Integrability in Gauge/String Theories

The generalized unitarity method reconstructs loop-level scattering amplitudes from tree-level scattering amplitudes. We review some of the steps of this method which are necessary for applications in any quantum field theory, discuss several points of view and emphasize some of the aspects that can be easily automated. We mainly use N=4 super-Yang-Mills theory to illustrate the discussion and comment on the necessary changes for applications to super-Yang-Mills theories with reduced supersymmetry.

In this talk I will report on the computation at strong coupling of the AdS contribution to the 3-point correlation function of operators corresponding to classical strings which rotate in the S5. This contribution is universal for all operators which have only SO(6) charges.

We compute three-point functions of single trace operators in planar N = 4 SYM. We consider the limit where one of the operators is much smaller than the other two. We find a precise match between weak and strong coupling in the Frolov-Tseytlin classical limit for a very general class of classical solutions. To achieve this match we clarify the issue of back-reaction and identify precisely which three-point functions are captured by a classical computation.

For N=4 super Yang-Mills theory, in the large-N limit and at strong coupling, Wilson loops can be computed using the AdS/CFT correspondence. In the case of flat Euclidean loops the dual computation consists in finding minimal area surfaces in Euclidean AdS3 space. In such case very few solutions were known. In this talk I will describe an infinite parameter family of minimal area surfaces that can be described analytically using Riemann Theta functions. Furthermore, for each Wilson loop a one parameter family of deformations that preserve the area can be exhibited explicitly. The area is given by a one dimensional integral over the world-sheet boundary.

In this talk we give a survey of recent developments concerning the fermionic structure in the sine-Gordon model. For the lattice counterpart (6 vertex model), we introduce fermions acting on the space of (quasi) local operators. The main theorem is a determinant formula for the expectation values of fermionic descendants of primary fields. In the continuum limit this construction gives rise to a basis of the space of all descendant fields, whose expectation values take a very simple form. Unexpectedly, it turns out that the action of our fermions on form factors coincides with yet another fermions which have been introduced some time ago by Babelon, Bernard and Smirnov.

In the so-called unitary limit of quantum gases, the scattering length diverges and the theory becomes scale invariant with dynamical exponent z=2. This point occurs precisely at the crossover between strongly coupled BEC and BCS. These systems are currently under intense experimental study using cold atoms and Feshbach resonances to tune the scattering length. We developed a new approach to the statistical mechanics of gases in higher dimensions modeled after the thermodynamic Bethe ansatz, i.e. based on the exact 2-body S-matrix. Calculations of the critical temperature Tc/T_F = 0.1 are in good agreement with experiments and Monte-Carlo studies. We also calculated the ratio of viscosity to entropy density and obtained 4.7 times the conjectured lower bound of 1/4 pi, in good agreement with very recent experiments. We also present evidence for a strongly interacting version of BEC.

The high-energy behavior of gauge theory amplitudes can be studied using the operator expansion in Wilson lines. I review the next-to-leading order calculations of the high-energy amplitudes in N=4 SYM and QCD.