Here mj are the eigenmasses, N the number of flavours, the corresponding CP eigenvalues (), and Uej the mixings of the mass eigenstates to the electron neutrino. The two electrons carry away all of the kinetic energy E0 released in the decay. Second, the study of double beta decay gives insight on the coupling of the neutrino to hypothetical light neutral bosons, generically named Majorons (). Such Majorons could be emitted in the double beta decay ( ). The sum energy spectrum of the two electrons extends from 0 to E0, as the Majoron carries away a fraction of the energy. Many models with Majorons have been built, including some in which two Majorons are emitted. In the oldest scheme the spectral shape is quite hard peaking at 2/3 E0. The half life depends on , the effective coupling constant of the Majoron to the neutrino:
For the sake of comparison, experimentalists usually interpret their results in terms of this model. The study of the allowed double beta decay ( ) does not address fundamental questions. It is nevertheless interesting in the sense that it probes the nuclear models with which the nuclear matrix elements for all the decay modes are calculated. The corresponding elements must be known in order to interpret the measured half lives on and decay in terms of neutrino masses and Majoron coupling. In decay the spectrum of the sum energy of the two electrons ranges from 0 to E0, and peaks at roughly 1/3 E0.