Gauge/String Duality, Hot QCD and Heavy Ion Collisions

TL;DR

This review synthesizes how gauge/string duality, especially AdS/CFT, informs the study of hot QCD matter and heavy ion phenomenology. It connects nonperturbative gravity-based calculations to lattice QCD data, detailing thermodynamic and transport properties of strongly coupled plasmas, and explores how energetic probes such as jets and quarkonia behave in these media. A key highlight is the universal η/s = 1/(4π) result in large-Nc, strongly coupled theories, and the absence of well-defined quasiparticles at strong coupling, with extensive implications for heavy ion observables. The work also surveys methods to incorporate finite temperature, chemical potential, and fundamental matter within holography, outlining both achievements and significant open questions for bridging holographic models with real-world QCD. Overall, the review sketches a coherent holographic picture of a strongly coupled quark–gluon plasma, offering qualitative and, in favorable regimes, quantitative guidance for interpreting heavy ion collision data and guiding future research directions.

Abstract

Over the last decade, both experimental and theoretical advances have brought the need for strong coupling techniques in the analysis of deconfined QCD matter and heavy ion collisions to the forefront. As a consequence, a fruitful interplay has developed between analyses of strongly-coupled non-abelian plasmas via the gauge/string duality (also referred to as the AdS/CFT correspondence) and the phenomenology of heavy ion collisions. We review some of the main insights gained from this interplay to date. To establish a common language, we start with an introduction to heavy ion phenomenology and finite-temperature QCD, and a corresponding introduction to important concepts and techniques in the gauge/string duality. These introductory sections are written for nonspecialists, with the goal of bringing readers ranging from beginning graduate students to experienced practitioners of either QCD or gauge/string duality to the point that they understand enough about both fields that they can then appreciate their interplay in all appropriate contexts. We then review the current state-of-the art in the application of the duality to the description of the dynamics of strongly coupled plasmas, with emphases that include: its thermodynamic, hydrodynamic and transport properties; the way it both modifies the dynamics of, and is perturbed by, high-energy or heavy quarks passing through it; and the physics of quarkonium mesons within it. We seek throughout to stress the lessons that can be extracted from these computations for heavy ion physics as well as to discuss future directions and open problems for the field.

Figures (71)

• Figure 1: Left: Charged particle multiplicity distributions for collisions at RHIC with four different energies as a function of pseudorapidity Back:2005hs. Very recently, a charged particle multiplicity $dN_{\rm ch}/d\eta = 1584\pm 4 ({\rm stat}) \pm 76 ({\rm sys})$ has been measured at $\eta=0$ in heavy ion collisions with $\sqrt{s}=2.76$ TeV by the ALICE detector at the LHC Aamodt:2010pb. Right: Charged particle spectrum as function of $p_T$ in gold-gold collisions at top RHIC energy at different values of the impact parameter, with the top (second-from-bottom) curve corresponding to nearly head-on (grazing) collisions Adams:2003kv. The bottom curve is data from proton-proton collisions at the same energy.
• Figure 2: Left: Spectra for identified pions, kaons and protons as a function of $p_T$ in head-on gold-gold collisions at top RHIC energy Adler:2003cb. Right: Spectra for identified pions, kaons and protons as a function of $p_T$ in (non single diffractive) proton-proton collisions at the same energy $\sqrt{s}=200~{\rm GeV}$Adams:2003qm.
• Figure 3: So-called thermal fit to different particle species. The relative abundance of different hadron species is well-described by a two-parameter grand canonical ensemble in terms of temperature $T$ and baryon chemical potential $\mu_b$Andronic:2005yp.
• Figure 4: Left: Chemical potential extracted from thermal fits at different center of mass energies Andronic:2005yp. Right: The number of protons minus number of antiprotons per unit rapidity for central heavy ion collisions Bearden:2003hx. This net proton number decreases with increasing center of mass energy from $\sqrt{s}=5~{\rm GeV}$ (at the AGS collider at BNL), via $\sqrt{s}=17~{\rm GeV}$ (at the SPS collider at CERN) to $\sqrt{s}=200~{\rm GeV}$ (at RHIC). (For each collision energy, $y_p$ indicates the rapidity of a hypothetical proton that has the same velocity after the collision as it did before.)
• Figure 5: Sketch of the collision of two nuclei, shown in the transverse plane perpendicular to the beam. The collision region is limited to the interaction almond in the center of the transverse plane. Spectator nucleons located in the white regions of the nuclei do not participate in the collision. Figure taken from Ref. Ollitrault:2006va.