Light sterile neutrinos

Most extensions of the standard model predict the existence of sterile neutrinos. For example, simple $SO(10)$ and $E_6$ 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 ($10^{-3}$ eV) Kaluza-Klein modes associated with a large (mm) extra dimension [15].

Figure: Neutrino oscillation plot, indicating the solar neutrino small mixing angle (SMA), large mixing angle (LMA), low mass (LOW), and vacuum (VAC) solutions, as well as the Superkamiokande atmospheric neutrino range, the LSND region, and various exclusion regions. From [16].