FUTURE SCIENCE AT THE RELATIVISTIC HEAVY ION COLLIDER.

Technical Report · Thu Dec 21 00:00:00 EST 2006

DOI:https://doi.org/10.2172/897801· OSTI ID:897801

LUDLAM, T

QCD was developed in the 1970's as a theory of the strong interaction describing the confinement of quarks in hadrons. An early consequence of this picture was the realization that at sufficiently high temperature, or energy density, the confining forces are overcome by color screening effects, resulting in a transition from hadronic matter to a new state--later named the Quark Gluon Plasma--whose bulk dynamical properties are determined by the quark and gluon degrees of freedom, rather than those of confined hadrons. The suggestion that this phase transition in a fundamental theory of nature might occur in the hot, dense nuclear matter created in heavy ion collisions triggered a series of experimental searches during the past two decades at CERN and at BNL, with successively higher-energy nuclear collisions. This has culminated in the present RHIC program. In their first five years of operation, the RHIC experiments have identified a new form of thermalized matter formed in Au+Au collisions at energy densities more than 100 times that of a cold atomic nucleus. Measurements and comparison with relativistic hydrodynamic models indicate that the matter thermalizes in an unexpectedly short time ( < 1 fm/c) , has an energy density at least 15 times larger than needed for color deconfinement, has a temperature about 2 times the critical temperature of {approx}170 MeV predicted by lattice QCD, and appears to exhibit collective motion with ideal hydrodynamic properties--a ''perfect liquid'' that appears to flow with a near-zero viscosity to entropy ratio - lower than any previously observed fluid and perhaps close to a universal lower bound. There are also indications that the new form of matter directly involves quarks. Comparison of measured relative hadron abundances with very successful statistical models indicates that hadrons chemically decouple at a temperature of 160-170 MeV. There is evidence suggesting that this happens very close to the quark-hadron phase transition, ie. that hadrons are born in the phase transition from quark matter, and abundance-changing interactions then quickly cease. Valence quark number scaling of the measured anisotropy parameter for all hadrons suggests that the collectively flowing matter involves quarks, not hadrons. And the striking observation of a universal, strong enhancement of baryons relative to mesons at intermediate transverse momentum has been interpreted as evidence of competition between quark coalescence of the bulk medium and jet fragmentation. It is generally agreed that the new matter is not describable in terms of ordinary color neutral hadrons, and that many observations are consistent with models that incorporate quark and gluon degrees of freedom. The evidence is consistent with the matter being a strongly coupled quark gluon plasma (sQGP), and thus it behaves quite differently from the perturbative QCD parton gas that was expected by most people prior to RHIC data. The extraordinary properties of this new state of matter demand further measurements to better understand its behavior, properties, origin and description.

• Research Organization: Brookhaven National Lab. (BNL), Upton, NY (United States)
• Sponsoring Organization: Doe - Office Of Science
• DOE Contract Number: DE-AC02-98CH10886
• OSTI ID: 897801
• Report Number(s): BNL-77334-2006-IR; R&D Project: PO-3; KB-02-01; TRN: US0701521
• Country of Publication: United States
• Language: English

Related Subjects

72 PHYSICS OF ELEMENTARY PARTICLES AND FIELDS
71 CLASSICAL AND QUANTUM MECHANICS
GENERAL PHYSICS
73 NUCLEAR PHYSICS AND RADIATION PHYSICS
74 ATOMIC AND MOLECULAR PHYSICS
ANISOTROPY
BARYONS
CRITICAL TEMPERATURE
DEGREES OF FREEDOM
ENERGY DENSITY
HEAVY IONS
HYDRODYNAMIC MODEL
NUCLEAR MATTER
QUANTUM CHROMODYNAMICS
QUARK MATTER
STATISTICAL MODELS
STRONG INTERACTIONS
TRANSVERSE MOMENTUM
VALENCE