Neutrino, parity violation, V-A: a historical survey
This is a concise story of the rise of the four fermion theory of the universal weak interaction and its experimental confirmation, with a special emphasis on the problems related to parity violation.
V-A was in a very good agreement with most of the experiments but its authors had to make the bold assumption that the predictions of the rest were actually erroneous; needless to say, this required a thorough work. Sudarshan and Marshak [SM1] concluded: ”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interaction which is possible and possesses many elegant features, it appears that one published [RR53] 25 and several unpublished experiments cannot be reconciled with this hypothesis... All of these experiments should be redone... If any of the above four experiments stands, it will be necessary to abandon the hypothesis of a universal V+A four-fermion interaction...” 26 .
In their review [G-MR] published in December 1957 (the survey of literature pertaining to the review being completed in July, 1957) Gell-Mann and Rosenfeld made the following comment: ”... there has been speculation that the form of the interaction might also be ’universal’. Such a situation seems to be ruled out if the β -decay coupling is primarily S and T and the µ decay coupling V and A... Since the β -decay picture is somewhat confused at the moment, let us discuss briefly the possibility that we may have V and A there too, instead of S and T with a possible admixture of V. We may call this V, A hypothesis the ’last stand’ of the UFI... We must first of all disregard much of the evidence on27 e − ν angular correlation in β -decay, especially the result of Rustad & Ruby on He 6 , which clearly indicates T rather than A. This is already a very serious objection to the UFI.”
Discussing the universality of the weak interaction with V-A in [FG-M], Feynman and Gell-Mann also stressed that ”... At the present time several β -decay experiments seem to be in disagreement with one another. Limiting ourselves to those that are well established, we find that the most serious disagreement with our theory is the recoil experiment in He 6 of Rustad and Ruby 28 indicating that the T interaction is more likely than the A. Further check on this is obviously very desirable.” Feynman’s picturesque story in [FL, Me] about experimental points at the edge of the data range reflects his personal battle with this problem.