Subatomic Physics

Part IV
Interactions
In the previous nine chapters, we have used the concept of interaction without
discussing it in detail. In the present part, we shall rectify this omission, and we
shall outline the important aspects of the interactions that rule subatomic physics.
It is useful in the treatment of interactions to distinguish between bosons and
fermions. Bosons can be created and destroyed singly. Lepton and baryon conservation
guarantee that fermions are always emitted or absorbed in pairs. The simplest
interaction is thus one in which a boson is emitted or absorbed. Two examples
are shown in Fig. IV.1. The interactions occur at the vertices where three particle
lines are joined. The fermion does not disappear, but the boson either is created or
destroyed. In both cases, the strength of the interaction can be characterized by a
coupling constant. This coupling constant is written next to the vertex. A boson
can also transform into another boson, as shown in Fig. IV.2. There a photon disappears,
and a vector meson, for instance, a rho, takes its place. Again the coupling
constant is indicated near the vertex.
�
e Vertex
Ground
state
Excited
state
Excited
nucleon
N *
Vertex
Nucleon
f �NN*
Figure IV.1: Emission and absorption
of a boson by a fermion. The coupling
constants are denoted by e and fπNN∗ .
N
� �
g ��
�
Vector
meson
Photon
Figure IV.2: Transformation of one
boson into another.
The force between two particles is usually assumed to be mediated by particles,
asdiscussedinSection5.8. Theexchangeof a pion between two nucleons, shown
in Fig. 5.19, is again represented in Fig. IV.3. The forces represented by Figs. IV.1
and IV.3 are, however, no longer considered to be elementary. As discussed in
Section 5.11, baryons and mesons are composed of quarks and the more fundamental