Subatomic Physics

15.2. Quarks as Building Blocks of Hadrons 473
a reasonable mass range. To estimate the reasonable mass range, we note that the
mass splitting due to the electromagnetic interaction is of the order of a few MeV,
as indicated in Fig. 15.1. Since the hadronic interaction is about 100 times stronger
than the electromagnetic one, a mass splitting of the order of a few hundred MeV
can be expected. Since the pion is the lightest hadron, it is tempting to look first
at the low-lying 0 − bosons. There are nine such particles below 1 GeV: three pions,
two kaons, two antikaons, the eta, and the eta-prime.
Figure 15.2: The nine pseudoscalar mesons with mass below 1 GeV.
At the left the masses are given as they occur in nature. Going to
the right, first the weak interaction is switched off, then the electromagnetic
interaction, and finally part of the hadronic interaction.
The mass splittings caused by Hw and Hem are exaggerated. The
position of the 0 − particle is unknown.
These particles are
shown to the left in
Fig. 15.2. In nature,
only the positive
and negative members
of the same isomultiplet
are degenerate,
and all other
particles possess different
masses. If
the weak interaction
is switched off, the
very small splitting
between K 0 and K 0
disappears. If in addition
Hem is turned
off, the neutral and
the charged members
of the same isospin
multiplet become degenerate.
Finally, it is assumed that all nine pseudoscalar mesons become degenerate if part
of the hadronic interaction is turned off. We call the resulting nine-fold degenerate
pseudoscalar state the 0 − particle. The mass of the 0 − particle is determined by
the part of the hadronic interaction that has not been switched off. According to
Fig. 15.2, the 0 − particle gives rise to a native family of nine different particles.
Closer inspection shows that three other particle multiplets can be discerned in
the region below a few GeV. The characteristics of the four multiplets are summarized
in Table 15.1.
The crucial question is now: Is this scheme useful and can it be brought into
more precise form? Does it then yield new predictions? To make the classification
more quantitative, we discuss it in terms of quarks.