A neutral fermion without the ordinary active-neutrino weak gauge couplings. Mixing can still let it interact or decay. Its possible dark-matter role depends on lifetime, production and free streaming, rather than on Majorana character alone.
A proposed sterile neutrino component of dark matter must survive for cosmological times. If its decay rate is , its lifetime 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.
A right-handed neutrino can transform as under colour, weak isospin and hypercharge. It has no direct Standard Model gauge coupling, but may mix with active neutrinos. Sterility under the low-energy group does not imply sterility under an enlarged unbroken gauge group.
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A sterile neutrino is a hypothetical type of neutrino that does not interact via the standard weak interactions like the three known types of neutrinos: electron neutrinos, muon neutrinos, and tau neutrinos. Instead, sterile neutrinos are proposed to interact only through gravity and possibly via mixing with active neutrinos, making them "sterile" because they do not participate in the weak force.