New astrophysical constraint on radiatively decaying neutrinos

Abstract

We calculate constraints on radiatively decaying neutrinos from the recent detection of singly ionized helium in the diffuse intergalactic medium (IGM) at z~=3.3. We consider a model in which neutrinos predominantly decay into invisible relativistic particles with a rate τ-1, and with a small branching ratio into the radiative mode. To satisfy the observation, which puts a lower bound on the number density of singly ionized helium atoms in the IGM, we show that, for τ>~1018 sec (1 eV/m2ν), transition moment of neutrinos, μ12, is constrained to be <=4-8×10-17 μB for 110 eV<~mν<~10 keV. Implications of this bound for particle physics and cosmology are briefly discussed.