Abstract
After developing a general criterion for deciding which neutrino mass models belong to the category of inverse seesaw models, we apply it to obtain the Dirac analogue of the canonical Majorana inverse seesaw model. We then generalize the inverse seesaw model and obtain a class of inverse seesaw mechanisms both for Majorana and Dirac neutrinos. We further show that many of the models have double or multiple suppressions coming from tiny symmetry breaking “μ-parameters”. These models can be tested both in colliders and with the observation of lepton flavour violating processes.
Highlights
We began by reviewing the famous canonical inverse seesaw before developing its Dirac analogue
We focused on developing the “multiplet” extensions of the inverse seesaw. We showed that both the canonical Majorana inverse seesaw and its Dirac analogue can be generalized by using fermions and scalars which transform as higher SU(2)L multiplets with appropriate hypercharges
We showed that one can generalize the idea of inverse seesaw to models where the neutrino mass is suppressed by multiple symmetry breaking μ-terms
Summary
The inverse seesaw is a popular approach for the generation of neutrino masses with the mediator masses potentially being close to the electroweak scale. To classify as a genuine inverse seesaw, the μ-parameter should be a “soft term” This means that if the μparameter is an explicit symmetry breaking term, it should have a positive mass dimension. 4. Extended fermionic sector: a genuine inverse seesaw model should always have an extended fermionic sector directly participating in the neutrino mass mechanism. Extended fermionic sector: a genuine inverse seesaw model should always have an extended fermionic sector directly participating in the neutrino mass mechanism This means fermions beyond the fermionic content of the SM should be involved in neutrino mass generation. The explicit Majorana mass term (a soft term) for this new fermion will break lepton number in two units explicitly and its smallness is protected by a symmetry. That scenarios with an explicit μ-term would lead to analogous conclusions, just replacing a VEV by a bare mass term.
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