Paper detail

Is hydrodynamics relevant to RHIC collisions?

The hydrodynamic (hydro) model applied to heavy ion data from the relativistic heavy ion collider (RHIC) in the form of single-particle spectra and correlations seems to indicate that a dense QCD medium nearly opaque to partons, a strongly-coupled quark-gluon plasma (sQGP), is formed in more-central Au-Au collisions, and that the sQGP may have a very small viscosity ("perfect liquid"). Measurements of radial and elliptic flows, with possible coalescence of "constituent quarks" to form hadrons, seem to support the conclusion. However, other measurements provide contradictory evidence. Unbiased angular correlations indicate that a large number of back-to-back jets from initial-state scattered partons with energies as low as 3 GeV survive as "minijet" hadron correlations even in central Au-Au collisions, suggesting near transparency. Two-component analysis of single-particle hadron spectra reveals a corresponding spectrum hard component (parton fragment distribution described by pQCD) which can masquerade as "radial flow" in some spectrum analysis. Reinterpretation of "elliptic flow" as a QCD scattering process resulting in fragmentation is also possible. In this paper I review analysis methods and results in the context of two paradigms: the conventional hydrodynamics/hard-probes paradigm and an alternative quadrupole/minijets paradigm. Based on re-interpretation of fiducial data I argue that hydrodynamics may not be relevant to RHIC collisions. Collision evolution may be dominated by parton scattering and fragmentation, albeit the fragmentation process is strongly modified in more-central A-A collisions.