Sterile neutrinos
| Composition | Elementary particle |
|---|---|
| Statistics | Fermionic |
| Generation | unknown |
| Interactions | gravity; other potential unknown interactions |
| Status | Hypothetical |
| Types | unknown |
| Mass | unknown |
| Electric charge | 0 |
| Color charge | none |
| Spin | 1/2 |
| Spin states | 2 |
| Weak isospin projection | 0 |
| Weak hypercharge | 0 |
| Chirality | right handed |
| B − L | depends on L charge assignment |
| X | −5 |
Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that interact only via gravity and do not interact via any of the fundamental interactions of the Standard Model. The term sterile neutrino is used to distinguish them from the known active neutrinos in the Standard Model, which are charged under the weak interaction.
This term usually refers to neutrinos with right-handed chirality (see right-handed neutrino), which may be added to the Standard Model. Occasionally it is used in a more general sense for any neutral fermion.
The existence of right-handed neutrinos is theoretically well-motivated, as all other known fermions have been observed with left and right chirality, and they can explain the observed active neutrino masses in a natural way. The mass of the right-handed neutrinos themselves is unknown and could have any value between 1015GeV and less than one eV.
The number of sterile neutrino types is unknown. This is in contrast to the number of active neutrino types, which has to equal that of charged leptons and quark generations to ensure the anomaly freedom of the electroweak interaction.
The search for sterile neutrinos is an active area of particle physics. If they exist and their mass is smaller than the energies of particles in the experiment, they can be produced in the laboratory, either by mixing between active and sterile neutrinos or in high energy particle collisions. If they are heavier, the only directly observable consequence of their existence would be the observed active neutrino masses. They may, however, be responsible for a number of unexplained phenomena in physical cosmology and astrophysics, including dark matter, baryogenesis or dark radiation.
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