Abstract
We discuss a radiative type-I seesaw. In these models, the radiative generation of Dirac neutrino masses allows to explain the smallness of the observed neutrino mass scale for rather light right-handed neutrino masses in a type-I seesaw. We first present the general idea in a model independent way. This allows us to estimate the typical scale of right-handed neutrino mass as a function of the number of loops. We then present two example models, one at one-loop and another one at two-loop, in which we discuss neutrino masses and lepton flavour violating constraints in more detail. For the two-loop example, right-handed neutrino masses must lie below 100 GeV, thus making this class of models testable in heavy neutral lepton searches.
Highlights
We have constructed a new realization of the type-I seesaw mechanism based on radiatively generated Dirac neutrino masses
We showed that this class of models can naturally generate a small neutrino mass for order-one couplings and relatively low mass scales
Compared to the standard type-I seesaw mechanism, for which the Majorana mass scale should be of the order of the grand unified theory scale, we found viable models even for MR below 100 GeV
Summary
As in our current paper, in these models neutrinos are Majorana particles with radiatively generated Dirac mass terms. Both of these papers presented just one particular one-loop example model, while we formulate general conditions for the implementation of the radiative seesaw type-I mechanism at any loop order. We discuss the radiative generation of neutrino Dirac couplings from a model-independent point of view This allows us to estimate the typical scales for the Majorana mass of neutrinos as a function of the loop level, at which the Dirac couplings are generated.
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