Subatomic Physics

Chapter 14
Strong Interactions
All good things must come to an end. In chapters 10 and 11 we have seen that
the electromagnetic and the weak interactions of leptons at low energies were characterized
each by a single coupling constant. Furthermore, nature uses only one
type of current for the electromagnetic interaction, a vector; and two for the weak
interaction, a vector and an axial vector. The situation with the strong interaction
at low energies is much more complicated. In the nucleon-nucleon interaction,
for instance, almost every term allowed by general symmetry principles appears
to be required to fit the experimental data. In addition, at energies �1 GeVthe
phenomenological strong interactions do not provide any evidence that they are
goverened by one universal coupling constant. Consider, for instance, Figs. IV.1
and IV.3. The strength of the interaction of the pion with the baryon is described
by the constant fπNN ∗ in the first case, and by fπNN in the second one. The two
constants are not identical. The interaction of the pion with pions is characterized
by yet another constant. Since many hadrons exist, a large number of coupling constants
occur. The corresponding interactions are all called strong because they all
are about two orders of magnitude stronger than the electromagnetic one. However,
they are not exactly alike. While some connections among the coupling constants
can be derived by using symmetry arguments, these relations are only approximate,
and many constants appear at present to be unrelated. The situation resembles a
jigsaw puzzle in which it is not known if all pieces are present and in which the
shape of some pieces cannot be seen clearly.
We found in chapter 13 how a clever idea led to a simplification and a unification
of the weak and electromagnetic interactions into a single interaction with only one
coupling constant. Is it possible that the strong interaction at these lower energies
masks simplicity that sets in at higher energies? In the past several decades
a theory has been developed in which the strong interactions at sufficiently high
energies (more precisely, high momentum transfers or short distances) are just as
simple as the electroweak theory and are described by a single coupling constant.
This theory is called quantum chromodynamics (1) (QCD) and has received over-
1 I.R. Aitchison, An Informal Introduction to Gauge Field Theories, Cambridge University
Press, Cambridge, 1982; I.R. Aitchison and A.J.G. Hey, Gauge theories in particle physics:
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