Weak decays of stable particles

The 'stable' elementary particles are those that do not decay through the strong interaction; thus all leptons are stable, as are the lightest hadrons with a given quark composition. Their lifetimes cover an enormous range of values; for example the proton lifetime is greater than 10 31 years, the neutron lifetime is approximately 15 minutes and the lifetime of the tau lepton is less than one picosecond. The gauge bosons which decay weakly are the W and Z particles whose lifetimes are of the order of 10 -25 seconds. Study of the weak decays of stable particles sheds light not only on the nature of the weak interaction itself but also on the internal structure of hadrons and the forces between their constituent quarks. In this article the author reviews recent advances in the field of weak decays and considers their implications for quantum chromodynamics (the theory of strong interactions) and electroweak theory (the combined theory of electromagnetic and weak interactions), which together form the 'Standard Model' of elementary particles.