1. S. Badier andC. Bouchiat:Nuovo Cimento,39, 1183 (1965).
2. L. Durand III andJ. Sandweiss,Phys. 0227 0225 V 3 Rev.,135, B 540 (1964).
3. E. M. Levin andL. L. Frankfurt:Sov. Phys. JETP Lett.,2, 65 (1965);H. J. Lipkin andF. Scheck:Phys. Rev. Lett.,16, 71 (1966);J. J. J. Kokkedee andL. Van Hove:Nuovo Cimento,42 A, 711 (1966).
4. The experimental cross-sections are given at fixed value ofp f * , the final momentum of the particles in the c.m. system. We have takenp f * =0.8 GeV/c as in ref. (1). Slightly different values take more recent data into account. See also ref. (5).
5. See the list given in ref. (1) andC. Baltay, J. Sandweiss, H. D. Taft, B. B. Culwick, J. K. Kopp, R. I. Louttit, R. P. Strutt, A. M. Thorndike andM S. Webster:Phys. Rev.,140, B 1027 (1965);O. Czyżewski, B. Escoubès, Y. Goldschmidt-Clermont, M. Guinea-Moorhead, D. R. O. Morrison andS. De Unamuno-Escoubès:Phys. Lett.,20, 554 (1966);K. Böckmann, B. Nellen, E. Paul, B. Wagini, I. Borecka, J. Diaz, U. Heeren, U. Liebermeister, E. Lohrmann, E. Raubold, P. Söding, S. Wolff, J. Kidd, L. Mandelli, L. Mosca, V. Pelosi, S. Ratti andL. Tallone:Nuovo Cimento,42 A, 954 (1966);R. K. Böck, W. A. Cooper, B. R. French, J. B. Kinson andR. Levi-Setti:Phys. Lett.,17, 166 (1965).