Subatomic Physics

15.8. Heavy Mesons: Charmonium, Upsilon, ... 491
15.8 Heavy Mesons: Charmonium, Upsilon, ...
The existence of quarks heavier than the strange one was predicted on the basis
of the electroweak theory introduced in Chapters 11–13. The absence of weak
strangeness-changing neutral currents required a new quark, charm. A whole
new era of physics was ushered in when Richter at SLAC (Stanford) and Ting at
Brookhaven and their collaborators almost simultaneously discovered the J/ψ. (25)
This meson, composed of cc, was found at SLAC in e + e − collisions as described
in Section 10.9, and at Brookhaven in the study of hadronically produced e + e − .
There could be little doubt that a new chapter of physics had been opened since
Figure 15.12: Spectrum of charmonium. The lines connecting energy levels represent photon
transitions. The shaded region is the continuum for decays into DD mesons. [From PDG.]
the decay width of the J/ψ is only about 70 keV and not of the order of a hundred
MeV. The decay width to the specific channel of e + e− is only of the order of 5 keV,
as expected for a vector meson. It is now known that the J/ψ is a 3S1(1− ) state
of cc. The excitement of the physics community over the new state of matter was
heightened further by the discovery of excited states of cc.
25J. J. Aubert et al., Phys. Rev. Lett. 33, 1404 (1974), J. E. Augustin et al., Phys. Rev. Lett.
33, 1406 (1974).