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

Abstract

Recent experimental evidence suggests that the neutrino (strictly, an antineutrino) emitted in the β-decay of tritium is a mixture of two mass eigen states |_v1 and |_v2 where |_ve=cosθ |_v1 + sinθ |_v2 with m_v1<50 eV and m_v2=17.1±0.2 keV and sin²θ =0.03. This conclusion is based on the observation of a 'kink' in the Fermi-Kurie plot 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. 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).