Abstract
The possible discovery of a 17 keV neutrino in β-decay experiments is a challenge to both astrophysics and cosmology. Should such a species exist, its contribution to the present mass density would play havoc among the heavens unless that neutrino decays with a life-time shorter than 30 000 yr. If so, its radiative decays should neither overcool red giant stars, nor exceed the upper bound set upon the γ-ray flux from supernova 1987A by the Solar Maximum Mission satellite. In addition, radiative decays should not distort the microwave background radiation, whose spectrum is quasi-planckian as observed by COBE. Combining those various arguments, we conclude that the branching ratio B γ to photons cannot exceed 4×10 −5. Such a limit severely constraints the most naive models of the 17 keV neutrino, and points towards unconventional physics.
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