Speaker
Denes Molnar
(Purdue University and RIKEN BNL Research Center)
Description
The past couple years have seen a lot of progress in the application of
causal dissipative hydrodynamics to model heavy-ion collisions at the
Relativistic Heavy Ion Collider (RHIC) at Brookhaven National Laboratory.
These advances benefited greatly from breakthrough results obtained in the
gravity (AdS/CFT) dual of theories based on N=4 supersymmetric Yang-Mills.
2+1D viscous hydrodynamic calculations have now converged between the
various groups, and the shear viscosity to entropy density ratio has been
estimated from RHIC data. The results are roughly comparable with the
AdS/CFT bounds (eta/s)_min = ~0.1, an order of magnitude below
perturbative QCD estimates. I will review some of this progress in the
first part of my talk.
It is, however, important to realize that viscous hydrodynamics, even the
second-order formulations (such as Israel-Stewart theory), is only an
approximation with a certain region of validity. Relaxation towards local
equilibrium competes with the rapid longitudinal and later also transverse
expansion of the heavy-ion system. I will report on investigations of this
relaxation-expansion competition in a nonequilibrium theory framework,
covariant transport. These studies find that Israel-Stewart hydrodynamics
is only accurate for RHIC applications when eta/s < ~few * 0.1. A useful
rule of thumb is that hydrodynamics becomes inaccurate when dissipative
corrections to pressure and entropy exceed about 20%. If bulk viscosity
plays a significant role in the dynamics, the additional entropy
generation further constrains the applicability of hydrodynamics.
Most of the above results center on the transverse momentum anisotropy (so
called "elliptic flow") observable. Conical flow generated by a supersonic
source has also been proposed as a sensitive experimental probe of
thermalization AND very low viscosities. In the third part of my talk I
will show, from covariant transport, what it takes to generate conical
flow. Finally, I will finish up with a discussion of key open problems
that need to be tackled in order to reliably estimate the shear viscosity
at RHIC from heavy-ion data.
Author
Denes Molnar
(Purdue University and RIKEN BNL Research Center)
