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.
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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.