Search For A Heavy Neutrino In The Beta-Decay Of S-35

Recent experimental evidence1 suggests that the neutrino (strictly, an antineutrino) emitted in the β-decay of tritium is a mixture of two mass eigen states ∣ν1〉 and ∣ν2〉 where ∣νe〉=cos θ∣ν1〉 + sin θ∣ν2〉 with <50 eV and =17.1±0.2 keV and sin2 θ=0.03. This conclusion is based on the observation of a ‘kink’ in the Fermi–Kurie plot2,3 of the electron energy spectrum at an energy 17.1 keV below the end point. Such an observation, if confirmed, will have far-reaching consequences in cosmology and particle physics4,5. In the experiment reported here, a search was made for such an admixture in the β-decay of 35S. We fail to confirm the presence of mixing of neutrinos with mass 17.1 keV and place an upper limit on such an admixture of 0.6% (90% confidence level).