Classification

The weak interactions between quarks and leptons are those mediated by the socalled intermediate bosons W± and Z0, with a coupling which we generically label g. As indicated in Figure 7.1, there is a strong similarity between the Feynman diagrams for electromagnetic interactions mediated by photon exchange and weak interactions mediated by the intermediate bosons.

A familiar interaction is that between two straight wires hanging close together when electric currents – i.e. a flow of electrons and ions – pass through them. So, we can view the interaction of Figure 7.1(a) as that of two conserved electric currents jem, carried in this case by the electron e and the quark Q. Similarly, the interactions shown in Figures 7.1(b) and (c) can be viewed as between weak currents jweak. The difference is that these currents consist of a flow of conserved weak charge g rather than electric charge, e. In either case, these currents will contain the product of the (normalised) wavefunctions of the ‘in’ and ‘out’ particles that occur in the matrix element (2.16) for the interaction. Thus j ∝ ψ* ψ, where ψ* denotes an incoming and ψ an outgoing amplitude.

In Figure 7.1(b) one sees that the electric charges associated with the weak current actually change in the interaction, while in Figure 7.1(c) they do not. Rather inaccurately, these two are referred to as charged-current weak interactions (those in which the electric charge of each weak current changes) and neutral-current weak interactions (those in which the electric charges do not change).