Experimental and Theoretical Challenges in the Search for the Quark Gluon Plasma: The STAR Collaboration's Critical Assessment of the Evidence from RHIC Collisions

TL;DR

This assessment critically evaluates whether RHIC STAR measurements constitute a compelling discovery of the Quark-Gluon Plasma. It integrates lattice-QCD expectations, hydrodynamic modeling, jet quenching, gluon-saturation ideas, and quark recombination to interpret bulk and hard-probe observables. The authors find strong, consistent indications of a dense, rapidly thermalizing, and opaque medium compatible with QGP-like behavior, but caution that no unambiguous deconfinement smoking gun exists and stress the need for crosschecks and a more unified theory. They outline concrete experimental and theoretical directions to sharpen the case, including heavy-flavor measurements, direct photons, and higher-energy tests at LHC to distinguish QGP-specific phenomena from hadronic or initial-state effects.

Abstract

We review the most important experimental results from the first three years of nucleus-nucleus collision studies at RHIC, with emphasis on results from the STAR experiment, and we assess their interpretation and comparison to theory. The theory-experiment comparison suggests that central Au+Au collisions at RHIC produce dense, rapidly thermalizing matter characterized by: (1) initial energy densities above the critical values predicted by lattice QCD for establishment of a Quark-Gluon Plasma (QGP); (2) nearly ideal fluid flow, marked by constituent interactions of very short mean free path, established most probably at a stage preceding hadron formation; and (3) opacity to jets. Many of the observations are consistent with models incorporating QGP formation in the early collision stages, and have not found ready explanation in a hadronic framework. However, the measurements themselves do not yet establish unequivocal evidence for a transition to this new form of matter. The theoretical treatment of the collision evolution, despite impressive successes, invokes a suite of distinct models, degrees of freedom and assumptions of as yet unknown quantitative consequence. We pose a set of important open questions, and suggest additional measurements, at least some of which should be addressed in order to establish a compelling basis to conclude definitively that thermalized, deconfined quark-gluon matter has been produced at RHIC.

Figures (37)

• Figure 1: LQCD calculation results from Ref. Karsch for the pressure divided by $T^4$ of strongly interacting matter as a function of temperature, and for several different choices of the number of dynamical quark flavors. The arrows near the right axis indicate the corresponding Stefan-Boltzmann pressures for the same quark flavor assumptions.
• Figure 2: Temperature-dependence of the heavy-quark screening mass (divided by temperature) as a function of temperature (in units of the phase transition temperature), from LQCD calculations in Ref. Kaczmarek. The curves represent perturbative expectations of the temperature-dependence.
• Figure 3: LQCD calculations for two dynamical quark flavors Karsch showing the coincidence of the chiral symmetry restoration (marked by the rapid decrease of chiral condensate $\langle \overline{\psi} \psi \rangle$ in the upper right-hand frame) and deconfinement (upper left frame) phase transitions. The lower plot shows that the chiral transition leads toward a mass degeneracy of the pion with scalar meson masses. All plots are as a function of the bare coupling strength $\beta$ used in the calculations; increasing $\beta$ corresponds to decreasing lattice spacing and to increasing temperature.
• Figure 4: LQCD calculation results for non-zero chemical potential Fodor-2, suggesting the existence of a critical point well above RHIC chemical potential values. The solid line indicates the locus of first-order phase transitions, while the dotted curve marks crossover transitions between the hadronic and QGP phases.
• Figure 5: Pressure as a function of energy density at vanishing net baryon density for three different equations of state of strongly interacting matter: a Hagedorn resonance gas (EOS H), an ideal gas of massless partons (EOS I) and a connection of the two via a first-order phase transition at $T_c$=164 MeV (EOS Q). These EOS are used in hydrodynamics calculations in Ref. kolbheinz, from which the figure is taken.