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.