In addition to their role in well-motivated mechanisms of neutrino mass generation, heavy neutral leptons (HNL) can be at the source of an extensive array of new phenomena, which are forbidden (or strongly suppressed) in the Standard Model (SM), several manifesting the possible Majorana nature of the extended neutral lepton sector.
These phenomena include lepton number violating (LNV) processes such as (semi) leptonic tau and meson decays, charged lepton flavour violating (cLFV) transitions (including radiative and three-body decays, $Z$ and Higgs bosons decays, as well as rare phenomena in the presence of matter).
Such heavy Majorana fermions are also expected to have a significant
impact concerning electroweak (EW) precision observables, and also lead to striking signatures at colliders (be it the LHC, or future lepton colliders). All the above new phenomena are rooted in the non-negligible mixings of the heavy Majorana states with the light active neutrinos, which in turn is responsible for modifying the flavour mixings in the lepton sector. Furthermore, within the framework of the inverse seesaw mechanism, we calculate the electroweak corrections to leptonic Higgs, $W$ and $Z$ decays, which are modified due to the presence of HNL in the loop diagrams. In certain regions of the currently viable parameter space significant deviations from the SM predictions can be observed. In view of the anticipated increased precision of experimental measurements of these decay modes at future $Z$ factories (such as FCC-ee), precision measurements of leptonic Higgs, $W$ and $Z$ decays can lead to complementary constraints on seesaw-like neutrino mass models.
Andrea Pelloni, Tanjona Rabemananjara, Tommaso Giani