What RHIC experiments and theory tell us about properties of quark–gluon plasma?

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 ultracold 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.

Drag force in a charged Script N = 4 SYM plasma

“…They found that the ratio of shear viscosity to entropy density equals 1/4π, a result that was later argued to be universal, in the sense that it applies to any gauge theory described by a supergravity dual, in the limit of large 't Hooft coupling [8]. 1 These results raised the tantalizing possibility of using AdS/CFT to study the QGP. Transport coefficients of different thermal gauge theories have been calculated in [7]- [22].…”

Shear and bulk viscosities of the gluon plasma in a quasiparticle description

“…Although the results of these calculations depend crucially on the employed freeze-out prescription as well as the assumed initial conditions [11][12][13][14][15][16][17][18], the success of ideal hydrodynamics suggested, at most, small dissipative effects in the medium produced at RHIC [19][20][21]. This observation led to the interpretation that the produced quark-gluon plasma (QGP) behaves as a strongly coupled fluid rather than a weakly interacting gas [22][23][24]. Detailed calculations within the framework of relativistic dissipative fluid dynamics were performed [13,[25][26][27][28][29][30][31][32][33][34][35][36][37][38], confirming indeed the smallness of dissipative effects.…”

From static to evolving geometries—R-charged hydrodynamics from supergravity

“…Since it appears that the gauge theory coupling in the plasma is large [1], it is interesting to develop nonperturbative methods for studying its properties.…”