Most extensions of the standard model predict the existence of sterile
neutrinos. For example, simple and
grand unified theories
predict one or two sterile neutrinos per family, respectively. The only real
questions are whether the ordinary and sterile neutrinos of the same chirality mix
significantly with each other, whether some or all of
the mass eigenstate neutrinos are
sufficiently light, and how many
(e.g., whether there are 1, 3, 6 or some other number of
light sterile neturinos).
When there are only Dirac masses, the ordinary and sterile states do not mix because of the conserved lepton number. Pure Majorana masses do not mix the ordinary and sterile sectors either. In the seesaw model the mixing is negligibly small, and the (mainly) sterile eigenstates are too heavy to be relevant to oscillations. The only way to have significant mixing and small mass eigenstates is for the Dirac and Majorana neutrino mass terms to be extremely small and to also be comparable to each other. This appears to require two miracles in conventional models of neutrino mass.
One promising possibility involves the generation of neutrino
masses from higher-dimensional operators in theories involving
an intermediate scale [8],
as described in Section .
Another [14] involves sterile neutrinos
associated with a parallel hidden sector of nature, as suggested in
some superstring and supergravity theories. Yet another associates
the sterile neutrino with the light (
eV) Kaluza-Klein modes associated
with a large (mm) extra dimension [15].
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