The QGP Discovered at RHIC

TL;DR

The paper argues that RHIC data reveal a strongly coupled Quark Gluon Plasma (sQGP) characterized by near-perfect fluid behavior at low $p_T$ and strong parton energy loss at high $p_T$. It combines three independent evidence streams—bulk collective flow ($v_2$, $v_1$), jet quenching ($R_{AA}$, $I_{AA}$ with GLV energy loss), and a D+Au control—to define the QGP as a synthesis of bulk QCD thermodynamics, short-wavelength perturbative dynamics, and controlled initial-state effects. The findings suggest a medium with low viscosity close to the fundamental bound, substantial gluon density, and a dynamical evolution governed by strong coupling, prompting future studies via short-wavelength correlators and heavy-flavor tomography. This work emphasizes that the sQGP is a new state of QCD matter with practical implications for understanding strong interactions under extreme conditions.

Abstract

Three empirical lines of evidence from RHIC have converged and point to the discovery of a strongly coupled Quark Gluon Plasma. The evidence includes (1) bulk collective elliptic flow and (2) jet quenching and mono-jet production, observed in Au+Au collisions at 200 AGeV, and (3) a critical control experiment using D+Au at 200 AGeV.

Figures (9)

• Figure 1: A recent Lattice QCD calculation Fodor:2004nz of the pressure, $P(T)/T^4$, and a measure of the deviation from the ideal Stefan-Boltzmann limit $(\epsilon(T)-3 P(T))/T^4$. Note that the scale on both graphs has not been corrected for finite lattice volume effects: see Allton:2003vx for discussion.
• Figure 2: Important features of the QGP equation of state. The speed of sound Gupta:2003be$c_s^2=d\epsilon/dP$ drops below $1/3$ for $T<2T_c \approx300$ MeV. Right panel shows a current estimate of the location of the tricritical point at finite baryon density Fodor:2004nz
• Figure 3: First line of evidence: Bulk collective flow is the barometric signature of QGP production. Left figure combines STAR Adams:2003zg-Adler:2002pu and PHENIX Adler:2003ktAdler:2003qi measurements of the azimuthal elliptic flow ($v_2(p_T)$) of $\pi,K,p,\Lambda$ in Au+Au at 200 AGeV. The predicted hydrodynamic flow pattern from Kolb:2000fh-Kolb:2003dz agrees well with observations in the bulk $p_T<1$ GeV domain. Right figure from Alt:2003ab shows $v_2$ scaled to the initial elliptic spatial anisotropy, $\epsilon$, as a function of the charge particle density per unit transverse area. The bulk hydrodynamic limit is only attained at RHIC.
• Figure 4: Left figure combines STAR and NA49 data Adams:2003zg and shows that the directed sidewards flow, $v_1(y)$, is correlated over 8 units of rapidity at RHIC. At SPS collectivity is dominated by the overlapping fragmentation regions while at RHIC the nearly identical directed flow of in the fragmentation regions is shifted to $|y|>2$. Right figure shows the pseudo rapidity dependence of elliptic from PHOBOS Back:2002ft.
• Figure 5: Evidence for dissipative collective flow below RHIC energies. Left: Non equilibrium BUU nuclear transport theory Danielewicz:2002puStoicea:2004kp can explain the observed elliptic squeeze-out (negative $v_2$) collectivity below 4 AGeV. Right: CERESAgakichiev:2003gg data on elliptic flow at SPS is well below hydrodynamic predictions with freeze-out $T_f=120$ MeV required to reproduce the single inclusive radial flow. Early freeze-out with $T_f=160$ MeV, simulating effects of dissipation, is needed to reproduce the data.