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What RHIC Experiments and Theory tell us about Properties of Quark-Gluon Plasma ?

E. V. Shuryak

TL;DR

The paper argues that RHIC data reveal a strongly coupled quark–gluon plasma (sQGP) near the QCD transition, whose bulk behavior is well described by ideal hydrodynamics using a lattice QCD EoS, while transport properties indicate a surprisingly low viscosity with $\eta/s \sim 1/10$. It contrasts weak-coupling expectations with observations such as strong elliptic and radial flow and substantial jet quenching, and draws connections to strongly coupled systems via AdS/CFT and unitary Fermi gases. A key proposal is that hundreds of bound states, many colored, exist in sQGP and contribute to the EoS and jet quenching, offering a cohesive picture of QGP dynamics. The outlook highlights remaining questions on heavy flavor, bound-state signatures, and LHC phenomenology, suggesting bound-state dynamics play a central role in QGP properties across temperatures $T_c < T < 4T_c$.

Abstract

This brief review summarizes the main experimental discoveries made at RHIC and then discusses their implications. The robust collective flow phenomena are well described by ideal hydrodynamics, with the Equation of State (EoS) predicted by lattice simulations. However the transport properties turned out to be unexpected, with rescattering cross section one-to-two orders of magnitude larger than expected from perturbative QCD. These and other theoretical developments indicate that Quark-Gluon Plasma (QGP) produced at RHIC, and probably in a wider temperature region $T_c<T<4T_c$, is not at all a weakly coupled quasiparticle gas, but is rather in a strongly coupled regime, sQGP for short. After reviewing two other ``strongly coupled systems'', (i) the strongly coupled supersymmetric theories studied via Maldacena duality; (ii) trapped ultra-cold atoms with very large scattering length, we return to sQGP and show that there should exist literally hundreds of bound states in it in the RHIC domain, most them colored. We then discuss recent ideas of their effect on the EoS, viscosity and jet quenching.

What RHIC Experiments and Theory tell us about Properties of Quark-Gluon Plasma ?

TL;DR

The paper argues that RHIC data reveal a strongly coupled quark–gluon plasma (sQGP) near the QCD transition, whose bulk behavior is well described by ideal hydrodynamics using a lattice QCD EoS, while transport properties indicate a surprisingly low viscosity with . It contrasts weak-coupling expectations with observations such as strong elliptic and radial flow and substantial jet quenching, and draws connections to strongly coupled systems via AdS/CFT and unitary Fermi gases. A key proposal is that hundreds of bound states, many colored, exist in sQGP and contribute to the EoS and jet quenching, offering a cohesive picture of QGP dynamics. The outlook highlights remaining questions on heavy flavor, bound-state signatures, and LHC phenomenology, suggesting bound-state dynamics play a central role in QGP properties across temperatures .

Abstract

This brief review summarizes the main experimental discoveries made at RHIC and then discusses their implications. The robust collective flow phenomena are well described by ideal hydrodynamics, with the Equation of State (EoS) predicted by lattice simulations. However the transport properties turned out to be unexpected, with rescattering cross section one-to-two orders of magnitude larger than expected from perturbative QCD. These and other theoretical developments indicate that Quark-Gluon Plasma (QGP) produced at RHIC, and probably in a wider temperature region , is not at all a weakly coupled quasiparticle gas, but is rather in a strongly coupled regime, sQGP for short. After reviewing two other ``strongly coupled systems'', (i) the strongly coupled supersymmetric theories studied via Maldacena duality; (ii) trapped ultra-cold atoms with very large scattering length, we return to sQGP and show that there should exist literally hundreds of bound states in it in the RHIC domain, most them colored. We then discuss recent ideas of their effect on the EoS, viscosity and jet quenching.

Paper Structure

This paper contains 11 sections, 8 equations, 9 figures, 1 table.

Table of Contents

  1. Two sets of major discoveries made at RHIC
  2. Evolving theoretical views on QGP properties
  3. Collective flows, EoS and transport properties
  4. Transverse flow
  5. Elliptic Flow
  6. The limits to ideal hydro
  7. Other strongly couples systems
  8. Finite $T$ ${\tt N}\,\,$=4 supersymmetric gauge field theory at strong coupling
  9. Strongly coupled trapped atoms
  10. New picture of QGP, with multiple bound states
  11. Brief summary and outlook

Figures (9)

  • Figure 1: (a) The pressure (divided by that for free gas) versus the temperature $T/T_c$, from lattice thermodynamics studied by Bielefeld group. (b) Schematic plot of the cut-off scales during the evolution of the system with time, from Shu_spha. At the collision time=0 the scale is presumably the saturation scale $Q_s$ in the incoming nuclei, which grows with the collision energy. Then the cutoff decreases reaching some nearly constant value in QGP at $T>T_c$, the thermal gluon mass $M_T$ (\ref{['eqn_thermal']}) and stay at this value till it rises again in the mixed phase to its vacuum value in the hadronic (H) phase $Q_{vac}\sim 1 \, GeV$.
  • Figure 2: (a) The "blast model" fits to STAR collaboration data. The values of the and freezeout temperatures are shown in (a) and the mean collective velocity. (b) Comparison between STAR and PHENIX data for protons with hydro calculation by Kolb and Rapp Kolb:2002ve (which correctly incorporates chemical freezeout).
  • Figure 3: The $p_t$-differential elliptic flow $v_2(p_t)$ from minimum bias Au+Au collisions at RHIC, for different identified hadron species (PHENIX). with negative (left) and positive (right) charge.The curves are hydrodynamic calculations.
  • Figure 4: (a) The compilation of elliptic flow (the ratio of $v_2/\epsilon$) dependence on collision energy (represented by the particle multiplicity). (b,c) Energy dependence of the elliptic flow predicted by hydro calculation by Teaney et al hydro for different EoS. The curve with the latent heat (LH) =800 $MeV/fm^3$ is the closest to the lattice EoS, and it is also the best fit to all flow data at SPS and RHIC
  • Figure 5: Elliptic flow $v_2$ as a function of $p_T$ for different values of $\Gamma_s/\tau_{o}$. The data points are four particle cumulants data from the STAR collaboration. Only statistical errors are shown.
  • ...and 4 more figures