1. For some time it has been considered theoretically that baryon number conservation may be violated in some processes at very low level. We shall discuss this in Chap. 9. Because of the known lower limit of the lifetime of the proton, τ
P > 1039 s, the effective coupling constant for such processes must be at least 24 orders of magnitude smaller than the Fermi constant of the weak interaction.
2. G. Lüders: Danske Vid. Selskab Mat. Fys. Medd. 28, 5 (1954); see also Vol.4 of this series — W. Greiner and J. Reinhardt: Quantum Electrodynamics (Springer, Berlin, Heidelberg, New York 1990).
3. M. Gell-Mann, A. Pais: Phys. Rev. 97, 1387 (1955).
4. J.H. Christensen, J.W. Cronin, V.L. Fitch, R. Turlay: Phys. Rev. Lett. 13, 138 (1964).
5. M. Kobayashi, K. Maskawa: Prog. Theor. Phys. 49, 656 (1973).