Subatomic Physics

94 The Subatomic Zoo
Table 5.2: Interactions and Subatomic Particles. Entries not in
parentheses are for particles that exist free in nature. The particles
in parentheses are permanently confined.
Particle Type Weak Electromagnetic Hadronic
Photon Gauge boson No Yes No
W ± ,Z0 Gauge bosons Yes Yes No
(Gluon)
Leptons
Gauge boson No No Yes
Neutrino Fermion Yes No No
Electron Fermion Yes Yes No
Muon Fermion Yes Yes No
Tau
Hadrons
Fermion Yes Yes No
Mesons Bosons Yes Yes Yes
Baryons Fermions Yes Yes Yes
(Quarks) Fermions Yes Yes Yes
a weak interaction. The charged leptons, in addition, are also subject to the electromagnetic
force. All other particles, including nuclei, are hadrons; their behavior
is governed by the strong, the electromagnetic, and the weak interactions. In the
following sections we describe the particles listed in Table 5.2 in more detail. We
include quarks and gluons; they cannot be observed directly but their existence is
basedonfirmarguments.
5.5 Gauge Bosons
The first group of particles in Table 5.2 lists three types of quanta, called gauge
bosons, the photon, the W + ,W − and Z 0 , and the gluons. We are all familiar
with the photon, but the other quanta and the name “gauge boson” require some
introductory remarks. These particles are the carriers of forces as will be discussed in
Section 5.8. Three types of forces are important in subatomic physics, the hadronic,
the electromagnetic, and the weak. We therefore expect three types of particles to
be responsible for the three forces between the leptons and quarks. Indeed, the
photon mediates the electromagnetic force, the massive bosons, W ± and Z 0 carry
the weak force, and the gluons are the field quanta of the hadronic force. As we
will show later, the form of the interaction is determined by a symmetry principle
called gauge invariance; hence the name gauge bosons. We begin the discussion of
the gauge bosons with the photon, the quantum of light. The particle properties of
light invariably lead to some confusion. It is not possible to eliminate all confusion
at an elementary level because a satisfactory treatment of photons requires quantum
electrodynamics. However, a few remarks may at least make some of the important
physical properties clearer.