Radiative energy loss and v2 spectra for viscous hydrodynamics

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 freeze-out surface is the largest part of the viscous corrections to v2(pT). However, the momentum dependence of the departure from equilibrium is not known a priori, and it is probably not proportional to pT2 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 pT3/2. The weaker pT dependence leads to straighter v2(pT) curves at the same value of viscosity. Furthermore, 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.