Integrability in Gauge/String Theories

I will give a brief review on the subject of scattering amplitudes in N=4 super Yang-Mills focussing on their infinite dimensional symmetry structure at tree-level and the fate of these symmetries at loop-level in particular employing a Highs-Regulator for the IR divergencies.

We study the four-point correlation function of stress-tensor supermultiplets in N=4 SYM using the method of Lagrangian insertions. We argue that, as a corollary of N=4 superconformal symmetry, the resulting all-loop integrand possesses an unexpected complete symmetry under the exchange of the four external and all the internal (integration) points. This alone allows us to predict the integrand of the three-loop correlation function up to four undetermined constants. Further, exploiting the conjectured amplitude/correlation function duality, we are able to fully determine the three-loop integrand in the planar limit. We perform an independent check of this result by verifying that it is consistent with the operator product expansion, in particular that it correctly reproduces the three-loop anomalous dimension of the Konishi operator. As a byproduct of our study, we also obtain the three-point function of two half-BPS operators and one Konishi operator at three-loop level. We use the same technique to work out a compact form for the four-loop four-point integrand and discuss the generalisation to higher loops.

The S-matrix of N=4 super Yang-Mills in the planar limit enjoys a remarkable duality with non-BPS null polygon Wilson loops in the same theory, but with the role of momenta and position interchanged. I will attempt to explain how such a duality works, by stressing how familiar notions such as factorization limits, unitarity and the loop integrand translate into simple and verifiable statements about Wilson loops. This requires a suitable supersymmetric extension of Wilson loops, which I will describe. I will then discuss recent progress in using the full supersymmetry of the theory on the Wilson loop side to simplify scattering amplitude computations

I review how N=4 SYM can be reformulated as a theory on twistor space, and explain various calculations that have been performed there. In particular, twistors turn out to be a powerful tool for investigating the duality between scattering amplitudes and null polygonal Wilson Loops in the planar limit. The BCFW recursion relations are interpreted as the loop equations for a supersymmetric generalization of the Wilson Loop.

An object which has been under attack from several fronts is the planar S-matrix of N=4 SYM. One approach towards addressing the computation of scattering amplitudes using integrability is by using an analogues of an Operator Product Expansion for these observables. It is a very general expansion that is based on the dual conformal symmetry of the amplitudes or their dual description in terms of null polygon Wilson loops. In this expansion the Wilson loop/amplitude is viewed as a transition amplitude for flux tube excitations. The flux tube in question is the color flux stretched between two fast moving quarks and the excitation are the excitations of that color flux. In the planar limit, it has an holographic description in terms of a two dimensional world sheet, known as the GKP string. For N=4 SYM, the dynamics of the flux excitation is integrable to all loops.

The TBA approach to the AdS/CFT spectral problem is used to compute scaling dimensions of several operators dual to two-particle states of the l.c. AdS5 x S5 string theory. The implementation of the psu(2,2|4) symmetry in the TBA framework is discussed.

I will show how to solve the AdS/CFT Y-system in terms of a finite set of nonlinear integral equations (FiNLIE). To uniquely define the solution we impose the set of constraints on the Y- and T-functions which can be summarized as: symmetry (PSU(2,2|4) + Z_4) + analyticity + large volume asymptotics. Some of these constraints describe previously unknown properties of the Y-system. As an important check of our approach, we showed that the proposed constraints can be also used to derive the infinite set of the TBA equations. We also successfully checked FiNLIE numerically for the case of Konishi operator.

I will present a first-principles derivation of the AdS5/CFT4 T-system up to first non-trivial order in the large 't Hooft coupling expansion. The proof relies on the computation of quantum effects in the fusion of some special line operators, namely the transfer matrices. This computation is done in the pure spinor formalism for the superstring in AdS5xS5. I will also discuss the generalization of this computation to other integrable 2D CFTs that define string theory in AdS backgrounds.

The motion of superstrings on symmetric space target spaces is classically equivalent, via the Pohlmeyer reduction, to a family of 2-d relativistic integrable field theories known as semisymmetric space sine-Gordon (SSSSG) theories. In this talk I will review recent progress in constructing the relativistic S-matrix corresponding to the quantum solution of the AdS5 x S5 SSSSG theory.

We study the dimensions of non-chiral operators in the Veneziano limit of N=1 supersymmetric QCD in the conformal window. We show that when acting on gauge-invariant operators built out of scalars, the 1-loop dilatation operator is equivalent to the spin chain Hamiltonian of the 1D Ising model in a transverse magnetic field, which is a nontrivial integrable system that is exactly solvable at finite length. Solutions with periodic boundary conditions give the anomalous dimensions of flavor-singlet operators and solutions with fixed boundary conditions give the anomalous dimensions of operators whose ends contain open flavor indices.