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∼40(η/s)4/3/T [L. Keegan, A. Kurkela, P. Romatschke, W. van der Schee, Y. Zhu, Weak and strong coupling equilibration in nonabelian gauge theories, JHEP 04 (2016) 031. arXiv:1512.05347, doi:10.1007/JHEP04(2016)031], 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 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∼(0.41+1.6(η/s−1/4π))/T [S. Grozdanov, W. van der Schee, Coupling constant corrections in holographic heavy ion collisions, arXiv:1610.08976]. Future improvements include the inclusion of the effects of the running coupling constant in QCD.
Highlights
The creation of a strongly coupled plasma at relativistic nucleus-nucleus collisions is one of the most striking discoveries at RHIC and LHC
This is surprising since the gradients at that time result in a small longitudinal pressure, even with the small viscosity present. This process of going to a regime described by hydrodynamics is called hydrodynamisation and depending on the gradients present this can take much shorter than the process of equilibration
The initial stage of heavy ion collisions progresses from weaker coupling at the moment of the collision to strong coupling in the later hydrodynamic regime
Summary
It is profoundly challenging to describe the entire process from collision to hydrodynamics fully within QCD itself This is partly because at high energy scales a perturbative treatment can be appropriate, while at energy scales of the temperature of the plasma formed a strong coupling picture should be used. At strong coupling there are currently no theoretical tools to do such a description within QCD itself, within holography it is possible to try and mimic QCD as close as possible [3] This talk will review both weak and strong coupling approaches from a far-from-equilibrium initial stage to a plasma described by relativistic hydrodynamics. This includes both the timescale of the process, as well as the evolution of the flow and energy density during the process
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