NEW ASTROPHYSICAL CONSTRAINT ON RADIATIVELY DECAYING NEUTRINOS

We calculate constraints on radiatively decaying neutrinos from the re cent detection of singly ionized helium in the diffuse intergalactic m edium (IGM) at z similar or equal to 3.3. We consider a model in which neutrinos predominantly decay into invisible relativistic particles w ith a rate tau(-1), and with a small branching ratio into the radiativ e mode. To satisfy the observation, which puts a lower bound on the nu mber density of singly ionized helium atoms in the IGM, we show that, for tau greater than or equal to 10(18) sec (1 eV/m(nu)(2)), transitio n moment of neutrinos, mu(12), is constrained to be less than or equal to 4-8 x 10(-17) mu(B) for 110 eV less than or equal to m(nu)less tha n or equal to 10 keV. Implications of this bound for particle physics and cosmology are briefly discussed.