Search for a heavy neutrino in the β-decay of ³⁵S

Abstract

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 ∣ν_e〉=cos θ∣ν₁〉 + sin θ∣ν₂〉 with $m_{{\nu }_1}$<50 eV and $m_{{\nu }_2}$=17.1±0.2 keV and sin² θ=0.03. This conclusion is based on the observation of a ‘kink’ in the Fermi–Kurie plot^2,3 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 physics^4,5. 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).