Subatomic Physics

5.9. Baryon Ground States 107
neutron number, N; isobars are nuclides with the same total number of nucleons,
element. The alpha particle, for
A. A particular nuclide is written as (A, Z) or A Z
instance, is characterized by (4, 2) or 4 2He or simply 4He. Stable nuclides, characterized by
N = A − Z and Z, are represented
as small squares in an N − Z plot
in Fig. 5.20. The plot indicates that
stable nuclides exist only in a small
band in the N − Z plane. The band
starts off at 45 ◦ (equal proton and neutron
numbers) and slowly veers toward
neutron-rich nuclides. This behavior
will provide a clue to an understanding
of properties of the nuclear force.
Figure 5.20 contains only stable nuclides.
In Section 5.7 we have pointed
out that stability is not an essential
criterion in considering hadrons. Unstable
nuclear ground states therefore
can also be added to the N − Z plot.
We shall explore some properties of
such an extended plot in Chapter 16.
Figure 5.20: Plot of the stable nuclides. Each
stable nuclide is indicated as a square in this
N − Z plot. The solid line would correspond to
nuclides with equal proton and neutron numbers.
(After D.L. Livesey, Atomic and Nuclear
Physics, Blaisdell, Waltham, MA, 1966.)
At the mass number A = 1, nuclear and particle physics meet. The proton and
the neutron, the two building blocks of all heavier nuclides, can either be considered
the simplest nuclei or they can be called particles. It is a surprising fact that the
two nucleons are not the only A = 1 hadrons. Other baryons with the mass number
A = 1 exist; they are called hyperons.
As an example of the investigation of hyperons, we
consider the production of the lambda. If negative
pions of a few GeV of energy pass through a hydrogen
bubble chamber, events such as the one shown
in Fig. 5.21 are observed: The negative pion “disappears,”
and further downstream two V -like events
appear. At first, the two V s seem to be very similar.
Figure 5.21: Observation of
the process pπ − → Λ 0 K 0 in
a hydrogen bubble chamber.
However, when the energies and momenta of the four particles are determined (Section
5.3) it turns out that one V consists of two pions, and the other of a pion
and a proton. Invariant mass plots, such as explained in Section 5.3, show that
the particle giving rise to the two pions has a mass of about 500 MeV/c 2 , while