Abstract
The recent results showing the presence of neutrino oscillations clearly indicate that the difference between the squared mass of neutrinos of different flavors is different from zero, but are unable to determine the nature and the absolute value of the neutrino mass. Neutrinoless double beta decay (DBD) is at present the most powerful tool to ascertain if the neutrino is a Majorana particle and to determine under this condition the absolute value of its mass. The results already obtained in this lepton violating process will be reported and the two presently running DBD experiments briefly discussed. The future second generation experiments will be reviewed with special emphasis to those already partially approved. In conclusion the peculiar and interdisciplinary nature of these searches will be stressed in their exciting aim to discover if neutrino is Dirac or Majorana particle.
Highlights
The double-beta decay is the rarest nuclear weak process
The two-neutrino decay conserves the lepton number and was originally proposed by Goeppert-Mayer in 1935 1. It is a secondorder weak process, this is the reason of its low rate, and the first direct laboratory detection was only achieved as recently as 1987 2
It has been measured for a dozen of Advances in High Energy Physics
Summary
The double-beta decay is the rarest nuclear weak process. It takes place between two eveneven isobars, when the decay to the intermediate nucleus is energetically forbidden due to the pairing interaction, which shifts the even-even and the odd-odd mass parabolas in a given isobaric chain; only due to the pairing interaction can the double-beta decay be observed. Since the half-life of the decay is determined, together with the effective Majorana neutrino mass defined later in Section 2 , by the nuclear matrix elements for the process NME, its knowledge is essential to predict the most favorable decays and, once detection is achieved, to settle the neutrino mass scale and hierarchy. Another process of interest is the resonant double-electron capture which could have lifetimes competitive with the neutrinoless double-beta decay ones only if there is a degeneracy of the atomic mass of the initial and final states at the eV level 9.
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