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A sterile neutrino is an neutrino that does not interact via any of the fundamental interactions of the Standard Model. Such a particle belongs to a singlet representation with respect to the strong interaction and the weak interaction and has zero weak hypercharge. Sterile neutrinos would still interact via gravity, so if they are heavy enough, they could explain cold dark matter.
In the Standard Model, sterile neutrinos may mix with ordinary neutrinos via a Dirac mass. The sterile neutrinos and ordinary neutrinos may also have Majorana masses. In certain models, both Dirac and Majorana masses are used in a seesaw mechanism, which drives ordinary neutrino masses down and makes the sterile neutrinos extremely heavy (around the GUT scale ~1012 GeV).
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