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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.

In the MSSM, superpartners change the three Standard Model gauge renormalization-group beta functions so that the inverse gauge couplings meet accurately near $2\times 10^{16}$ GeV, up to threshold and higher-loop corrections. The corresponding Standard Model one-loop lines miss a common intersection.