Selected Citation

The Nature of the Four-Fermion Interaction

Authors(s):E. C. G. Sudarshan and R. E. Marshak Publication:Proc. of the Conference on Mesons and Newly-Discovered Particles, Padua-Venice Publication Date:September 22, 1957 Publisher: Citation: Link:PDF

The near equality of the effective coupling constants in the processes of beta decay, muon decay and muon capture has led to the postulation of a Universal Fermi Interaction between the pairs of spinor fields np, µν, eν. The weak decays of the strange particles are consistent with an effective coupling constant of the same order of magnitude and can be understood by adding the Λ 0 − p pair to the above pairs of spinors. The possibility of a universal four-fermion interaction has been reexamined in the light of the recent experimental results on the nonconservation of parity and charge conjugation in weak interactions. From measurements on beta, muon, pion and kaon decay and the assumptions of the two-component neutrino and the law of conservation of leptons, it is concluded that the only possible universal fourfermion interaction is an equal admixture of vector plus axial vector interaction. Several experiments appear to contradict this hypothesis and their status is reexamined.

One verifies immediately that there is no choice of coupling types consistent with all the experimental data. For example, the measurement of angular correlations in mixed transitions as well as the measurements of emission asymmetry or B-y correlations in mixed transitions favor a VA or ST combination; but the measurements of angular correlation in He6 and A35 give T, V respectively. Nevertheless, the fact that in practically all the measurements of pseudoscalars the values obtained are consistent with the maximum value possible must be highly significant.
The choice between AV and ST thus hinges essentially on the electron-neutrino angular correlations of equivalently on the determination of the spirality of the neutral particle emitted in beta decay. As regards the electron-neutrino angular correlations, this implies a choice between the A35 and He6 experiments. It seems fair to suggest that both the A35 and He6 experiments ought to be redone. x [footnote] One possibility is for the ground state spin of He6 to be 1 instead of 0. The transition would then become a mixed 1-1 transition and the angular correlation would then become consistent with the VA assignment. However, a spin of 1 for He6 would violate our present views of nuclear structure.
While it is clear that a mixture of vector and axial vector is the only universal four-fermion interaction which is possible and possesses many elegant features, it appears that one published and several unpublished experiments cannot be reconciled with this hypothesis. These experiments are: (a) The electron neutrino angular correlation in He6 (Rustad and Ruby).... All of these experiments should be redone, particularly since some of them contradict the result of other recent experiments on the weak interactions. If any of the above four experiments stands, it will be necessary to abandon the hypothesis of a universal V+A four fermion interaction or either or both of the assumptions of a two-component neutrino and/or the Law of Conservation of Leptons.