Non-perturbative approaches to Scattering Amplitudes

This thesis is devoted to the study of scattering amplitudes using two non-perturbative approaches. In Part I we focus on a particular theory known as N = 4 Super-Yang-Mills in four spacetime dimensions. The scattering amplitudes in this theory are dual to the expectation value of null polygonal Wilson loops which can be computed non-perturbatively using integrability. The Wilson loop is decomposed into smaller polygons and computed as an evolution of the color flux tube of the theory, summing over all intermediate flux tube states. By a suitable generalization of the building blocks called pentagons we describe how this program can describe all helicity configurations of the amplitude. We also show how the contribution from all flux tube excitations can be resummed to reproduce the general kinematics result at weak coupling. In Part II we take a different approach and study the space of Quantum Field Theories (QFTs). We focus on two-dimensional theories with a mass gap and a global symmetry. By studying the consequences of unitarity, crossing symmetry and analyticity of the two-to-two scattering matrix element we are able to constrain the space of allowed QFTs. At the boundary of this space we find several interesting features of the S-matrices and identify various integrable points.