J/psi Meson - Background To Discovery

Background To Discovery

The background to the discovery of the J/ψ was both theoretical and experimental. In the 1960s, the first quark models of elementary particle physics were proposed, which said that protons, neutrons and all other baryons, and also all mesons, are made from three kinds of fractionally-charged particles, the "quarks", that come in three different types or "flavors", called up, down, and strange. Despite the impressive ability of quark models to bring order to the "elementary particle zoo", their status was considered something like mathematical fiction at the time, a simple artifact of deeper physical reasons.

Starting in 1969, deep inelastic scattering experiments at SLAC revealed surprising experimental evidence for particles inside of protons. Whether these were quarks or something else was not known at first. Many experiments were needed to fully identify the properties of the subprotonic components. To a first approximation, they were indeed the already-described quarks.

On the theoretical front, gauge theories with broken symmetry became the first fully viable contenders for explaining the weak interaction after Gerardus 't Hooft discovered in 1971 how to calculate with them beyond tree level. The first experimental evidence for these electroweak unification theories was the discovery of the weak neutral current in 1973. Gauge theories with quarks became a viable contender for the strong interaction in 1973 when the concept of asymptotic freedom was identified.

However, a naive mixture of electroweak theory and the quark model led to calculations about known decay modes that contradicted observation: in particular, it predicted Z boson-mediated flavor-changing decays of a strange quark into a down quark, which were not observed. A 1970 idea of Sheldon Glashow, John Iliopoulos, and Luciano Maiani, known as the GIM mechanism, showed that the flavor-changing decays would be eliminated if there were a fourth quark, charm, that paired with the strange quark. This work led, by the summer of 1974, to theoretical predictions of what a charm/anticharm meson would be like. These predictions were ignored. The work of Richter and Ting was done for other reasons, mostly to explore new energy regimes.