Phenomenological and Cosmological Aspects of Axions and Other Nambu-Goldstone Bosons

Though the Standard Model of Particle Physics is an impressive theory with countless phenomena correctly predicted, it still presents some open problems. In this thesis we explore the power of some Nambu-Goldstone Bosons to deal with two of these problems: the Strong CP Problem and the Flavour Puzzle, while studying at the same time some of their cosmological features.

The Strong CP Problem is addressed at the same time as part of the Flavour Puzzle with an invisible QCD Axion arising from the Minimal Flavour Violation ansatz. This model explains the mass ratios for third-generation fermions while providing a solution to the Strong CP Problem and possible DM candidate, the Minimal Flavour Violating Axion.
Neutrino masses are then addressed with a Majoron that also alleviates the Hubble tension and can arise from Minimal Flavour Violation, compatible thus with the previous model. This model also presents a relatively light new scalar and heavy neutrinos below the usual Type-I see-saw scale, with interesting phenomenology at colliders and beam dump experiments.
Finally, we study the impact hot axions can have on the number of relativistic degrees of freedom. Performing a rigorous and smooth analysis across the Electroweak Phase Transition, we study both a model-independent scenario and three specific models: DFSZ, KSVZ and the previously presented Minimal Flavour Violating Axion. Additionally, the compatibility of this observable with the XENON1T excess is considered. Results measurable by the CMB-S4 experiment, some compatible with axion cold Dark Matter, are obtained.