Chasing the Unicorn: RHIC and the QGP

Abstract

At nonzero temperature, it is expected that QCD undergoes a phase transition to a deconfined, chirally symmetric phase, the Quark-Gluon Plasma (QGP). I review what we expect theoretically about this possible transition, and what we have learned from heavy ion experiments at RHIC. I argue that while there are unambiguous signals for qualitatively new behavior at RHIC, versus experiments at lower energies, that in detail, no simple theoretical model can explain all salient features of the data.

Large transient states in quantum field theory

“…5 The point can be more clearly illustrated with a discrete sum. Consider the sets of N positive numbers {a j } and {b j }.…”

“…The evidence suggests that QCD undergoes a phase transition from a gas of hadrons to a gas of quarks and gluons at a temperature T c = 190 MeV. The properties of the quark-gluon plasma have been examined considerably [4,5,18,19,36], with particular emphasis on a gauge invariant description. The second question is much less clearly understood, since there are a few complicating factors to this simple picture.…”

“…The advent of QCD and its success through the parton model, eventually brought forth the conjecture that fundamental quarks and gluons may be capable of existing in a plasma phase at higher temperatures and densities to the hadron gas [2][3][4]. To test this idea, collisions between heavy nuclei have been a central source of experimental input [5], with at present the Relativistic Heavy Ion Collider (RHIC) operating at 200 GeV/nucleon [6] and in the near fu- ture A Large Ion Collider Experiment (ALICE) at LHC to operate at 5.5 TeV/nucleon [7]. So far experimental data seems to rule out the hadron gas phase in very high energy heavy nuclei collisions, and the data is also inconclusive about the quark-gluon plasma phase [5,6].…”

“…To test this idea, collisions between heavy nuclei have been a central source of experimental input [5], with at present the Relativistic Heavy Ion Collider (RHIC) operating at 200 GeV/nucleon [6] and in the near fu- ture A Large Ion Collider Experiment (ALICE) at LHC to operate at 5.5 TeV/nucleon [7]. So far experimental data seems to rule out the hadron gas phase in very high energy heavy nuclei collisions, and the data is also inconclusive about the quark-gluon plasma phase [5,6]. Thus all parts of the theoretical picture have not fit as yet, but there is suggestive evidence of a richer particle-like substructure beyond the conventional hadron gas.…”

Nonextensive perfect hydrodynamics — a model of dissipative relativistic hydrodynamics?

“…The characteristic feature of such a processes is the production of a large number of secondaries (multiplicities at present approach ∼ 10 3 ). Already in 1953, when there were only ∼ 10 particles produced and registered, mostly in cosmic rays experiments, it was found [1][2][3] that such processes can interest to us (being currently investigated at RHIC at Brookhaven, with the newly commissioned LHC at CERN joining soon) because it is widely believed that in such collisions a new form of hadronic matter, the so called Quark-Gluon-Plasma (QGP), will be produced [15][16][17][18].…”