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 $\tau^{-1}$, 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 $\tau \ga 10^{18} \,{\rm sec}(1 \,{\rm eV}/m_\nu^2)$, transition moment of neutrinos $\mu_{12}$ is constrained to be $\le 4 \hbox{--}8 \times 10^{-17} \,\mu_{\sc B}$ for $110 \,{\rm eV} \la m_\nu \la 10 \,{\rm keV}$. We compare this bound with other astrophysical and cosmological bounds on radiatively decaying neutrinos.