$ curves at the same value of viscosity. Further, the departure from equilibrium is generally species dependent. A species dependent equilibration rate, with baryons equilibrating faster than mesons, can explain ``constituent quark scaling'' without invoking coalescence models.