1. J. Ashkin, T. Fazzini, G. Fidecaro, N. H. Lipman, A. W. Merrison andH. Paul:Nuovo Cimento,14, 1266 (1959);D. Berley, J. Lee andM. Bardon:Phys. Rev. Lett.,2, 357 (1959); also for previous work.
2. J. Lee andN. P. Samios:Phys. Rev. Lett.,3, 55 (1959).
3. J. Steinberger andH. B. Wolfe:Phys. Rev.,100, 1490 (1955); also for previous work.
4. S. Weinberg andG. Feinberg:Phys. Rev. Lett.,3, 111, 244 (1959);N. Cabibbo andR. Gatto:Phys. Rev.,116, 1334 (1959).
5. A. Citron et al.:Berkeley Meeting (September 1960). We wish to acknowledge the co-operation of all members of the Citron group who have developed this beam.