Subatomic Physics

422 Strong Interactions
Table 14.1: Analogies of QCD and QED.
QED QCD
Fundamental particles Charged leptons Quarks
Gauge quanta Photon Gluons
Source of interaction Charge Color charge
Basic strength α = e 2 /�c αs
whelming support from experiments carried out at the highest energies available.
QCD has features that are analogous to those of the theory for the electron and
the electromagnetic field (quantum electrodynamics), but has also important differences
therefrom. The analogy is shown in Table 14.1. The fundamental particles of
QED are the leptons, those of QCD the quarks. The gauge quanta of the electromagnetic
field is the photon and that of the QCD field the gluon. The strength of
the electromagnetic field is determined by the electric charge, that of QCD by the
color charge. QCD, like QED, is described by a single coupling constant the square
of which, αS, is the analogue of the fine structure constant α = e 2 /�c. There are
also differences between QED and QCD. Whereas the photon is electrically neutral
and therefore transfers no charge, the gluon carries color. This difference is crucial.
The strong squared coupling constant, αS, depends on momentum transfer or distance
probed, and becomes progressively weaker as the former is increased and the
latter becomes smaller. By contrast, the squared coupling constant of QED, α, has
only a weak dependence on momentum transfer, and increases as that momentum
grows. For QCD at very high momentum transfers perturbation theory becomes
practical so that the theory can be easily tested in this realm. The theory is said
to be “asymptotically free” in that the coupling constant is predicted to vanish as
the distance probed shrinks to zero. On the other hand, the squared coupling constant
αS becomes very large for large distances, which leads to quark confinement
or what is often called “infrared slavery.” The word infrared connotes large wavelengths
or distances. This feature of QCD implies that neither single quarks nor
gluons can be observed as free particles. The theory is thus highly nonlinear, and
it is the large-distance behavior that is probed at low energies where it is depicted
more effectively by meson exchanges and their couplings to baryons. In this limit
the theory is solved numerically by ‘lattice’ calculations (section 14.9.) We will
describe some features of the low-energy theory in Section 14.1.
14.1 Range and Strength of the Low-Energy Strong Interactions
Some features are common to all low-energy strong interactions, and in this section
we shall describe two of the most important ones, range and strength. The range is
the distance over which the force is effective. Historically, much of the information
A practical introduction Philadelphia, Institute of Physics Pub., 2003; C. Quigg, Gauge Theories
of the Strong, Weak and Electromagnetic Interactions, Benjamin, Reading, Ma 1983; K.
Gottfried and V.F. Weisskopf, Concepts of Particle Physics, Oxford University Press, New York,
1984.