Phenomenological aspects of majoron seesaw models

The origin of the small but nonzero neutrino masses remains a fundamental challenge in particle physics, strongly suggesting the existence of physics beyond the Standard Model. A well-motivated framework is offered by the Type-I Seesaw mechanism, in which lepton number violation and the introduction of heavy neutral leptons (HNLs) provide a natural explanation for tiny neutrino masses. A compelling scenario is that this mechanism originates from high-energy dynamics, manifesting at low energies as active neutrino masses. In this setup, Majorana masses can arise dynamically through the spontaneous symmetry breaking (SSB) of a global U(1) symmetry, leading to the emergence of the Majoron as the associated Goldstone boson. This thesis explores the phenomenological aspects of Majoron models within the Type-I Seesaw mechanism, with a particular focus on the minimal massive Majoron (mmM) model. This minimal framework introduces only two HNLs, leading to a predictive scenario in which neutrino and Majoron masses are intrinsically linked. A detailed analysis of the model’s parameter space is conducted, extending beyond previously explored regions to assess new viable scenarios. Furthermore, the implications of neutrinoless double beta decay within the mmM model are investigated, considering contributions from both active and sterile neutrino states, as well as potential Majoron-emitting decay mode. The constraints imposed by current experimental limits on the model’s parameter space are examined, along with potential refinements from future experimental advancements.