Subatomic Physics

114 The Subatomic Zoo
not accepted by Planck for at least 15 years. The neutrino, postulated by Pauli in
1930, was considered to be speculative for many years even by Bohr. In the case of
the photon, the observation of the Compton effect dispelled the doubt; in the case
of the neutrino, detection in absorption by Reines and coworkers in 1956 convinced
the last disbelievers. As we have said in Section 5.3, we will never see the rho
“directly”. Can we still consider it a particle? We will not establish a firm criterion
here, but instead introduce particles for which either the experimental evidence is
very strong, or for which the theoretical arguments are convincing. In either case,
the introduction of these particles makes the discussion of experiments and results
much more elegant.
We have already stated in the introduction to Part II that experiments at energies
above 10 GeV reveal that the proton, for instance, is not elementary but
composed of subunits. These experiments, discussed in Section 6.7, and many additional
data provide unambiguous evidence for the existence of quarks. (35) We
will treat quarks in detail in Chapter 15. Here we only describe the properties
that we will need for a preliminary understanding. Baryons are fermions built primarily
from three quarks, and mesons are bosons built from a quark (q) andan
antiquark (¯q):
baryon (qqq)
meson (q¯q).
In order to describe the presently known baryons and mesons, six quarks and the
corresponding antiquarks are needed. In Table 5.7, we give the most important
properties of quarks. At the same time we list the leptons again, in order to point
out a striking similarity in the grouping of the two otherwise very different sets of
particles.
Leptons and quarks are fermions; all particles in Table 5.7 have spin 1/2 and
possess antiparticles. The particles divide into three generations or families, light,
intermediate, and heavy. Recent evidence from the decay of the Z 0 shows conclusively
that there are only three generations of neutrinos of small mass. (36) Within
each family, there are two different “flavors”, and the table contains six flavors of
leptons and six of quarks.
The quark property that immediately catches the eye is the electric charge:
quarks have charges (2/3)e and −(1/3)e! These charges, of course, permit the assignment
Eq. (5.66). With the charges given in Table 5.7 it is easy to see that the
combination (uud) has the correct charge to be a proton, (udd) a neutron. Despite
great efforts to catch a free quark, none has been seen (see Chapter 15); strong
theoretical arguments imply that quarks must remain confined within hadrons. (37)
35 S. L. Glashow, Sci. Amer. 33, 38 (October 1975).
36 The present limit is Nν =2.984 ± 0.008 See PDG.
37 Y. Nambu, Sci. Amer. 235, 48 (November 1976); K. A. Johnson, Sci. Amer. 241, 112 (July
1979); C. Rebbi, Sci. Amer. 248, 54 (February 1982).