Neutrinos are elusive particles: for many years their very existence was only inferred from the part that they play in β-decay. However, we have seen in Chapter 10 and Chapter 11 that they are of great importance in astrophysics, and in the forging of the nuclei of the heavy elements in supernovae. Apart from β-decay, other experimental results on neutrinos are accumulating. In this chapter we describe some of these results, and their possible interpretation.

13.1 Neutrino cross-sections

To design neutrino detectors, for example to measure the flux of neutrinos striking the Earth, it is important to know their interaction crosssections with atomic nuclei and electrons. Unless the neutrino energy is so high that its de Broglie wavelength ƛ = (2ℼ h/p) is comparable with or less than the nuclear radius, the nuclear cross-sections for processes which convert a neutrino to its charged lepton partner will involve matrix elements of the same form as those which appear in the theory of β-decay. For example the total cross-section for the reaction