Equilibration and hydrodynamics at strong and weak coupling

Abstract

We give an updated overview of both weak and strong coupling methods to describe the approach to a plasma described by viscous hydrodynamics, a process now called hydrodynamisation. At weak coupling the very first moments after a heavy ion collision is described by the colour-glass condensate framework, but quickly thereafter the mean free path is long enough for kinetic theory to become applicable. Recent simulations indicate thermalization in a time $t\sim40(η/s)^{4/3}/T$ [1], with $T$ the temperature at that time and $η/s$ the shear viscosity divided by the entropy density. At (infinitely) strong coupling it is possible to mimic heavy ion collisions by using holography, which leads to a dual description of colliding gravitational shock waves. The plasma formed hydrodynamises within a time of $0.41/T$. A recent extension found corrections to this result for finite values of the coupling, when $η/s$ is bigger than the canonical value of $1/4π$, which leads to $t\sim(0.41+1.6(η/s-1/4π))/T$ [2]. Future improvements include the inclusion of the effects of the running coupling constant in QCD.