Subatomic Physics

426 Strong Interactions
Nevertheless, the pion and its interaction
with nucleons play a
special role. First, the pion lives
long enough so that intense pion
beams can be prepared and the
interaction of pions with nucleons
can be studied in detail. Second,
the pion is the lightest meson;
it is more than three times
lighter than the next heavier one.
In the energy range up to 500
MeV the pion–nucleon interaction
can be studied without interference
from other mesons. Moreover,
because the range of a force,
R = �/mc, is inversely proportional
to the mass of the quantum,
the pion alone is responsible
for the long-range part of the
nuclear force. In principle, the
properties of the nuclear force beyond
a distance of about 1.5 fm
canbecomparedwiththetheoretical
predictions without severe
complications from other mesons.
Figure 14.3: Pions can
be emitted and absorbed
singly. The strength of the
pion–nucleon interaction
is characterized by the
coupling constant fπNN.
Figure 14.4: Typical diagrams
for pion–nucleon
scattering and for pion
photoproduction.
Experimentally and theoretically, then, the pion–nucleon force plays the role
of a test case, and we shall therefore discuss some of the important aspects here.
Pions, being bosons, can be emitted and absorbed singly, as shown in Fig. 14.3.
The actual experimental exploration of the pion–nucleon force is performed, for
instance, through studies of pion–nucleon scattering and of the photoproduction of
pions. Two typical diagrams are shown in Fig. 14.4. In principle, many different
pion–nucleon scattering processes can be observed, but only the following three can
be readily investigated at low energies:
π + p −→ π + p (14.2)
π − p −→ π − p (14.3)
π − p −→ π 0 n. (14.4)
5 V. Flaminio et al., Compilation of Cross Sections I: π + and π − Induced Reactions,
CERN/HERA Report 83-01, 1983. See also G. Höhler, Pion–Nucleon Scattering, (H. Schopper,
ed.) Landoldt–Bernstein New Series I/9 b1 (1982) and I/9 b2 (1983).