Naturally Light Dirac and Pseudo-Dirac Neutrinos from Left-Right Symmetry

TL;DR

This paper proposes a left-right symmetric model where neutrinos are naturally Dirac particles with small masses generated by two-loop corrections, fitting neutrino oscillation data and predicting observable cosmological effects.

Contribution

It introduces a novel class of left-right symmetric theories with a generalized seesaw mechanism, explaining neutrino masses and flavor structure without relying on high-scale symmetry breaking.

Findings

Excellent fit to neutrino oscillation data independent of symmetry breaking scale

Neutrinos are naturally Dirac particles with masses from two-loop corrections

Predicted cosmological parameter ΔN_eff ≈ 0.14, testable in future measurements

Abstract

We develop a class of left-right symmetric theories based on the gauge group $SU(3)_c \times SU(2)_L \times SU(2)_R \times U(1)$ with a generalized seesaw mechanism for generating the charged fermion masses. Neutrinos are naturally Dirac particles in this setup with their small masses arising from two-loop quantum corrections. We evaluate these two-loop diagrams exactly and analyze the flavor structure of the lepton sector. We find excellent fits to neutrino oscillation data, independent of the right-handed gauge symmetry breaking scale. We also explore the possibility that neutrinos are pseudo-Dirac particles in this framework, with the tiny mass splittings between active and sterile neutrinos arising from Planck-induced corrections and find possible realizations. These models can be tested in the near future with precision cosmological measurements of $\Delta N_{\rm eff}$ in CMB which is predicted to be $\simeq 0.14$. This class of models allows for a solution to the strong CP problem via parity symmetry without the need for an axion.