Why does the Quark-Gluon Plasma at RHIC behave as a nearly ideal fluid ?

TL;DR

RHIC reveals a Quark-Gluon Plasma that behaves as a nearly perfect fluid, challenging weakly coupled expectations. The paper synthesizes experimental evidence for strong collective flow with hydrodynamic modeling, and argues that near Tc the plasma hosts loosely bound states and exhibits large scattering, yielding a small η/s close to the conjectured bound. It connects QCD phenomena to strong-coupling theories via AdS/CFT, showing η/s = 1/(4π) in N=4 SYM and a modified Coulomb regime, and extends the discussion to cold-atom systems that display analogous hydrodynamic behavior. Together, these perspectives support a unified view of strongly coupled fluids across high-energy and condensed-mense physics, with significant implications for transport properties and the interpretation of heavy-ion collisions.

Abstract

The lecture is a brief review of the following topics: (i) collective flow phenomena in heavy ion collisions. The data from RHIC indicate robust collective flows, well described by hydrodynamics with expected Equation of State. The transport properties turned out to be unexpected, with very small viscosity; (ii) physics of highly excited matter produced in heavy ion collisions at T_c<T<4T_c is different from weakly coupled quark-gluon plasma because of relatively strong coupling generating bound states of quasiparticles; (iii) wider discussion of other ``strongly coupled systems'' including strongly coupled supersymmetric theories studied via Maldacena duality, as well as recent progress in trapped atoms with very large scattering length.

Figures (14)

• Figure 1: (a) Compilation of slopes of the $m_t$ spectra from pp collisions (ISR) (open circles), SS and PbPb collisions at SPS. (b) Comparison between STAR and PHENIX data for protons with hydro calculation by Kolb and Rapp Kolb:2002ve (which correctly incorporates chemical freezeout).
• Figure 2: Pion, kaon and nucleon spectra from STAR collaboration (upper panels), together with the "blast model" fits. The values of the and freezeout temperatures are shown in (a) and the mean collective velocity in (b) part of the lower panel.
• Figure 3: The dependence of the pressure-to-energy-density $p/\epsilon$ ratio on the energy density along the adiabatic paths with different baryon-to-entropy ratio. RHIC corresponds to the curve with the lowest baryon density, for which the contrast is the largest.
• Figure 4: Time evolution of the spatial ellipticity $\epsilon_x$, the momentum anisotropy $\epsilon_p$, and the radial flow $< v_\perp >$. The labels a, b, c and d denote systems with initial energy densities of 9, 25, 175 and 25000 GeV/fm$^3$, respectively, expanding under the influence of EOS Q. Curves e show the limiting behavior for EOS I as $e_0\to\infty$ (see text). In the second panel the two vertical lines below each of the curves a-d limit the time interval during which the fireball center is in the mixed phase. In the first panel the dots (crosses) indicate the time at which the center of the reaction zone passes from the QGP to the mixed phase (from the mixed to the HG phase). For curves a and b the stars indicate the freeze-out point.; for curves c-e freeze-out happens outside the diagram.
• Figure 5: (a) The compilation of elliptic flow (the ratio of $v_2/s_2$) dependence on collision energy (represented by the particle multiplicity). (b,c) Elliptic flow predicted by hydro calculation by Teaney et al 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