In fact, it has been recognized that the small but finite neutrino masses can be understood naturally by the See-Saw mechanism 2) – 4) by introducing super-heavy neutral particles. On the other hand, it has also been recognized that there is no strong theoretical reason that postulates vanishing neutrino mass. This theory has been extraordinary successful in explaining all the existing data from recent particle physics experiments that have been carried out using high energy accelerators. Indeed, the standard model of elementary particle physics has been formulated assuming that the masses of the neutrinos are exactly zero. These results suggested that neutrinos could be mass-less. All the experiments, which observed the energy spectrum of the electrons very accurately, showed that the mass of the neutrino is consistent with zero within the experimental accuracies. If the mass of the neutrino is heavy, one expects that the maximum energy of the observed electron is lower than the case of a mass-less neutrino.
A 3H decays to a 3He, an e −, and an anti-electron-neutrino ( ν ̄ e). As an example, we consider a beta decay of a tritium, i.e., 3H. A remarkable feature of neutrinos is their masses. Since it has been extremely difficult to study neutrinos, details of the properties of neutrinos have not been known. For example, a neutrino produced in the Earth’s atmosphere can easily travel through the whole Earth. If a neutrino is produced, it travels straightly in any matter as if it is traveling in the vacuum. Neutrinos only interact via the weak force, which is indeed very weak. Likewise, they do not interact via the strong nuclear force, by which nucleons are bound to a nucleus. They have no electric charge hence they do not interact via the electro-magnetic force. They, however, are very difficult to observe. Neutrinos are one of the most abundant particles in the Universe.
Pauli in 1930 1) in order to secure the energy, momentum and spin conservations in the nuclear beta decays.
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