1. G. Danby, J.-M. Gaillard, K. Goulianos, L. M. Lederman, N. Mistry, M. Schwartz andJ. Steinberg:Phys. Rev. Lett.,9, 36 (1962).
2. See for exampleO. Klein:Ark. f. Fys.,16, 191 (1959); A. Gamba, R. Marshak and S. Okubo:Proc. Nat. Acad. Sei.,45, 881 (1959); W. Królikowski:Nuovo Cimento,24, 52 (1962).
3. See, for example,R. Marshak andE. C. G. Sudarshan:Introduction to Elementary Particle Physics (New York, 1961), Chap. 5, Section 5.
4. Our metric is g11= g22=g33 = -g44= + 1. In general our notation and the cross-section calculations of Section 3 will follow as closely as possible those ofJ. Jauch andP. Eohelich:The Theory of Photons and Electrons (Cambridge, 1955).
5. Cf.M. Gell-Mann:Phys. Rev.,125, 1067 (1962), eqs. (1.1) and (1.2) (the prefixed minus sign in eq. (1.6) is irrelevant).