ABSTRACT
Experiments
at the Relativistic Heavy-Ion Collider (RHIC) colliding
gold nuclei have shown that the hot and dense fireball formed in the
collision exhibits hydrodynamic behaviour. Surprisingly, the bulk of
the spectra of hadrons produced in these collisions, including fine
details such as the hadron mass dependence of the anisotropic
(elliptic)
flow seen in non-central collisions, can be described with a model that
assumes that the matter behaves as in ideal, non-viscous fluid. This
requires a very short thermalization time scale. I will show that
thermalization must be reached after a time of less than 1 fm/c, at
a peak energy density of about 30 GeV/fm**3 and a temperature which
is a factor 2 above the critical value for color deconfinement.
Increasing deviations from perfect fluidity are seen as one studies
more and more non-central collisions, collisions at lower beam energy,
and as one goes farther and farther away from midrapidity. I will
show calculations that suggest that all these deviations can be fully
explained by properly taking into account that the hadron resonance
gas during the late stage of the collision is highly viscous, whereas
the quark-gluon present in the early expansion stage is indeed
essentially inviscid.