1. H. Yukawa, Proc. Phys.-Math. Soc.(Japan), 17, 48 (1935).
2. C.M.C. Lattes et al., Nature 159, 694 (1947).
3. Review of Particle Properties, Phys. Lett. 170B, 1 (1986).
4. M. Gell-Mann, Phys. Lett. 8, 214 (1964);
5. G. Zweig, CERN preprint 8182/TH 401 (1964).