How to avoid unnatural hierarchical thermal leptogenesis

TL;DR

This paper demonstrates that minimal three-flavour hierarchical thermal leptogenesis cannot naturally explain neutrino masses and baryon asymmetry, and explores potential resolutions involving a second Higgs doublet.

Contribution

It proves the impossibility of avoiding unnatural hierarchical thermal leptogenesis in minimal three-flavour Type I seesaw models and discusses a solution with a second Higgs doublet.

Findings

Hierarchical thermal leptogenesis cannot be naturally realized in minimal three-flavour models.

A second Higgs doublet can enable low- to intermediate-scale leptogenesis.

Models predict a SM-like Higgs and TeV-scale scalar states.

Abstract

A one-flavour naturalness argument suggests that the Type I seesaw model cannot naturally explain neutrino masses and the baryon asymmetry of the Universe via hierarchical thermal leptogenesis. We prove that there is no way to avoid this conclusion in a minimal three-flavour setup. We then comment on the simplest ways out. In particular, we focus on a resolution utilising a second Higgs doublet. Such models predict an automatically SM-like Higgs boson, (maximally) TeV-scale scalar states, and low- to intermediate-scale hierarchical leptogenesis with $10^3\text{ GeV}\lesssim M_{N_1}\lesssim 10^7\text{ GeV}$.