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Parton fragmentation in nuclear collisions

The hydrodynamic (hydro) model has been extensively applied to heavy ion data from the relativistic heavy ion collider (RHIC). Results are interpreted to conclude that a dense QCD medium nearly opaque to most partons, a strongly-coupled quark-gluon plasma (sQGP), is formed in more-central Au-Au collisions. The sQGP may have a very small viscosity (``perfect liquid''). However, other analysis methods provide contradictory evidence. Two-component analysis of single-particle hadron spectra reveals a spectrum hard component consistent with a parton fragment distribution described by pQCD which can masquerade as ``radial flow'' in some hydro-motivated analysis. Minimum-bias angular correlations reveal 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 to partons. In this talk I present methods by which almost all spectrum and correlation structure, even in the most-central Au-Au collisions at 200 GeV, can be described quantitatively by pQCD calculations. The evolution of nuclear collisions is apparently dominated by parton scattering and fragmentation even in the most-central A-A collisions, albeit the fragmentation process is strongly modified.