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> a proposed additional type of neutrino, called a sterile neutrino

Does this imply the existence of a new lepton?

Do you mean a new generation of matter, with two new leptons and two new quarks?

Cosmological data show that there should be only three generations of matter.

> Do you mean a new generation of matter, with two new leptons and two new quarks?

Yeah, that's what I was getting at: one of the leptons being the sterile neutrino and the other being the "sterile particle".