On the Departure from Equilibrium of the Quasi-Particle Distribution Functions in High Energy Nuclear Collisions

P. Chakraborty, J. I. Kapusta

Published 2016-10-26Version 1

In high energy heavy ion collisions, single particle distributions are distorted from their thermal equilibrium form due to gradients in the flow velocity. These are closely related to the formulas for the shear and bulk viscosities in the quasi-particle approximation. Distorted single particle distributions are now commonly used to calculate the emission of photons and dilepton pairs, and in the late stage to calculate the conversion of a continuous fluid to individual particles. In practice this is done only in a very approximate way. We show how it can be done rigorously in the quasi-particle approximation and illustrate it with the linear $\sigma$ model at finite temperature for both the shear and bulk contributions.