ABSTRACT
This chapter describes the weak interaction that is responsible for reactions in which quarks and leptons change flavour. The weak interaction is experienced by all quarks, as well as all charged leptons and neutral leptons. In weak interactions, one of three massive particles, known as the positive and negative W bosons and the neutral Z boson, is exchanged. Beta-minus and beta-plus decays are used to exemplify how transformations between protons and neutrons ultimately result from transformations between up and down quarks in which W bosons are emitted before decaying into electrons or positrons and electron antineutrinos or electron neutrinos. The relatively long half-life of the neutron, and its survival beyond the first few minutes after the Big Bang, are shown to result from the relatively large mass of the W boson.
