Subatomic Physics

14.8. The Standard Model, Quantum Chromodynamics 451
It is important to note that the high energy results presented earlier dealt with
total cross sections. For hadrons, these appear to be dominated by large distance
phenomena. It is not clear that this feature will persist at ever higher energies.
Moreover, high momentum transfer, or short distance, collisions are different, and
serve as tests of the underlying theory, quantum chromodynamics or QCD, since
the interaction is predicted to become ever weaker.
14.8 The Standard Model, Quantum Chromodynamics
There is now good evidence that the theory of the strong forces is quantum chromodynamics
(QCD), so named because of its analogy to quantum electrodynamics,
the quantum theory of electricity and magnetism. The term “chromodynamics”
refers to the key ingredient of color in the theory. In Section 10.9 we saw that
the experimental production of hadrons in e + e − collisions provides evidence that
quarks must come in three colors. This additional degree of freedom is responsible
for the forces between quarks.
Table 14.1 presents the analogy
between QCD and QED;
the gauge field quantum, the
gluon, like its counterpart, the
photon, is massless and has
a spin of 1�; thus there are
color electric and color magnetic
forces.
Figure 14.20: (a) Gluon coupling to quarks and (b), (c)
gluon self-couplings.
However, there are also crucial differences between QCD and QED. The gluons
themselves are “color charged” and not neutral as is the photon. Indeed, the gluons
can be considered to be bicolored, that is, to be made up of a color and an anticolor.
The gluon color leads to a non-Abelian (noncommuting) theory. There are eight
colored gluons. Out of three colors and their anticolors, we can make up nine
possible combinations; one of these, rr + gg + bb is colorless and the remaining eight
correspond to the gluons.
Because the gluons themselves are color-charged, they can interact with each
other and there are not only quark–gluon couplings as shown in Fig. 14.20(a), but
also gluon–gluon couplings as shown in Figs. 14.20(b) and 14.20(c). The source of
the gluon fields need not be quarks, but can be other gluons! This self-coupling
gives rise to a highly nonlinear theory with no “free” gluon field. There also arises
the possible existence of mesons made up of gluons only. Such objects are called
glueballs; they have been sought but have not yet been found, and may not exist in
pure form. The color combinations carried by the gluons can be described in terms
of the three colors of the quarks. In Fig. 14.21 we show two ways of drawing the
exchange of a gluon between a quark and an antiquark. The exchange leads from
a red–antired to a blue–antiblue combination. The red quark is changed to a blue