Pseudo Dirac Neutrino

Abstract

There is a growing interest in the neutrino masses. Theoretically, there is much freedom in neutrino mass patterns in contrast to quarks and charged leptons because neutrinos are in general considered Majorana particles. Experimentally, the v-oscillation!) and the /3/3 decay2),3) give serious constraints to neutrino masses and their mixing scheme. A special case where there are degeneracies of Majorana neutrino masses has recently been interesting. If the degeneracy happens as a consequence of the special symmetry in the mass matrix, a pair of degenerate Majorana neutrinos may form a Dirac neutrino. This Dirac neutrino is called the pseudo Dirac (PD) neutrino,4)-6) if the symmetry of the mass matrix is not the symmetry of the weak interaction. That is, the lepton number assigned to this Dirac neutrino is not conserved in the weak interaction and thus the mass degeneracy is removed by the higher order perturbation of the weak interaction. A PD neutrino has recently drawn special attention due to the tight neutrino mass bound obtained from the analysis of the /3/3 decay,2),3)