"RHIC serves the perfect fluid" -- Hydrodynamic flow of the QGP

TL;DR

RHIC results indicate the bulk QCD matter formed in heavy-ion collisions behaves as an almost ideal fluid, with rapid thermalization and collective flow governed by the QCD equation of state obtained from lattice QCD. Hydrodynamic modeling captures radial and elliptic flow and the evolution of spatial eccentricity, but the late hadronic stage introduces viscous effects that require hybrid hydro+cascade approaches to reconcile all observables and to constrain transport properties. The observed near-minimal viscosity suggests a strongly coupled quark-gluon plasma close to the lower bound ${\eta}/{s} \gtrsim \hbar/(4\pi)$, while hadronic dissipation and high-$p_T$ dynamics provide crucial tests of the transport coefficients. The work advocates 3+1D viscous hydrodynamics and expansive data analysis to extract the EOS, thermalization time, and transport properties, deepening our understanding of the QCD phase diagram and the nature of the QGP.

Abstract

The bulk of the hot and dense matter created at RHIC behaves like an almost ideal fluid. I present the evidence for this and also discuss what we can learn about the transport properties of the quark-gluon plasma (QGP) from the gradual breakdown of ideal fluid dynamic behavior at large transverse momenta, lower beam energies, larger impact parameters, and forward rapidities.

Figures (7)

• Figure 1: The normalized energy density $e/T^4$ (left) and pressure $p/T^4$ (right) from lattice QCD KL04 for 0, 2 and 3 light quark flavors, as well as for 2 light + 1 heavier (strange) quark flavors. Horizontal arrows on the right indicate the corresponding Stefan-Boltzmann values for a non-interacting quark-gluon gas.
• Figure 2: The square of the speed of sound $c_s^2$ from lattice QCD above $T_{\rm cr}$KL04 (left) and from models above and below $T_{\rm cr}$Choj04 (right).
• Figure 3: Negative pion, kaon, antiproton, and $\Omega$ spectra from central Au+Au collisions at $\sqrt{s}{\,=\,}200\,A$ GeV measured at RHIC spec200. The curves show hydrodynamical calculations KR03 (see text).
• Figure 4: Left: $p_\perp$-averaged elliptic flow for all charged hadrons from 130 $A$ GeV Au+Au collisions, as a function of collision centrality ($n_{\rm ch}$ is the charged multiplicity at $y{\,=\,}0$). The curves are hydrodynamic calculations with different choices for the initial energy density profile (see KHHET). Right: Differential elliptic flow $v_2(p_\perp)$ for identified hadrons from minimum bias Au+Au collisions at 200 $A$ GeV Ackermann:2001trPHENIXv2Sorensen:2003kp, together with hydrodynamic curves from Huovinen:2001cy.
• Figure 5: Left: Elliptic flow from a parton cascade Molnar:2001ux, compared with STAR data, for different parton-parton scattering cross sections. Larger cross sections lead to smaller mean free paths. Right: Perturbative effects of shear viscosity on the elliptic flow $v_2(p_\perp)$T03 (see text).