Subatomic Physics

15.9. Outlook and Problems 493
whereas the allowed decay into charmed mesons, DD above the 3.7 GeV/c threshold
is shown in Fig. 15.13(b). Below this threshold, the decay into hadrons is primarily
electromagnetic, or via three gluons, in order to be colorless. For charmed mesons
with c = 1, the preferred virtual decay is weak, c → sW + , shown in Fig. 15.13(c).
The charmed quark was postulated on theoretical grounds, and the discovery of
the cc meson, the J/ψ, was a theoretical and experimental triumph. In contrast,
the discovery of the bottom quark was unexpected. Preliminary evidence for a fifth
quark had been obtained as early as 1968, but only the observation of a narrow
dimuon resonance in the scattering of 400 GeV protons from nuclei at Fermilab
in 1977 provided the conclusive evidence for a new particle, the upsilon. (28) The
upsilon is the bound state of a bottom (or beauty) quark with its antiquark, bb.
Similar to charmonium, “bottonium” possesses a spectrum of excited positroniumlike
resonances. The spectrum of bb resonances can be understood on the basis of
Eq. (14.59) with the same constant A and with k as predicted by the one-gluon
exchange potential. Thus the long range confining force is flavor-independent. As
seen in Table 15.2 the partner of the bottom quark, called top, has a very large
mass of ∼ 179 GeV/c 2 and toponium can be studied as well.
15.9 Outlook and Problems
We have only scratched the surface of particle models. The detailed discussion goes
far deeper and involves more than the composition of hadrons in terms of quarks
and gluons. It includes particle properties such as static moments, decays, form
factors, and couplings of mesons to baryons.
The description of hadrons in terms of quarks is very successful. The success
leads to a number of questions; a few of these are listed here:
1. The indirect evidence for quarks is overwhelming, but quark confinement is
not yet understood fully. It is expected to follow from QCD, and numerical
calculations (on a lattice rather than for a continuum space) suggest that it
does occur. (29)
2. Are the quarks themselves structureless particles? Since there are at least 18
quarks, composed of six flavors and three colors, we must wonder whether the
quarks are really the fundamental constituents of hadrons.
3. What is the relationship of the strong multiplets and the electroweak families?
Is there a relationship, as suggested by grand unified theories (GUTs)?)
4. Color is the important attribute of quarks for the strong interactions. Flavor
is more important for the electroweak interaction. Why is this so? What is
28 S. W. Herb et al., Phys. Rev. Lett. 39, 252 (1977); W. R. Innes et al., Phys. Rev. Lett. 39,
1240 (1977); L. Lederman, Sci. Amer. 239, 72, (October, 1978).
29 N.A. Campbell, L.A. Huntley, and C. Michael, Nucl. Phys. B306, 51 (1978).