Selected Citation
Helicity of Neutrinos
A combined analysis of circular polarization and resonant scattering of γ rays following orbital electron capture measures the helicity of the neutrino. We have carried out such a measurement with Eu152m, which decays by orbital electron capture. If we assume the most plausible spin-parity assignment for this isomer with its compatible decay scheme, ' , 0 —, we find that the neutrino is "left-handed, i. e. a„p„= 1 (negative helicity) .
In all formulations of β-decay theory no distinction is made between the neutrino emitted in orbital electron capture and that emitted in P+ decay. Taken together with the fact that the helicity of the positrons in a Gamow-Teller transition is positive4 or with the fact that positrons are emitted from oriented nuclei in the direction in which the nuclear spin is pointing, ' our result indicates that the Gamow-Teller interaction is axial vector (A) for positron emitters, in agreement with the conclusions of Hermannsfeldt et al.' These authors show that all recoil experiments with p+ emitters are compatible with AV, but not with TS interactions which have been reported for P emitters (largely based on He' recoil experiments). The AV combination may be compatible with lepton conservation and a universal Fermi interaction as pointed out by Sudarshan and Marshak' and by Feynman and Gell-Mann. This view is strengthened by the recent results showing positive helicity for the positrons from p+ decay. It would therefore seem desirable to apply the method described here to a p emitter in order to determine the helicity of the antineutrino. Although the analysis of such an experiment is considerably more complicated, it may prove possible to reach a decision between A and T, which is independent of the "classical" recoil experiments.