Subatomic Physics

118 The Subatomic Zoo
of the W (∼ 80 GeV/c 2 )andoftheZ(∼ 90 GeV/c 2 ) imply that their production
requires extremely high energies. The long search for the W finally came to an
end in 1983 when five clear cases of W production and decay were observed in pp
collisions at 2 × 270 GeV at the CERN SPS (Fig. 2.12). (41) The Z 0 was found
shortly thereafter. (42)
Why have we not listed the pion as a field quantum in Table 5.9? In the picture
we have presented, the pion itself is viewed as a quark–antiquark state and the
long-range force between nucleons, mediated by the pion, is not elementary. At the
more basic level, all three forces—strong, electromagnetic, and weak—are mediated
by gauge bosons of spin one.
Together with the basic constituents of matter, the three subatomic forces make
up the so-called “standard model.” Its basic features have been introduced in this
and previous sections and will be discussed in more detail in later chapters. We
summarize its main features in Table 5.10.
The standard model is believed to be a rather accurate description of nature:
The basic constituents of matter are three families of point quarks and three of point
leptons. There are also three basic non-gravitational gauge-type forces. The quarks
interact through all three forces and the (charged) leptons interact only through
the electromagnetic and weak forces. All three forces are carried by gauge bosons.
5.12 Excited States and Resonances
In atomic physics, the development of concepts and theories is intimately linked
with the exploration of excited states, in particular those of the hydrogen atom.
The Balmer series, the Ritz combination principle, the Bohr theory, the Schrödinger
equation, the Dirac equation, and the Lamb shift are all connected with the hydrogen
spectrum. Without the simplicity and the richness of the hydrogen spectrum,
progress would have been slower. In subatomic physics, the situation is more complex.
The nuclear system that most closely resembles the hydrogen atom is the
deuteron, a bound system consisting of a proton and a neutron. This system has
only one bound state and consequently does not provide the richness of information
that the hydrogen atom yielded. It is necessary to consider the excited states of
more complicated systems, such as heavier nuclides. Moreover, excited states of
baryons and mesons exist, and they must be studied in detail in the hope that they
will provide clues to an understanding of hadronic physics.
An understanding of the features of excited hadronic states requires a knowledge
of some results of quantum mechanics, and these can be discussed most easily by
41G. Arnison et al., Phys. Lett. 122B, 103 (1983); M. Banner et al., Phys. Lett. 122B, 476
(1983).
42G. Arnison et al., Phys. Lett. 126B, 398 (1983); P. Bagnaia et al., Phys. Lett. 129B,
130 (1983); for a summary, see E. Rademacher in Progress Particle Nuclear Physics, Vol 14
(A. Faessler, ed) (Pergamon, New York), p. 231 (1985) and P. Watkins, Story of the W and Z
(Cambridge University Press, Cambridge, 1986).