The phenomenon of neutrino oscillation discussed in Chapter 18 implies that neutrinos have finite mass. The phenomenology of neutrino oscillation determines only the mass square differences but not the absolute masses of the neutrinos. The scalar and fermionic structure of the standard model does not allow neutrinos to have non-zero mass. There is also the possibility that neutrinos may have magnetic moments (intrinsic or transition) considerably larger than the prediction of the standard model. Moreover, there are anomalous results in the measurements of neutrino and antineutrino oscillation parameters in various regions of energies which suggest that the standard model description in terms of three weak flavor doublets of leptons and quarks is not adequate to describe weak interactions and there may exist additional flavor of neutrinos which are non-interacting i.e. sterile. All these processes suggest that although the standard model has been a spectacular success in describing most electroweak processes, there is need for physics beyond the standard model.
Moreover, there are many other rare physical processes driven by weak interactions which have been studied for a long time theoretically as well as experimentally and are not explained by the standard model. The experimental observations of these processes would establish the physics beyond the standard model. The subject of physics beyond the standard model is too vast to be described in space of a chapter but we discuss here, some of the neutrino processes to introduce the subject.
(i) Neutrinoless double beta decay (NDBD) and Majorana neutrinos
(ii) Lepton flavor violating (LFV) decays of elementary particles
(iii) Flavor changing neutral current (FCNC)
(iv) Existence of non-standard interaction in high precision weak processes.
Netrinoless Double-beta Decay
The problem of double-beta decays (DBD) involving two-neutrino double-beta decay (2νββ) and neutrinoless double-beta decay (0νββ) has been with us for more than 80 years after it was first discussed by Goeppert-Mayer in the case of (2νββ) in 1935  soon after the Fermi theory of β-decay was formulated  and the process of 0νββ was discussed by Furry in 1939 after a new theory of neutrino was given by Majorana .