Subatomic Physics

Chapter 13
The Electroweak Theory of
the Standard Model
13.1 Introduction
In this chapter, we provide an introduction to the “standard model of the electroweak
interactions.” The subject is complex; for details, we refer the reader to
texts and reviews listed at the end of the chapter.
The phenomenological current–current interaction as described in chapter 11
gives excellent agreement with low energy experiments. It is not, however, a welldefined
theory. All calculations are performed to lowest order in the effective coupling
constant, GF , i.e., to lowest (first) order in perturbation theory. Computations
of higher order, or of radiative corrections lead to physically meaningless infinities
which we do not know how to remove. On the other hand, it is experimentally known
that the higher order weak processes are extremely small. For instance, the mass
difference between KL and KS is of second order in GF and is tiny (Section 9.7).
Consequently, the “theory” in the form given in chapter 11 is unsatisfactory. No
adequate theory of the weak interaction, alone, has been discovered. This shortcoming
was a challenge to solve a wider problem, and produced a more fundamental
theory that describes the weak interactions unified with the electromagnetic one.
The electroweak theory is a major triumph. In 1879 James Clerk Maxwell
formulated a unified theory of electricity and magnetism; and exactly one hundred
years later Sheldon Glashow, Abdus Salam, and Steven Weinberg received the Nobel
prize for a comparable achievement, the unification of the weak and electromagnetic
forces. (1) As we saw in chapter 11, the two interactions are of the current–current
form and require vector (also axial vector for the weak interactions) currents. Both
the weak vector and electromagnetic currents are conserved. Despite these and other
similarities, the two forces appear at first sight to have little in common. The electromagnetic
force has an infinite range and is carried by massless photons, whereas
the weak force has a very short range and is mediated by very heavy vector bosons.
1 S. Weinberg, Rev. Mod. Phys. 52, 515 (1980); A. Salam, Rev. Mod. Phys. 52, 525 (1980);
S. L. Glashow, Rev. Mod. Phys. 52, 539 (1980).
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