Hydrodynamics in RHIC collisions:
Perfect liquid or desert mirage?

Tom Trainor
University of Washington

Hydrodynamic phenomena, including radial and elliptic flow, are commonly 
believed to play a dominant role in the soft physics of RHIC 
collisions. Conclusions are drawn from elliptic flow data about early 
thermalization, and claims are made for a bulk medium  with anomalously 
small viscosity (perfect liquid). However, contradictory results have 
emerged from recent correlation analysis. Minimum-bias parton fragment 
(minijet) yields are observed to be consistent with binary-collision 
scaling (a transparent system). Large-statistics measurements of 2D 
angular correlations at 62 and 200 GeV reveal a strong and sharp 
transition of minijet properties at a mid-peripheral Au-Au centrality 
point depending on collision energy. The same angular correlations 
provide model-independent separation of "flow" from "nonflow" 
(minijets). Accurate azimuth quadrupole measurements (directly related 
to v2) depend only on the initial A-A collision geometry (impact 
parameter b) and collision energy for all energies above 13 GeV. There 
is no apparent sensitivity to the sharp transition experienced by 
minijet correlations. A two-component (soft+hard) analysis of the 
centrality evolution of Au-Au pt spectra to 12 GeV/c for pions and 
protons confirms (in the hard-component systematics) the sharp 
transition observed for minijet correlations. And, there is no evidence 
in the spectra for radial flow. Those results call into question the 
interpretation of RHIC collisions in terms of a flowing medium with 
small viscosity and provide new insights into parton energy loss.