The next simplest application of the standard model to understanding the properties of the observed elementary particles is to compute the decay lifetimes of the other weakly interacting particles of the model. The only remaining particles that do not participate in the strong interactions are the leptonic fermions. This chapter is devoted to a calculation of their decay properties.

The purpose of this chapter is threefold. Two of these are straightforward. Lepton decays furnish our first example of a “second-order” decay that proceeds via a virtual particle, and so provide a good motivation for a full description of the Feynman rules of the theory. This calculation also provides some insight into the observed properties of real leptons and so allows more contact with experimentally accessible quantities. Indeed, the weak decays of the known fundamental particles provide much of our current information concerning the electroweak couplings. The third and final motivation is to provide the first illustration of the utility of the technique of effective Lagrangians for computing the virtual effects of heavy particles.

Qualitative features

The six flavors of fundamental leptons are e, µ, τ, νe, νµ, and ντ. Four of these are absolutely stable in the standard model by virtue of exact or extremely good approximate conservation laws of the model. The stable species are the three neutrino types and the electron.