Sterile neutrino

Particles that possess the quantum numbers of sterile neutrinos and masses great enough such that they do not interfere with the current theory of Big Bang nucleosynthesis are often called neutral heavy leptons (NHLs) or heavy neutral leptons (HNLs).

[4] To comply with theories of leptogenesis and dark matter, there must be at least 3 flavors of sterile neutrinos (if they exist).

[12] However, a particle with mass that starts out with left-handed chirality can develop a right-handed component as it travels – unless it is massless, chirality is not conserved during the propagation of a free particle through space (nominally, through interaction with the Higgs field).

[13] If the Standard Model is embedded in a hypothetical SO(10) grand unified theory, they can be assigned an X charge of −5.

That makes it possible to produce them in experiments, if they are light enough to be within the reach of current particle accelerators.

, spontaneously breaking its SU(2)L × U(1) symmetry, and thus yielding non-zero Yukawa couplings: Such is the case for charged leptons, like the electron, but within the Standard Model the right-handed neutrino does not exist.

[a] Sterile neutrinos allow the introduction of a Dirac mass term as usual.

Similar problems (although less severe) are observed in the quark sector, where the top and bottom masses differ by a factor of 40.

For Dirac neutrinos, the dipole moments are proportional to mass and would vanish for a massless particle.

terms provide a route for some small part of the sterile neutrinos' enormous mass,

Apart from empirical evidence, there is also a theoretical justification for the seesaw mechanism in various extensions to the Standard Model.

For a particle to be considered a dark matter candidate, it must have non-zero mass and no electromagnetic charge.

[17] Naturally, neutrinos and neutrino-like particles are of interest in the search for dark matter because they possess both these properties.

The active neutrinos of the Standard Model, having very low mass (and therefore very high speeds) are therefore unlikely to account for all dark matter.

Firstly, in order to produce the structure of the universe observed today the mass of the sterile neutrino would need to be on the keV scale, based on parameter space of the remaining supersymmetric models that have not yet been excluded by experiment.

[19] Secondly, while it is not required that dark matter be stable, the lifetime of the particles must be longer than the current age of the universe.

This places an upper bound on the strength of the mixing between sterile and active neutrinos in the seesaw mechanism.

If they are indeed a constituent of dark matter, sensitive X-ray detectors would be needed to observe the radiation emitted by their decays.

On 11 April 2007, researchers at the MiniBooNE experiment at Fermilab announced that they had not found any evidence supporting the existence of such a sterile neutrino.

[25] More-recent results and analysis have provided some support for the existence of the sterile neutrino.

[27] Daya Bay has also searched for a light sterile neutrino and excluded some mass regions.

The number of neutrinos and the masses of the particles can have large-scale effects that shape the appearance of the cosmic microwave background.

The Planck Satellite 2013 data release is compatible with the existence of a sterile neutrino.

In June 2022, the BEST experiment released two papers observing a 20–24% deficit in the production of the isotope germanium expected from the reaction 71Ga + νe → e− + 71Ge.

The so-called "Gallium anomaly" suggests that a sterile neutrino explanation could be consistent with the data.

[33][34][35] In January 2023, the STEREO experiment published its final result, reporting the most precise measurement of the antineutrino energy spectrum associated with the fission of uranium-235.

The data is consistent with the Standard Model and rejects the hypothesis of a light sterile neutrino with a mass of around 1 eV.

[36] In 2023 results of searches by the CMS set new limits for sterile neutrinos with masses of 2–3 GeV.