The Dark Side of the Littlest Seesaw: freeze-in, the two right-handed neutrino portal and leptogenesis-friendly fimpzillas

TL;DR

This paper introduces a minimal extension of the Littlest Seesaw model linking neutrino physics and dark matter via freeze-in production, allowing for high-scale leptogenesis-compatible right-handed neutrinos and dark matter particles called 'fimpzillas'.

Contribution

It proposes a unified model connecting neutrino masses, mixing, and dark matter relic density through right-handed neutrino portal couplings and freeze-in mechanism.

Findings

The model can produce correct neutrino masses and dark matter abundance simultaneously.

Right-handed neutrino masses can reach $10^{10}-10^{11}$ GeV, compatible with leptogenesis.

Dark matter particles ('fimpzillas') can have masses around the scale of right-handed neutrinos.

Abstract

We propose a minimal model to simultaneously account for a realistic neutrino spectrum through a type-I seesaw mechanism and a viable dark matter relic density. The model is an extension of the Littlest Seesaw model in which the two right-handed neutrinos of the model are coupled to a $Z_2$-odd dark sector via right-handed neutrino portal couplings. In this model, a highly constrained and direct link between dark matter and neutrino physics is achieved by considering the freeze-in production mechanism of dark matter. We show that the neutrino Yukawa couplings which describe neutrino mass and mixing may also play a dominant role in the dark matter production. We investigate the allowed regions in the parameter space of the model that provide the correct neutrino masses and mixing and simultaneously give the correct dark matter relic abundance. In certain cases the right-handed neutrino mass may be arbitrarily large, for example in the range $10^{10}-10^{11}$ GeV required for vanilla leptogenesis, with a successful relic density arising from frozen-in dark matter particles with masses around this scale, which we refer to as "fimpzillas".