Neutrino mass generation with extra right-handed fields in a Dirac scenario via the type-I seesaw mechanism

TL;DR

This paper proposes a model extending the Standard Model with two right-handed neutrinos per generation, using a combined high-scale seesaw and low-scale pseudo-Dirac scenario to naturally generate tiny Dirac neutrino masses.

Contribution

It introduces a novel framework combining high-scale type-I seesaw and low-scale pseudo-Dirac mechanisms within a symmetric fermionic content hypothesis.

Findings

High-scale RH neutrino decouples at the seesaw scale

Low-scale RH neutrino forms a Dirac pair with LH neutrino

Tiny neutrino masses explained by fermionic content symmetry

Abstract

An extension of the Standard Model (SM) is studied in which two right-handed (RH) neutrinos per generation are incorporated, but considering the hypothesis of the symmetry of lepton and quark contents in order to deprive the number of RH neutrinos of freedom, generate Dirac neutrinos and accommodate naturally tiny values for their masses. The high scale type-I seesaw regime is applied to the first, ordinary RH neutrino, whereas a low scale pseudo-Dirac scenario is used for the second, adulterant RH neutrino, implying that the first RH neutrino decouples at the high scale, while the second RH neutrino survives down to the low scale to pair off in a Dirac-like form with the corresponding left-handed (LH) neutrino. The small mass and couplings of this extra RH neutrino are explained by means of the statement of the symmetry of fermionic content, only regarded as a guideline to the natural choice of parameters since it is not a proper symmetry in the Lagrangian.