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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
. We consider a model in which neutrinos predominantly decay into
invisible relativistic particles with a rate , 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 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.Comment: uuencoded file containing TeX file and postscript figure