1. A wide review on this subject can be found in:Rapporteur's talk byW. J. Willis:Proceedings of the International Conference on Elementary Particles, Heidelberg, 1967 (Amsterdam, 1968). An up-to-date bibliography on experimental works can be found in: RPP/H/37 Nucl. Ph. Labor. Oxford.
2. Aachen-Bari-Bergen-CERN-Nijmegen-Orsay-Padova-Paris-Torino Collaboration: preprint (9 May 1968).
3. E. S. Ginsberg:Phys. Rev.,142, 1035 (1966).
4. In the following we will use the metric and the notations stated inJ. D. Bjorken andS. D. Drell:Relativistic Quantum Fields (New York, 1965), except for the spinors normalization: our normalization isu + u=v + v=1.
5. See, for instanceA. I. Akhiezer andV. B. Berestetskii:Quantum Electrodynamics, inInterscience Monographs and Texts in Physics and Astronomy, vol. 11 (New York, 1965);R. H. Beherends, R. J. Finkelstein andA. Sirlin:Phys. Rev.,101, 886 (1956).