The Flavour of the ALPs

This thesis explores the phenomenological aspects of Axion-Like Particles (ALPs), understood as the Nambu-Goldstone bosons (NGBs) of spontaneously broken U(1) global symmetries coupled to the Standard Model (SM) fields. A particular emphasis is put on models connecting these light degrees of freedom with flavour sectors of the SM. We start by reviewing the general properties of pseudo-Nambu Goldstone bosons (pNGBs) and the relevant aspects of the SM for this thesis. Then we discuss the extension of the SM with NGBs via the Peccei-Quinn (PQ) mechanism, which introduces the axion. Subsequently, we study quark and lepton axion flavour-violation in connection with its ultraviolet (UV) completions, finding a close relation between the observables involving the axion and those generated by its UV completion. Next, we study the connection between the neutrino sector with a massive majoron, the pNGB associated with the spontaneous breaking of lepton number. We analyse different models capable of predicting a massive majoron while reproducing the active neutrino spectrum. Then, we explore the phenomenology and possibility of probing these models in future experiments. Finally, we examine the Effective Field Theory (EFT) of the ALPs in meson decays in the mass region of the 100 MeV-10GeV. We do a comprehensive analysis of the constraints imposed by quarkonia radiative decays in various scenarios, leading to the most complete set of bounds coming from this type of probes. Lastly, we make a connection between ALP meson flavour-violating decays and those involving generic vector bosons, showing how it is possible to translate the bounds using the equivalence theorem.