Seminarium "High Energy, Cosmology and Astro-particle Physics (HECA)"

We introduce a series of theoretical frameworks for neutrino mass generation that not only address the lightness of neutrinos but also offer rich phenomenology as well as a connection to dark matter. First, we present the minimal linear seesaw mechanism where small neutrino masses can be sourced by a tiny vacuum expectation value of a leptophilic Higgs doublet, and mediated by Quasi-Dirac heavy neutrinos. The neutrino mass mediators can be accessible to colliders. Moreover, lepton number violation can be large despite the smallness of neutrino masses, and may shed light on the Majorana nature of neutrinos.

Building on this, we extend the analysis to a dark sector seeded linear seesaw, where a Z2-odd sector introduces lepton number violation at the loop level through naturally suppressed scalar couplings. This scenario not only preserves the essential features of the linear seesaw but also introduces a viable dark matter candidate in the form of the lightest neutral scalar, with promising signals in lepton flavor violation experiments.

Finally, we explore axion frameworks in which colored mediators act as neutrino-mass mediators. In one realization, colored fermions and scalars act as two-loop Majorana neutrino-mass mediators. Alternatively, these exotic states generate Dirac neutrino masses where the global Peccei-Quinn symmetry that addresses the strong CP problem also ensures the Dirac nature of neutrinos. In both cases, various setups can be distinguished by probing the axion-to-photon coupling at helioscopes and haloscopes as well as potentially significant flavor-violating quark-axion interactions. The axion also accounts for the observed cosmological dark matter.