Flavour mixing neutrino oscillations

Abstract

The β spectrum recorded in a recent experiment is consistent with the emission of a heavy neutrino of mass about 17.1 keV and the mixing probability with the conventional massless neutrino is about 3%. This phenomenon appears as a manifestation of a new low-mass scale at which the global symmetry of lepton number is spontaneously broken. The model we introduce here in order to incorporate experimental data has been elaborated earlier by Gelmini and Roncadelli. Its remarkable feature is the presence of a massless neutral scalar particle known as majoron—the Goldstone boson of broken global symmetry. This majoron and a neutral Higgs with low-mass scalar also predicted appear coupled strongly to neutrinos and very weakly to quarks and leptons so that they cannot be detected at present laboratory energies. The neutrino observed by Simpson cannot be simply a Majorana particle, since this would imply that the neutrinoless double β-decay should go at a rate approximately 2000 times the present limit. This tends to suggest that neutrinos observed here are Dirac particles—components of a degenerate Majorana pair with oppositeCP eigenvalues giving net zero contribution to the double β-decay amplitude. This experiment by Simpson suggests that these neutrinos cannot be associated with the muon or the τ lepton. Our model contains one mixing angle and one mass parameter fixed by observation of Simpson in order to predict flavour mixing neutrino oscillations. A theoretical framework presented here states that the electron neutrino wave function ∣ve>L= sin θ oscillates with another neutrino of different flavour where ∣v1>L and ∣vn>L are massless and heavy neutrinos observed in tritium β-decay, respectively and the mixing angle sin θ is about 0.14÷0.20. Cosmological constraints on the 17 keV neutrino are discussed. Models with majorons or familons are found to be attractive possibilities.