INSTITUTE FOR NUCLEAR THEORY News
Home  Contact  Search  News archive  Site Map 

From Strings to Things
(INT program March 24  June 6, 2008)
Reported by Dam T. Son, Misha Stephanov, Matthew Strassler, Derek Teaney
Date posted July 28, 2008
The quest for understanding strong interactions has a long history. The challenge to construct a fundamental theory of the interactions which bind together protons and neutrons in atomic nuclei, and which are responsible for hundreds of hadronic resonances, led to the advent and development of theoretical methods which later found applications in many fields of physics. The list includes powerful tools such as the renormalization group, lattice field theory, and effective field theories. One line of such theoretical developments brought about in the course of this quest lead to the Regge phenomenology of hadrons and the Veneziano amplitude, culminating in the discovery of string theory. It is a wellknown fact that string theory emerged as a candidate for the theory of strong interactions before the advent of QCD. The discovery of asymptotic freedom, followed by many impressive phenomenological successes, firmly established QCD as the fundamental theory of strong interactions. Soon after, string theory metamorphosed into a theory of quantum gravity. Nevertheless, the connection between string theory and strong interactions has never been completely lost—the existence of QCD flux tubes and the planar diagram dominance in largeNc QCD hinted that string theory might still be a needed mathematical tool for understanding confinement. The strong coupling regime of QCD has always remained a challenge to analytical firstprinciple QCD approaches, and it has long been clear that qualitatively new methods and ideas are necessary to treat such nonperturbative phenomena as confinement and chiral symmetry breaking, and to understand the hadron spectra and interactions. The modern period of exploration of the QCD/string connection started with the formulation of the conjecture of gauge/string duality, or AdS/CFT correspondence, in 1997–98 by Maldacena and others. A weaker form of the conjecture is the gauge/gravity correspondence, which makes use of only some features of the string theory. This correspondence allows one to solve many problems in a class of strongly coupled gauge theories, including some QCDlike theories. In principle, gauge/gravity duality should apply to asymptotically free theories like QCD. In practice, however, no rigorous calculational methods for such theories are known. Despite this drawback, the methods of gauge/gravity duality have recently become very popular among QCD practitioners. The power of these methods is that they are ideally suited for the strongly interacting regime. Moreover, the string methods could be applied where the traditional “brick and mortar” numerical lattice approach is not effective, such as, e.g., nonequilibrium and realtime dynamics. The purpose of the INT program “From Strings to Things” was to further the development of these novel methods and bring them to bear on most pressing problems in QCD and strong interactions. This is the second time a meeting dedicated to string theory methods has taken place at the INT: a fourday workshop “QCD and String Theory” was held at the INT in February 2002. During the program “From Strings to Things” , one of the most actively discussed applications of string theory methods was to describe the realtime dynamics of strongly coupled quantum field theories at high temperature and density. The major discovery of RHIC experiments is that the matter created in heavy ion collisions at RHIC is not a weakly interacting plasma of quarks and gluons (QGP) but rather a strongly interacting medium (sQGP), which defies traditional kinetic description in terms of welldefined quasiparticles. How does this medium expand and cool? What happens when a very energetic quark or gluon, occasionally created in the initial stages of the collision, traverses this medium? How and on what time scales does thermalization occur? Can quarks coalesce into hadrons inside such a medium and under what conditions? These and many other important and intriguing questions need answers from QCD, and yet in the strong coupling regime the necessary mathematical apparatus is lacking. AdS/CFT methods allow one to study these questions and obtain reliable answers in theories which share many of the QCD properties, and which are strongly interacting.
Fig 1:The wake of a quark moving with supersonic velocity in the N=4 superYangMills plasma. It is hoped that the properties of this plasma is not too different from the quarkgluon plasma created at RHIC (source: Paul Chesler's talk at the program).
Another active avenue of research is the development of models which go beyond the original AdS/CFT correspondence by incorporating such features of QCD as confinement and chiral symmetry breaking—AdS/QCD models or holographic QCD. Two complementary approaches have emerged. The topdown approach uses the original AdS/CFT setup as a starting point, removes supersymmetry by modifying the background geometry and builds in quarks by introducing additional extended objects (branes). A complementary, bottomup approach starts from QCD and attempts to build the model which matches relevant QCD correlation functions by using the rules of the holographic correspondence in reverse. One of the distinctive automatic features of these models is the natural matching between sums over resonances and the partonmodel behavior of correlation functions. Another important feature is a natural explanation of vector meson dominance. Such models, even in their barest form, enjoy considerable phenomenological success. The most recent developments have been reported and discussed during the INT program such as phenomenology of the baryons (spectra, form factors), the finite density and temperature properties (phase diagrams), the response to external electric and magnetic fields.
Fig. 2: A comparision of the experimental data on the pion form factor with the prediction of various AdS/QCD models. AdS/QCD models work surprisingly well for many lowenergy quantities (source: Anatoly Radyushkin's talk at the program).
The application of string theory methods to strong interactions is a fast developing subject. The INT program summarized the recent developments, outlined short and longterm challenges and provided a fruitful ground for exchange of ideas. Further reading:
