The string/gauge theory correspondence in QCD

TL;DR

The paper surveys how gauge/gravity duality, epitomized by the AdS/CFT correspondence, furnishes a concrete string-based description of strongly coupled gauge theories, including QCD-like dynamics. It explains how confinement and chiral symmetry breaking arise in holographic backgrounds and how flavor degrees of freedom are incorporated via branes, yielding meson spectra that capture qualitative universality across models. Finite-temperature holography reveals phase transitions between confining and deconfined regimes, with meson melting and chiral restoration in temperature-dependent backgrounds. In the quark–gluon fluid, holography provides tractable calculations of drag forces and screening lengths, linking horizon physics to transport and dissipation in the plasma. Collectively, these developments offer a powerful geometrical framework for exploring nonperturbative QCD phenomena and RHIC-relevant physics, guiding both qualitative understanding and approximate quantitative modeling.

Abstract

Ideas about a duality between gauge fields and strings have been around for many decades. During the last ten years, these ideas have taken a much more concrete mathematical form. String descriptions of the strongly coupled dynamics of semi-realistic gauge theories, exhibiting confinement and chiral symmetry breaking, are now available. These provide remarkably simple ways to compute properties of the strongly coupled quark-gluon fluid phase, and also shed new light on various phenomenological models of hadron fragmentation. We present a review and highlight some exciting recent developments.

Figures (19)

• Figure 1: Feynman rules for SU($N$) gauge theory in the double-line notation. The powers of $g_{\text{YM}}$ come from the propagator and vertices, while every closed loop of a black edge yields a power $N_c$.
• Figure 2: Counting powers of $g_{\text{YM}}$ and $N_c$. By combining the coupling constant and the number of colours into the so-called 't Hooft coupling $\lambda = g_{\text{YM}}^2 N_c$, one sees that $N_c^{-2}$ counts the degree of non-planarity (left). Planar diagrams can be drawn planar on a sphere, while non-planar diagrams require higher-genus surfaces in order to be drawn as planar graphs (right).
• Figure 3: Regge trajectories of mesons relate the spin $J$ to the mass-squared $M^2$ (left) or the excitation level $n$ to the mass-squared (right) PDBook.
• Figure 4: Symbolic depiction of the basic idea of the AdS/CFT correspondence: the string theory dual to gauge theory is higher-dimensional. The string lives in a curved space-time, and there is a specific map which relates the physics of the string to the physics in our four-dimensional world. More recent extensions of this conjecture have produced string geometries dual to confining and thermal theories (right).
• Figure 5: The theory of open strings contains various heavy objects in its spectrum, which can be interpreted as boundary conditions on the endpoints of the light open strings.