= Neutrino dark-matter longevity constraint
A proposed <sterile neutrino> component of <dark matter> must survive for cosmological times. If its decay rate is $\Gamma$, its lifetime $\tau=1/\Gamma$ must greatly exceed the time over which the required abundance is to persist. Otherwise the assumed present-day dark-matter density would be depleted and the decay products could alter cosmological observations.
Mixing with active <neutrinos> can permit decays even when the sterile state has no direct ordinary weak gauge coupling. The same mixing can therefore affect both its production and its survival. A lifetime check must use the allowed channels for the stated <mass> and model, rather than inferring stability from electrical neutrality or <Majorana fermion> character. Cosmological abundance and constraints on observable decay products impose additional requirements beyond longevity alone.
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