Pseudo-Dirac neutrinos

Abstract

"Pseudo-Dirac" neutrinos, which are almost Dirac particles with tiny Majorana masses, are argued to provide a unique clue to the Majorana nature of neutrinos. The manifestation of the Majorana nature is through rich neutrino oscillation phenomena of pseudo-Dirac neutrinos, including the neutrino ↔ anti-neutrino oscillation due to the lepton-number violating Majorana masses, even inside each generation. The formulae for the various oscillations of pseudo-Dirac neutrinos are obtained. We find that the reported specific features of neutrino mixings in the data of neutrino oscillation experiments, such as almost maximal mixing in the atmospheric neutrino oscillation, is naturally explained in the pseudo-Dirac scenario with small generation mixings, just as in the quark sector. We, however, also point out that there remain some problems to be settled for this scenario to be viable. As another interesting application, it is argued that the tiny pseudo-Dirac mass splitting due to the Majorana mass can be revealed, looking at high energy astrophysical neutrinos by measuring the flavor ratios as the function of L/E.