Subatomic Physics

474 Quark Models of Mesons and Baryons
Table 15.1: Hadrons. The four lowest-lying multiplets of hadrons are
listed. They give rise to a total of 36 particles. The rest energy is arbitrarily
taken as the central energy of the multiplet.
Spin-parity, Rest Energy Members of Number of
J π (GeV) Type the Multiplet Members
0 − 0.5 Boson πKKηη ′ 9
1− 0.8 Boson ρK∗K ∗ ωφ 9
1 +
2
1.1 Fermion NΛΣΞ 8
3 +
2
1.4 Fermion ∆Σ∗Ξ∗Ω 10
15.3 Hunting the Quark
Do quarks exist? Considerable effort has been spent by many experimental groups
since 1964 to find quarks in nature, but no conclusive positive evidence has yet been
uncovered. Fortunately the fractional electric charges would make quark signatures
in careful experiments unambiguous.
In principle, quarks can be produced by high-energy protons through reactions
of the type
pN −→ NNqq + bosons, pN −→ Nqqq + bosons. (15.1)
The thresholds of these reactions depend on the mass mq of the quarks; the magnitudes
of the cross sections are determined by the forces between the hadrons and
the quarks. (Since neither the forces nor the quark masses are known, the search is
an uncertain affair. If quarks are not found, one never knows if it is because they
do not exist, because their mass is too high, or because the production cross section
is too low.)
The high energies required to produce massive particles are available in the
biggest accelerators, in high-energy colliding beams, and in cosmic rays. Moreover, if
free quarks exist, and if the world was created in a “big bang,” it is likely that quarks
were produced during a very early stage when the temperature was exceedingly high.
Some of these original quarks could still be around; searches in sedimentary rocks
have not found any.
Quarks can be hunted at accelerators (4) and in cosmic rays. Moreover, since at
least one quark must be stable, they should have accumulated in the earth’s crust,
in meteorites, or in moon rocks. Quarks can be distinguished from other particles
either by their fractional charge or by their mass. If the mass is studied, stability is
taken as an additional criterion. If the charge is used as signature, the idea is simple.
Equation (3.2) shows that the energy loss of a particle in matter is proportional to
the square of its charge. A quark of charge e/3 would produce one ninth of the ionization
of a singly charged particle of the same velocity. If the particle is relativistic,
4 M. Banner et al., (UA2 Collaboration) Phys. Lett. 121B, 187 (1983).