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

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.