The electroweak interaction

The development of the theoretical model that led to the electroweak unification started at the end of the 1960s. In this model, a single gauge theory, with the symmetry group SU(2) ⊗ U(1), includes the electromagnetic and weak interactions, both neutral current (NC) and charged current (CC). In particular, the electromagnetic and weak coupling constants are not independent but correlated by the theory. On the other hand, electroweak theory and QCD, both being gauge theories, are unified by the theoretical framework while their coupling constants are independent. Electroweak theory and QCD together form the Standard Model of fundamental interactions.

In the first part of this chapter we shall introduce the electroweak theory, as usual without any theoretical rigour. The unification characteristics appear mainly in the NC processes. The transition probabilities of all these processes are predicted by the theory with a single free parameter, the electroweak mixing angle. We shall discuss an example of its determination.

A crucial prediction of the theory is the existence of three vector bosons, W+, W− and Z0, together with predictions of their masses, widths and branching ratios in all their decay channels. All these predictions have been experimentally verified with high accuracy. We shall finally see the experimental proof of the fact that the vector bosons have weak charges themselves and that consequently they interact directly.