Thermalization at RHIC

TL;DR

The paper argues that RHIC collisions create thermalized, high-density matter whose bulk behavior is best described by ideal relativistic hydrodynamics, implying rapid thermalization and a strongly coupled quark-gluon plasma (sQGP). It links early pressure buildup to observable collective flow, notably elliptic flow v_2, and shows that radial flow and hadron spectra are reproduced only with swift equilibration and a stiff equation of state featuring deconfinement. Evidence for deconfinement is reinforced by quark coalescence, which yields a universal v_2 per valence quark when scaled by p_T/n, pointing to uncorrelated deconfined quarks prior to hadronization, and by successful grand-canonical descriptions of hadron yields. Collectively, these results position the QGP at RHIC as a strongly interacting, nearly perfect fluid with transport properties far from a weakly interacting gas, challenging perturbative expectations and highlighting non-perturbative dynamics in the early collision phase.

Abstract

Ideal hydroynamics provides an excellent description of all aspects of the single-particle spectra of all hadrons with transverse momenta below about 1.5-2 GeV/c at RHIC. This is shown to require rapid local thermalization at a time scale below 1 fm/c and at energy densities which exceed the critical value for color deconfinement by an order of magnitude. The only known thermalized state at such energy densities is the quark-gluon plasma (QGP). The rapid thermalization indicates that the QGP is a strongly interacting liquid rather than the weakly interacting gas of quarks and gluons that was previously expected.

Figures (4)

• Figure 1: (color online) Negative pion, kaon, antiproton, and $\Omega$ spectra from central Au+Au collisions at $\sqrt{s}{\,=\,}200\,A$ GeV, as measured by the four RHIC experiments PHENIX03spec200STAR03spec200PHOBOS03spec200BRAHMS03spec200STAR03omega. The curves show hydrodynamical calculations as described in the text.
• Figure 2: (color online) Elliptic flow coefficient $v_2$ for all charged particles from 130 $A$ GeV Au+Au collisions (left panel KHHET) and for several identified hadron species from 200 $A$ GeV Au+Au collisions (right panel Sorensen:2003kp), compared with hydrodynamic predictions. The left panel shows the $p_\perp$-averaged elliptic flow as a function of collision centrality, parametrized by the charged multiplicity density $n_{\rm ch}$ at midrapidity ($n_{\rm max}$ corresponds to the largest value in central collisions). The right panel shows the differential elliptic flow $v_2(p_\perp)$ for minimum bias collisions. The data were collected by STAR and PHENIX Ackermann:2001trPHENIXv2Adams:2003am. The curves in the left panel are hydrodynamic calculations corresponding to different choices for the initial energy density profile (see KHHET for details). The curves in the right panel were first published in Huovinen:2001cy.
• Figure 3: (color online) Scaled elliptic flow $v_2/\epsilon_x$ as a function of $(1/S)\, dN_{\rm ch}/dy$ (i.e. the charged multiplicity density per unit transverse overlap area $S$), for Pb+Pb and Au+Au collisions of different centralities at various collision energies, as compiled by the NA49 Collaboration NA49v2PRC. The lines indicate the hydrodynamically predicted values at RHIC, SPS and AGS energies KSH00 (see text for details).
• Figure 4: (color online) Elliptic flow per valence quark, $v_2/n$, as a function of transverse momentum per valence quark, $p_\perp/n$, for pions, kaons, protons, $\Lambda$ and $\Xi$ hyperons and their antiparticles. Data are from 200 $A$ GeV Au+Au collisions measured by PHENIX PHENIXv2 (left panel) and STAR Castillo:2004jy (right panel). Universality of this curve suggests that it represents the partonic elliptic flow $v_2^{\rm parton}(p_\perp^{\rm parton})$, with no apparent difference between light and strange quark elliptic flow.