Figure. The finalstate momentum anisotropy v_{2} (elliptic flow)
divided by the spatial anisotropy of the initial overlap (participant)
zone. The almondshaped participant zone is depicted in the upper part of
the figure. The horizontal axis is a relative measure of centrality —
for any given experiment, the most headon collisions correspond to the
rightmost point. Hydrodynamic models predict values at RHIC given by the
lower shaded band.

Limiting Hydrodynamic Behavior in Central GoldGold Collisions at RHIC
In nuclear collisions that are not exactly headon, the collision
process
converts the initialstate spatial anisotropy into momentum anisotropy in
the final state. A quantitative measure of the final momentum anisotropy
(flow) offers important insights into the bulk properties of the excited
phase of nuclear matter that the collision produces. Theorists argue that
observation of the large flow predicted by hydrodynamic models is highly
relevant to inferences about production of Quark Gluon Plasma, although
this observation is insufficient on its own. Hydrodynamics represents one
possible limiting case in describing nuclear collisions — the limit
where the mean free path for interaction of the constituents represented
by the fluid cells is very small compared with the region of nuclear
overlap.
The ratio of anisotropies plotted in the figure is especially useful for
comparisons of data with models; in the important class of hydrodynamic
model with constant sound speed, this ratio is largely independent of
collision centrality.
STAR measures the momentum anisotropy v_{2} directly, whereas the
initial spatial anisotropy is inferred indirectly from the centrality of
the collision. The flow measurement in this work represents a technical
advance, because of improvements in the method for removing correlations
unrelated to fluidlike flow and reduced systematic uncertainties
associated
with this correction. Overall, we conclude that at RHIC energies, the most
central collisions do indeed reach the hydrodynamic limit, unlike the case
of the other data plotted in the figure, which come from the lower energy
SPS accelerator at CERN and the AGS at Brookhaven.
Related STAR papers
Elliptic flow from two and fourparticle correlations in Au + Au collisions at sqrt(s_{nn}) = 130 GeV
Phys. Rev. C 66 (2002) 034904
ePrint Archives (nuclex/0206001):
Abstract 
PS 
PDF
Journal article:
Phys. Rev. C server
Identified Particle Elliptic Flow in Au+Au Collisions at sqrt(s_{nn}) = 130 GeV
Phys. Rev. Lett. 87 (2001) 182301
ePrint Archives (nuclex/0107003):
Abstract 
PS 
PDF
Journal article:
Phys. Rev. Lett. server
Elliptic Flow in Au+Au Collisions at sqrt(s_{nn}) = 130 GeV
Phys. Rev. Lett. 86 (2001) 402
ePrint Archives (nuclex/0009011):
Abstract 
PS 
PDF
Journal article:
Phys. Rev. Lett. server
