Pseudo-Dirac Neutrinos in the New Standard Model

TL;DR

This paper investigates pseudo-Dirac neutrinos within an extended Standard Model, showing how tiny Majorana masses affect neutrino oscillations and deriving experimental constraints on these masses from solar data.

Contribution

It introduces a framework for analyzing pseudo-Dirac neutrino mixing effects and relates experimental bounds to the Majorana mass parameters in the model.

Findings

Solar data constrains Majorana masses to less than 10^{-9} eV.

A simple relationship between Majorana masses and mixing parameters is established.

Future experiments could probe even smaller gauge singlet fermion masses.

Abstract

The addition of gauge singlet fermions to the Standard Model Lagrangian renders the neutrinos massive and allows one to explain all that is experimentally known about neutrino masses and lepton mixing by varying the values of the Majorana mass parameters M for the gauge singlets and the neutrino Yukawa couplings. Here we explore the region of parameter space where M values are much smaller than the neutrino Dirac masses. In this region, neutrinos are pseudo-Dirac fermions. We find that current solar data constrain M values to be less than at least 1E-9 eV, and discuss the sensitivity of future experiments to tiny gauge singlet fermion masses. We also discuss a useful basis for analyzing pseudo-Dirac neutrino mixing effects. In particular, we identify a simple relationship between elements of M and the induced enlarged mixing matrix and new mass-squared differences. These allow one to directly relate bounds on the new mass-squared differences to bounds on the singlet fermion Majorana masses.