Undemocratic Dirac seesaw

TL;DR

This paper proposes a novel Dirac seesaw mechanism where the mixing parameters are undemocratic, allowing larger left-handed neutrino mixings, and links neutrino masses to baryon asymmetry and dark matter within extended gauge symmetries.

Contribution

It introduces an undemocratic Dirac seesaw model that connects neutrino masses, baryon asymmetry, and dark matter through new gauge symmetries and embedding into larger theories.

Findings

Large left-handed neutrino mixings are phenomenologically viable.

Small mixings with heavy fermions can explain baryon asymmetry.

Dark matter stability is linked to a new $U(1)$ gauge symmetry.

Abstract

The standard model left-handed neutrinos and several right-handed neutrinos can obtain a tiny Dirac mass matrix through their mixings with relatively heavy Dirac fermions. In this Dirac seesaw scenario, the mixings involving the left-handed neutrinos can be allowed much larger than those involving the right-handed neutrinos. This undemocratic parameter choice is attractive to phenomenology. We show that the small mixings between the heavy Dirac fermions and the right-handed neutrinos can have a common origin with the observed baryon asymmetry in the universe. We also connect the introduction of right-handed neutrinos to the existence and stability of dark matter by a new $U(1)$ gauge symmetry for dark photon or baryon-minus-lepton number. We then specify how to embed our scenario into a left-right symmetric theory or a grand unification theory.