Abstract
It is by now generally accepted that the family of ψ particles is direct evidence that there are new conserved quantum numbers in the strong interaction and that the magnitudes of the partial decay widths of ψ(3.1) are consistent with virtual electromagnetic effects (Richter 1976). The most popular explanation is that the ψ particle spectrum arises from a charmed quark and its anti-quark bound in a non-relativistic potential, in analogy with positronium. The ψ particles, on this view, do not themselves carry the new quantum number and the very narrow width of ψ(3.1) is attributed to a dynamical effect (Zweig rule), which inserts
ad hoc
a damping factor of nearly 10
-4
but still leaves the conventional quantum numbers of the strong interaction conserved (Dalitz 1977). We wish to discuss briefly the alternative class of theories in which the ψ spectrum stems from the extension of the underlying strong group in just the same way that the ρ, ω and Φ particles are understood in terms of the multiplet structure of SU(3). In such theories the ψ(3.1) is narrow because it is stable with respect to the strong interaction and decays electromagnetically. As an example we wish to consider the ‘colour’ theory of Han & Nambu (1965) although many features considered below are generally true of this type of explanation. (For a more detailed discussion written in July 1975 see Feldman & Matthews (1976). We here incorporate the data which have emerged since that date. See also Marinescu & Stech (1975).)