Subatomic Physics

92 The Subatomic Zoo
Figure 5.11: Invariant mass spectrum if pion pairs are produced independently (phase space) or if
they result from the decay of a rho of small decay width.
The invariant mass spectrum is not restricted to particle physics; it has also
been used in nuclear physics. Consider, for instance, the reaction
p + 11 B →
� 3α
8Be + α.
(5.28)
Since 8 Be lives only for 2 × 10 −16 sec before decaying into two alpha particles, three
alphas are observed in either case. Nevertheless, the formation of 8 Be can be studied
with the invariant mass spectrum.
5.4 A First Glance at the Subatomic Zoo
The techniques discussed so far have led to the discovery of well over 100 particles
and a much larger number of nuclei. How can these be ordered in a meaningful
way? A first separation is achieved by considering the interactions that act on each
particle. Four interactions are known to exist, as pointed out in Section 1.1. In order
of increasing strength they are the gravitational, the weak, the electromagnetic, and
the hadronic interaction. (10) In principle, then, the four interactions can be used to
classify subatomic particles. However, the gravitational interaction is so weak that
it plays no role in present-day subatomic physics. For this reason we shall restrict
10We shall see later that all but the gravitational interaction are connected within the standard
model.