22
The Glashow–Salam–Weinberg Gauge
Theory of Electroweak Interactions
22.1 Difficulties with the current–current and ‘naive’ IVB
models
In chapter 20 we developed the ‘V-A current–current’ phenomenology of weak
interactions. We saw that this gives a remarkably accurate account of a wide
range of data – so much so, in fact, that one might well wonder why it should
not be regarded as a fully-fledged theory. One good reason for wanting to do
this would be in order to carry out calculations beyond the lowest order, which
is essentially all we have used it for so far (with the significant exceptions of
¯
the GIM argument, and box diagrams in M-M mixing). Such higher-order calculations are indeed required by the precision attained in modern high energy
experiments. But the electroweak theory of Glashow, Salam and Weinberg,
now recognized as one of the pillars of the Standard Model, was formulated
long before such precision measurements existed, under the impetus of quite
compelling theoretical arguments. These had to do, mainly, with certain inprinciple difficulties associated with the current–current model, if viewed as a
‘theory’. Since we now believe that the GSW theory is the correct description
of electroweak interactions up to currently tested energies, further discussions
of these old issues concerning the current–current model might seem irrelevant. However, these difficulties do raise several important points of principle.
An understanding of them provides valuable motivation for the GSW theory
– and some idea of what is ‘at stake’ in regard to experiments relating to
the Higgs sector, which has only recently begun to be explored (see section
22.8.3).
Before reviewing the difficulties, however, it is worth emphasizing once
again a more positive motivation for a gauge theory of weak interactions
(Glashow 1961). This is the remarkable ‘universality’ structure noted in chapter 20, not only as between different types of lepton, but also (within the context of CKM mixing) between the quarks and the leptons. This recalls very
strongly the ‘universality’ property of QED, and the generalization of this
property in the non-Abelian theories of chapter 13. A gauge theory would
provide a natural framework for such universal couplings.
367
The Glashow–Salam–Weinberg Gauge
Theory of Electroweak Interactions
22.1 Difficulties with the current–current and ‘naive’ IVB
models
In chapter 20 we developed the ‘V-A current–current’ phenomenology of weak
interactions. We saw that this gives a remarkably accurate account of a wide
range of data – so much so, in fact, that one might well wonder why it should
not be regarded as a fully-fledged theory. One good reason for wanting to do
this would be in order to carry out calculations beyond the lowest order, which
is essentially all we have used it for so far (with the significant exceptions of
¯
the GIM argument, and box diagrams in M-M mixing). Such higher-order calculations are indeed required by the precision attained in modern high energy
experiments. But the electroweak theory of Glashow, Salam and Weinberg,
now recognized as one of the pillars of the Standard Model, was formulated
long before such precision measurements existed, under the impetus of quite
compelling theoretical arguments. These had to do, mainly, with certain inprinciple difficulties associated with the current–current model, if viewed as a
‘theory’. Since we now believe that the GSW theory is the correct description
of electroweak interactions up to currently tested energies, further discussions
of these old issues concerning the current–current model might seem irrelevant. However, these difficulties do raise several important points of principle.
An understanding of them provides valuable motivation for the GSW theory
– and some idea of what is ‘at stake’ in regard to experiments relating to
the Higgs sector, which has only recently begun to be explored (see section
22.8.3).
Before reviewing the difficulties, however, it is worth emphasizing once
again a more positive motivation for a gauge theory of weak interactions
(Glashow 1961). This is the remarkable ‘universality’ structure noted in chapter 20, not only as between different types of lepton, but also (within the context of CKM mixing) between the quarks and the leptons. This recalls very
strongly the ‘universality’ property of QED, and the generalization of this
property in the non-Abelian theories of chapter 13. A gauge theory would
provide a natural framework for such universal couplings.
367