Nuclear Physics Seminar
The role of minijets in RHIC collisions
Lanny Ray (University of Texas at Austin)
Collisions between very high energy atomic nuclei at the Relativistic
Heavy Ion Collider (RHIC) are producing the most dense, strongly
interacting systems ever made in the laboratory. The RHIC research
program offers the opportunity to extend our understanding of QCD
(quantum chromodynamics) into kinematic domains far beyond that
explored in high energy particle physics. A prevalent interpretation
in the literature of the RHIC data is that quark and gluon (partons)
interactions just after the nuclear impact are very strong; enough
to produce rapid thermalization and intense thermodynamic pressure
which causes the system to expand collectively, cool and hadronize
(convert back into observable particles like pions, kaons, protons).
This scenario is embodied in non-viscous hydrodynamic models which
have been touted as providing evidence for the formation of a strong
interacting quark-gluon plasma (sQGP), and even a "Perfect Liquid."
In this talk I will present an alternative view of RHIC collisions
using the copious production of semi-hard partonic scattering, or
minijets, to probe the system. Using STAR and PHENIX data I will show
that as much as 1/3 of the particles produced in RHIC collisions are
associated with minijets, that momentum is very strongly dissipated
though the system does not appear to be thermalized, and that by
studying the systematic dependences of minijet production the data
reveal that the system undergoes a sudden, dramatic and qualitative
change at a specific density.
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