Abstract:

Mechanisms for generating the tiny observed neutrino mass scale via approximate global symmetries are often accompanied by psuedo-Nambu-Goldstone bosons (pNGBs) such as the well known Majoron or the less familiar Diraxion. These additional degrees of freedom could have important cosmological ramifications and are well-motivated dark matter candidates. In most approaches the prerequisite explicit symmetry breaking needed to generate their mass is usually put in by hand. Furthermore these particles are much harder to detect in laboratory experiments compared to the famous QCD axion, since their dominant interaction is usually with neutrinos and not photons due to the vectorial nature of U(1)_B-L. We provide a novel framework that addresses both of these issues by gauging the approximate SU(3)_H lepton flavor symmetry of the Standard Model. We introduce a global, chiral "Lepto-Peccei-Quinn" symmetry under which only the lepton sector is charged. SU(3)_H instantons render the associated pNGB, that we named the “Lepto-Axion”, massive. We discuss two distinct cosmological scenarios: In the first regime the coherent motion of a very heavy Lepto-axion catalyses spontaneous baryogensis by biasing the SU(3)_H sphaleron transition. In the second regime the Lepto-Axion has a mass far below the weak scale and its relic abundance from coherent oscillations can be drastically modified by the intermediate presence of lepton flavor monopoles via the Witten effect. These exotic defects - which do not couple to electromagnetism - can arise from the spontaneous breaking of SU(3)_H and later confine when they attach to gauged cosmic strings.