After about 70 years since the Beta Decay Theory was given to light, after the marvellous successes of the Standar Model Theory and about 50 years of neutrino experiments . . . we still miss one of the more fundamental information concerning the character of neutrinos, and still “Double Beta Decay provides” an almost unique “means of finding which form of the beta decay theory is correct”. If Neutrinoless Doube Beta Decay (ββ(0ν)) exists the correct form of the theory is the one in which the ν is a massive Majorana particle identical to its antiparticle. If ββ(0ν) does not exist — and if this is not the result of accidental “cancellations” — ν is a massive Dirac particle. Double Beta Decay is the only spontaneous decay that lead some unstable nuclei to a lowest energy isomer. In the decay two electrons are emitted transforming two neutrons into two protons. If the decay proceeds like two simultaneous beta decay two neutrinos are emitted toghether with the electrons but if ν is a Majorana particle the decay can proceed also through the exchange of a virtual ν and the emission of just electrons. Unable to detect the eventually emitted neutrinos the only signature that the experimentalist can search, to distinguish between ββ(2ν) and ββ(0ν), relies in the 2 electron energy spectrum: the sum of the electron kinetic energies is fixed at the Q-value in the ββ(0ν) (the nuclear recoil is negligible) while it has a continuum probability distribution, zeroing above the Q-value, in the ββ(2ν).
KeywordsNeutrino Oscillation Neutrino Experiment Inverted Hierarchy Neutrino Mass Hierarchy Nuclear Recoil
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