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A New Astrophysical Constraint on Radiatively Decaying Neutrinos

We calculate constraints on radiatively decaying neutrinos from the recent detection of singly ionized helium in the diffuse intergalactic medium (IGM) at $z \simeq 3.3$. 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 of singly ionized helium, which puts a lower bound on the number density of singly ionized helium in the IGM, we show that: for $τ\ga 10^{18} \,{\rm sec}(1 \,{\rm eV}/m_ν^2)$, transition moment of neutrinos $μ_{12}$ is constrained to be $\le 4 \hbox{--}8 \times 10^{-17} \,μ_{\sc B}$ for $ 110 \,{\rm eV} \la m_ν\la 10 \,{\rm keV}$. We compare this bound with other astrophysical and cosmological bounds on radiatively decaying neutrinos.