Physics of Massive Neutrinos

The existence of an almost massless neutral particle (later on called neutrino by Fermi) was postulated by Pauli in 1932 to account for the continuous energy spectrum of the electrons emitted in nuclear ( β decay. This particle was required to be a fermion in order to conserve angular momentum. Fermi incorporated this particle into a detailed theory of nuclear beta decay which could account for the observed shape of the electron energy distribution found in many nuclear beta decays. With availability of more experimental results, the original Fermi theory underwent many changes and finally culminated into a simple and elegant V — A theory [1, 2] which universally describes all the known (charged) weak interaction processes at low energy [3, 4, 5, 6]. The V — A theory is basically an effective theory which allows reliable calculations of weak interaction processes at energies ≪ O (100) GeV. The basic structure of this theory was later on generalized into a full fledged quantum theory based on ideas of spontaneously broken local gauge invariance [7]. It became possible to unify the weak and electromagnetic interactions within this framework. The resulting theory is now known as the standard electroweak model.