Dam T. Son

Misha Stephanov

Matthew Strassler

Derek Teaney

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Laura Lee
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Workshop: String Methods
for the Real World

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From Strings to Things: String Theory Methods in QCD and Hadron Physics

March 24 - June 6, 2008

Inca Quipu
Inca Quipu, Larco Museum

The interest in the applications of AdS/CFT techniques to QCD has exploded recently. One of the goals of this program is to bring coherence into this rapidly developing new field by comparing different approaches and ideas while grounding them firmly into the fundamental formalism.

A wide range of topics are currently under development, and several more seem likely to come within reach. We briefly mention a few of these topics below.

Hadron Physics Modeling and AdS/QCD

An important development in the AdS/CFT correspondence is the introduction of a simple method for adding quarks in fundamental representation to the original AdS/CFT setting. This technical advance has made it possible to construct analytically tractable models whose hadrons are very similar to that of QCD. A complementary approach has also emerged, which attempts to construct phenomenologically viable five-dimensional models of hadrons in QCD in a "bottom-up" fashion. During the program we plan to discuss these approaches.

Finite Temperature and Finite Density

Thermodynamic and kinetic observables of strongly coupled gauge plasmas which have gravity duals can be computed using the AdS/CFT correspondence. In these theories the viscosity to entropy ratio is universal and small. The value of this ratio has attracted considerable attention since the collective flow observed at RHIC implies a similarly small transport time scale in thermal QCD. Further, the AdS/CFT correspondence has been used to compute the energy loss of a heavy quark moving in a hot gauge plasma and the "jet-quenching parameter." In addition, many questions remain. For example, can gravity duals describe the dynamics of this thermalization? Should one expect AdS/CFT techniques to be less relevant or more relevant at LHC energies?

High Energy Scattering

One of the historical connections between string theory and QCD is in the large s, fixed t regime of high-energy scatterings, where data can be fit with a soft Pomeron Regge trajectory. Recently there has been a lot of interest in high-energy scattering, black holes as fireballs, and the eikonal approximation. Within the context of single Pomeron exchange, deeper formal connections between the strong-coupling results of string theory and the work of experts on the perturbative hard Pomeron are still being uncovered. Beyond this, there are nonlinear effects involving multi-Pomeron exchange which remain to be understood. The work on Pomerons is relevant for various issues in heavy ion physics.

Overcoming the Limitations of Gauge/Gravity Duality

The original example of gauge/gravity duality N=4 super-Yang-Mills theory, which is a conformal, and not confining, field theory. Several years ago it was shown how to deform this theory to make it more similar to QCD; theories with confinement, chiral symmetry breaking, and other key features of QCD have been found that have string theoretic dual descriptions. However, in order for the dual description to be tractable, the 't Hooft coupling of the gauge theory has to be large all the way to the ultraviolet, in contrast with the asymptotic freedom of QCD. To make closer contact with QCD, more details of string theory on highly curved spaces must be understood. This is a very difficult string theory problem, but slow progress is currently being made in a number of contexts.

General AdS/CFT Formalism Questions

There have been many discoveries in the formal aspects of gauge/gravity and gauge/string duality that could be of potential use for QCD, and many theoretical developments await active study. Interesting topics would include the role of approximate conformal invariance, the appearance of hidden integrability (perhaps in new as well as known contexts), and special features of the large-Nc and large 't Hooft coupling expansions. Other questions of great importance involve the universality, or lack theorof, of results obtained in the AdS/CFT correspondence. The gauge/string correspondence can help us clarify which aspects of QCD are unique to QCD, and which ones are true of larger classes of theories. It therefore has the potential to greatly improve out conceptual understanding of QCD.