Experimental limits for heavy neutrino admixture deduced from ¹⁷⁷Lu β decay and constraints on the life time of a radiative neutrino decay mode

From cosmological constraints, the requirement for stable neutrinos is to have masses less than 30 eV. In the case that neutrino masses exceed this bound, neutrinos must decay sufficiently fast in order to satisfy the presently observed energy density of the universe. The experiments presented in this contribution consist of the complementary search for heavy neutrino admixture in nuclear beta decay of ¹⁷⁷Lu and of the search for a radiative neutrino decay mode at the nuclear power station in Bugey, France. The data obtained from the ¹⁷⁷Lu beta decay restrict the mixing probability of a heavy neutrino to the electron |U_eh|² < 0.2 - 0.3% (90% Cl) for neutrino masses between 10 and 95 keV. The radiative lifetime is constrained to exceed t/m > 180 × |U_eh|² sec/eV which is one order of magnitude more restrictive than previous laboratory limits.