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#

Double beta decay

The study of nuclear double beta decay sheds light on physics beyond the
standard model. First, it provides a unique way to look for neutrino masses
of the Majorana type. With such masses neutrinoless double beta decay ( )
can occur. It violates lepton number conservation by two units. The half
life is directly related to the effective neutrino mass:
Here *m*_{j} are the eigenmasses, *N* the number of
flavours,
the corresponding CP eigenvalues (),
and *U*_{ej} the mixings of the mass eigenstates to the electron
neutrino. The two electrons carry away all of the kinetic energy *E*_{0}
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 *E*_{0},
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 *E*_{0}.
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 *E*_{0}, and peaks at roughly 1/3 *E*_{0}.

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