Quasi Dirac neutrino oscillations

Understand

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.