Chapter 2. A New Stage of Elementary Particle Theory

Abstract

1. The number of elementary particles has increased very rapidly smce World War II. It is said that many theoretically uninvited guests have appeared. At present moment, there are photons (r), neutrinos (v), electrons (e) and muons (p) as leptons, and pions (rc) and strange K-mesons as mesons. There are also protons (p) and neutrons (n) as heavy particles, and lamda-particles (A), sigma-particles (.S) and xi-particles (8) as strange particles. It seems to be one of the present-day central problems how to understand these particles in a unified way. 2. Japanese elementary particle physicists first paid attention to the strange properties of a series of new particles very soon after they had been discovered. The most typical feature seen in the case of strange particle production is that they are always produced in pairs. Later, in 1953, the strangeness quantum number was introduced by Nakano-Nishijima and inde­ pendently by Gell-Mann, so that a rule of the elementary particle world was obtained. 3. As to the structure or the property of elementary particles the research work has been done from several directions. At the two meetings, which were held at the Research Institute for Fundamental Physics (Kyoto) in November and in December, 1959, separately, some possibility to connect these directions was disclosed and a further outlook of the future development was pointed out. Namely, the following directions might be unified in an orgamc way: (a) To attack the problem from the outside region by making use of the meson theory of nuclear forces and nucleon structures; (b) To clarify the electromagnetic structures of elementary particles, and to investigate the effects of weak interactions on the structure of elementary particles; (c) To study the Sakata model, which is a composite model of mesons and heavy particles, and to develop the Nagoya model, which is a