Exact Results in the Theory of New Particles

Abstract

In this written version of my lectures, which are in a way a continuation of my lectures at Erice in 1977, I shall not repeat the general description of the situation of “quarkonia”, the J/ψ family, or the Υ family. Figure 1 summarizes the present knowledge of the J/ψ spectrum. The most noticeable improvement is the disappearance of the state at 2.83 GeV, decaying mostly into 2γ, and its replacement by a state X at 2.97 GeV, observed in ψ′ → γ + X and J/ψ → γ + X, decaying mostly into hadrons. For theorists this is a relief, because it was difficult to understand the large energy gap between 2.83 and the J/ψ, the too low transition rate observed, and the dominance of the γγ decay mode. On the other hand, as I said during my lectures, it would have been a catastrophe if a candidate for the 0⁻⁺ brother of the J/ψ had not been observed.