Double beta decay and Majorana neutrino
In 1937, Ettore Majorana, an italian physicist, elaborated
a symmetrical theory of neutrino and anti-neutrino.
If its theory is right, the neutrino should then be identical to the anti-neutrino.
Then, a special decay is possible: the double beta decay without neutrino.
The neutron inside the nucleus decays by emitting an anti-neutrino, which, if identical to the
neutrino, can be re-absorbed by a proton of the nucleus, that becomes a neutron by emitting
a positron (anti-electron).
The final balance is: no neutrino, but an electron and a positron.
The experiments which are searching or have searched this type of beta decay
have not found yet any signal. In such experiments, the noisy background signal
is huge and this type of decay is very rare (if it exists!).
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Last update: 26/06/1999: http://wwwlapp.in2p3.fr/neutrinos/adoublebeta.html