Wednesday, March 20, 2013

1303.4573 (Kristian L. McDonald)

Minimal Tree-Level Seesaws with a Heavy Intermediate Fermion    [PDF]

Kristian L. McDonald
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 lepton number violation occurs at the mass insertion or through a Yukawa coupling. 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.
View original: http://arxiv.org/abs/1303.4573

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