Minimal Tree-Level Seesaws with a Heavy Intermediate Fermion
Kristian L. McDonald

TL;DR
This paper classifies minimal tree-level seesaw models with a heavy fermion mediator that naturally generate small neutrino masses, extending known models and identifying a finite set of natural, non-tuned variants.
Contribution
It systematically enumerates all minimal, natural tree-level seesaw models with a heavy fermion, including new variants beyond Type-I and Type-III.
Findings
Finite set of minimal models identified
New models generalizing Type-I and Type-III seesaws presented
Exhaustive classification of non-tuned variants achieved
Abstract
There exists a generic minimal tree-level diagram, with two external scalars and a heavy intermediate fermion, that can generate naturally small neutrino masses via a seesaw. This diagram has a mass insertion on the internal fermion line, and the set of such diagrams can be partitioned according to whether the mass insertion is of the Majorana or Dirac type. We show that, once subjected to the demands of naturalness (i.e. precluding small scalar vacuum expectation values that require fine-tuning), this set is finite, and contains a relatively small number of elements. Some of the corresponding models have appeared in the literature. We present the remaining original models, thus generalizing the Type-I and Type-III seesaws, and apparently exhausting the list of their minimal non-tuned variants.
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Taxonomy
TopicsParticle physics theoretical and experimental studies · Neutrino Physics Research · Dark Matter and Cosmic Phenomena
