Topics beyond the Standard Model : axions, supersymmetry, string theory

Overview & Synopsis

The aim of this thesis is to study various but interconnected theories for new physics beyond the standard model of particle physics. Those are theories of a new kind of particles, axions, a new symmetry principle, supersymmetry, and a new description of fundamental degrees of freedom, string theory. Constant instrumental and theoretical progresses made over the years maintain those already old subjects as leading BSM candidates.Axions are first reviewed and studied from a phenomenological perspective: we present models which disentangle the different scales which define the axion parameter space, and we discuss axions which arise in models of flavour physics. Motivated by swampland considerations, we insist on using gauge, and not global, symmetries as model building inputs.The focus then shifts to supersymmetry. We study its breaking, both in explicit ultraviolet models which generate a low supersymmetry breaking scale from high-scale matter, and at the effective field theory level using non-linearly realized supersymmetry. In our study of the latter topic, we focus on the constrained superfield approach. Finally, we present exact classical solutions of a supersymmetric model with broad application scope, the Wess-Zumino model of a chiral superfield.Last, we discuss string theory. We compute string spectra as illustrations of the structure of the theory and as starting points to compute one-loop vacuum amplitudes. Those are used to understand supersymmetry breaking in string theory, as well as brane interactions. Then, the latter enable us to test one of the swampland criteria, the weak gravity conjecture, in a string theory setup with broken supersymmetry. Finally, axions in string theory are scrutinized, in particular when they are charged under an anomalous abelian factor of the gauge group.