Tests of the V-A structure and lepton universality

In a series of beautiful measurements of all sorts of spin correlations and distributions the SIN group have done a remarkable job of pinning down the values of the g, . In a strictly V — A theory one expects 

The experimental limits (at the 90% confidence level) on the coupling constants are shown in Fig. 1.8. Note in particular the coupling which would correspond to aV A interaction. 

Later we shall encounter a third lepton, the r, discovered only in 1975. Its properties are discussed in detail in Chapter 14, but we note here that it is supposed to interact in exactly the same fashion as do the p and e. It can betardecay into either a muon or an electron and a comparison of 

The evidence that the r coupling is of the V-A type is by now fairly conclusive (see Chapter 14). The shape of the energy spectrmn of the emitted lepton in beta-decay contains a parameter known as the Michel parameter , which is sensitive to the spin structure of the coupling (see Chapter 3 of Bailin, 1982). For example one has  

---