Parton fragmentation in nuclear collisions

Abstract

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.