Weak interactions

This chapter gives an introduction to the phenomenology and the theory of weak interaction processes involving leptons and hadrons. In sections 1 and 2 the most prominent and characteristic properties of weak interactions are collected as they follow from the analysis of a set of key experiments, old and recent. The following sections 3 to 5 deal with the elements of non-Abelian local gauge theories in general, and with the unified theory of electroweak interactions of Glashow, Salam and Weinberg (GSW), in particular. The GSW theory is a minimal theory in the sense that it incorporates all known phenomenology and successfully describes all known electromagnetic and weak interaction processes, including the masses and properties of the recently discovered W- and Z0-bosons. However, there are good reasons to expect that this minimal model is only an approximation to some, more global, theory of leptons and hadrons. The minimal GSW theory needs to be tested in precision experiments at all available energies. Sections 6 and 8 of this chapter are devoted to a discussion of such tests and to specific possibilities of searching for possible deviations from its predictions. Assuming the structure of the weak couplings to be known and to be given by the typical vector and axial vector current couplings, the matrix elements of such currents, taken between hadronic states, can be used to extract information on strong interaction quantities. This aspect of weak interactions at medium energies is dealt with in our discussion of various semileptonic decays of hadrons in several sections below. (Auth.)