ABSTRACT
This chapter provides a detailed treatment of those particles which have been closely associated with the weak interaction since the days of Enrico Fermi and Pauli and which have a key role to play in grand unification theories, namely neutrinos. In the mathematical description of the neutrino an ambiguity arises which does not exist in the case of the electron. The chapter considers firstly the simplest case of vanishing neutrino mass and then considers specific neutrino models. Many experiments have searched for the phenomenon of neutrino oscillations. This phenomenon was first discussed by Pontecorvo. There are two ways of searching for double beta decay: the geochemical method; and direct observation of the decay using an appropriate detector. The geochemical method is based on the fact that the daughter isotopes from the double beta decay of nuclei which are contained in minerals have accumulated over the total geological period since the formation of these minerals.
