Quasi Dirac neutrino oscillations

TL;DR

This paper explores the phenomenology of quasi-Dirac neutrinos, analyzing how small lepton number violating perturbations affect neutrino oscillations and deriving experimental bounds on their parameters.

Contribution

It provides a detailed analysis of quasi-Dirac neutrino oscillations, including how perturbations influence neutrino properties and the derivation of experimental constraints on their parameter space.

Findings

Stringent bounds on neutrino mass splittings in certain perturbation regimes

Weak or no limits on mass splittings with specific lepton mixing matrix modifications

Current experiments constrain non-standard lepton mixing angles

Abstract

Dirac neutrino masses require two distinct neutral Weyl spinors per generation, with a special arrangement of masses and interactions with charged leptons. Once this arrangement is perturbed, lepton number is no longer conserved and neutrinos become Majorana particles. If these lepton number violating perturbations are small compared to the Dirac mass terms, neutrinos are quasi-Dirac particles. Alternatively, this scenario can be characterized by the existence of pairs of neutrinos with almost degenerate masses, and a lepton mixing matrix which has 12 angles and 12 phases. In this work we discuss the phenomenology of quasi-Dirac neutrino oscillations and derive limits on the relevant parameter space from various experiments. In one parameter perturbations of the Dirac limit, very stringent bounds can be derived on the mass splittings between the almost degenerate pairs of neutrinos. However, we also demonstrate that with suitable changes to the lepton mixing matrix, limits on such mass splittings are much weaker, or even completely absent. Finally, we consider the possibility that the mass splittings are too small to be measured and discuss bounds on the new, non-standard lepton mixing angles from current experiments for this case.