1. M. Gell-Mann andR. P. Feynman:Annual International Conference on High Energy Physics at CERN, edited byB. Ferretti (Geneva, 1958), p. 261.
2. N. Cabibbo andR. Gatto:Phys. Rev.,116, 1134 (1959); see alsoR. Gatto:Lectures at the International School of Physics in Varenna, June 1959 (to appear inSuppl. Nuovo Cimento).
3. G. Feinberg, P. Kabir andS. Weinberg:Phys. Rev. Lett.,3, 527 (1959).
4. One may adopt the procedure used, for example, byN. N. Bogoljubov:Introduction to the Theory of Quantized Fields, (New York, 1959) p. 123, for free spinor fields. One obtains the energy as an integral over terms of the typea † ±(k)(A + Bγ 5)·a ±(k|k 0| where thea ±(k) are essentially Fourier components of the operators ψ±(r). The relationship between the positive definiteness of the energy and of (A-Bγ 5) then follows directly.