239Chapter 24Elementary Particles, Quarks and
Quantum Chromodynamics
In this chapter we will examine the nature of elementary particles. But
before doing that we need to note that the definition of what is an
elementary particle has changed in recent years. In the days that chemists
and physicists discovered that the basic units of chemistry and the
structure of gases, liquids and solids were atoms and molecules it was the
atoms that were considered to be the most fundamental particles of the
universe and that molecules were composites of atoms. The term atom is
derived from the ancient Greek term ἄτομος (átomos) meaning not
divisible. As it turned out the atom was divisible and was shown to be
composed of protons, neutrons and electrons, which were thought to be
the smallest possible particles and were dubbed the elementary particles.
The neutrino, which first appeared in the beta decay of neutrons, was
added to this category.
As we will soon see the number of elementary particles that were
discovered exploded and that many of these particles including the
proton, the neutron and many other particles including mesons and
baryons that interact through the nuclear force and are known as hadrons
were shown to be composites of still more elementary particles called
quarks and antiquarks. The electron and neutrino and other members of
their class known as leptons are not made of quarks and they retain their
designation as elementary particles. In addition to the quarks and leptons
there are a small number of bosons that give rise to the fundamental
forces of electromagnetism, the nuclear force or strong interaction, and
the weak interaction. These bosons include the photon, which gives rise
to electromagnetism, the gluon, which gives rise to the strong interaction
and the W and Z bosons, which give rise to the weak interaction. Strictly