The unified theory of weak and electromagnetic interactions was formulated independently by Weinberg  and Salam  for leptons. It was later extended to the quark sector using the GIM mechanism of quark mixing proposed by Glashow, Iliopoulos, and Maiani (GIM) . The theory is popularly known as the “standard model of electroweak interactions”. (ii) L. Glashow, A. Salam, and S. Weinberg were awarded the Nobel Prize in Physics in 1979 for the formulation of this theory. In this chapter, we describe the formulation of the standard model for leptons and quarks and its applications to other interactions. The predictions of the model and their experimental confirmations are also presented. The limitations of the model and some processes suggesting the need for the physics beyond the standard model are discussed later in Chapter 20.
In this section, the essential features of the electromagnetic interactions and the phenomenological V – A theory of electroweak interactions, well-known from the experimental and theoretical studies of the various weak processes are summarized. They are used as inputs in formulating the standard model. They are as follows:
1. Both electromagnetic and weak interactions involve, in general, all the elementary particles, that is, leptons and quarks, unlike the strong interaction, which affect only the quarks and the hadrons built from these quarks.
2. Both electromagnetic and weak interactions are mediated by vector fields. Electromagnetic interaction is known to be mediated by photons described by the electromagnetic field Aμ, which is massless. The observed weak interactions are presumed to be mediated by charged intermediate vector bosons (IVB), Which are massive with mass MW.
3. Weak interactions involve a pair of leptons, like in which the charged vector bosons, interact with the charged lepton currents, where only the left-handed leptons participate, that is, right-handed leptons, that is, are not involved in the interaction. The interaction Lagrangian is written as:
4. Electromagnetic interactions involve the electromagnetic field Am, which interacts with the electromagnetic current, implying that both the left- and the righthanded components of the electron participate, because The interaction Lagrangian is written as:
5. Both electromagnetic and weak interactions are universal interactions, that is, they have the same coupling strength for leptons and quarks. However, the coupling strengths of the electromagnetic and weak interactions to the quarks (leptons) are different from each other.