On attractor and fixed points in Bjorken flows

TL;DR

This paper investigates the transition from free streaming to hydrodynamics in a Bjorken expanding plasma using kinetic theory, fixed point analysis, and attractor solutions to improve early-time hydrodynamic descriptions.

Contribution

It introduces a fixed point and attractor framework for understanding the transition to hydrodynamics in Bjorken flows, incorporating time-dependent relaxation times and fixed point analysis.

Findings

Identification of an attractor solution connecting collisionless and hydrodynamic regimes.

Demonstration that renormalizing a transport coefficient improves early-time hydrodynamics.

Validation that the approach reproduces kinetic theory results accurately even far from equilibrium.

Abstract

We consider a plasma of massless particles undergoing Bjorken expansion, mimicking the matter created in ultra-relativistic heavy ion collisions. We study the transition to hydrodynamics using kinetic theory in the relaxation time approximation. By allowing the relaxation time to depend on time, we can monitor the speed of the transition from the collisionless regime to hydrodynamics. By using a special set of moments of the momentum distribution, we reduce the kinetic equation to a coupled mode problem which encompasses all versions of second order viscous hydrodynamics for Bjorken flows. This coupled mode problem is analysed first using techniques of linear algebra. Then we transform this two mode problem into a single non linear differential equation and proceed to a fixed point analysis. We identify an attractor solution as the particular solution of this non linear equation that joins two fixed points: one corresponding to the collisionless, early time regime, the other corresponding to late time hydrodynamics. We exploit the analytic solution of this equation in order to test several approximations and to identify generic features of the transition to hydrodynamics. We argue that extending the accuracy of hydrodynamics to early time, i.e. to the region of large gradients, amounts essentially to improve the accuracy of the location of the collisionless fixed point. This is demonstrated by showing that a simple renormalisation of a second order transport coefficient puts the free streaming fixed point at the right location, and allows us to reproduce accurately the full solution of the kinetic equation within second order viscous hydrodynamics, even in regimes far from local equilibrium.