### Speaker

Denes Molnar
(Purdue University)

### Description

Much of our understanding of the collision dynamics at RHIC and LHC relies
on contrasting hydrodynamic or hydro+transport calculations with
experimental data. For example, early evidence for rapid thermalization
and quark-gluon plasma phase transition at RHIC came from particle spectra
and the pion-proton splitting of differential elliptic flow. An inevitable
component in these calculations is the conversion of the fluid to
particles. For an ideal fluid the conversion is straightforward (the usual
caveats of the Cooper-Frye treatment aside) because the phase space
distributions are locally thermal for each species. For a viscous fluid,
however, an infinite class of phase space corrections can reproduce the
same hydrodynamic variables, even in a one-component system.
Present viscous hydrodynamic calculations routinely assume that phase
space corrections induced by shear stress are quadratic in momentum and
that they have the same coefficient for all particle species ("democratic"
Grad ansatz), independently of microscopic details. However, in a gas of
hadrons, equilibration is driven by scattering rates - species that
scatter rarely tend to be further away from local equilibrium than those
that scatter often.
We will present results from fully nonlinear covariant transport theory
for the phase space corrections in an expanding multicomponent gas, and
test the validity of Grad's quadratic ansatz and of the "democratic"
assumption for sharing viscous effects between species. The findings will
be compared to phase space corrections from linear response theory, which
is applicable for small gradients and small deviations from local
equilibrium. Finally, we will show how dynamical phase space corrections
affect basic identified particle observables (spectra and elliptic flow)
in the framework of viscous hydrodynamics.

### Primary author

Denes Molnar
(Purdue University)