Lowest-Order Weak Interactions

Abstract

Weak interactions are responsible for the radioactivity of nuclear elements as well as the decays of most of the stable elementary particles. A perturbation-theory phenomenological approach to low-energy weak interactions is even more successful than it is for strong interactions because the coupling is weak. But it is on an even less firm theoretical footing due to the “nonrenormalizability” of the current-current weak hamiltonian. Nevertheless, such a perturbation theory again reveals the fundamental role of (weak) currents—both of the lepton and of the hadron type. Accordingly, we discuss weak leptonic, semileptonic, and nonleptonic decays in the language of currents and lowest-order covariant (Feynman) diagrams. As a consequence we shall be able to extract the weak-interaction scale or coupling constant, along with the scales of hadronic currents which we exploited in the last chapter. We also attempt to unify strangeness-conserving and strangeness-changing weak hadronic transitions by introduction of the Cabibbo angle. Finally we look into the nonleptonic weak transitions, which are complicated by strong- as well as weak-interaction dynamics. We shall use the tools of hadronic currents, isospin, and Feynman pole graphs to analyze such nonleptonic decays.