neutrin

Neutrin is a hypothetical elementary particle posited in certain beyond-Standard-Model theories. The term is used to denote a light, electrically neutral fermion that shares some properties with the known neutrinos but is distinguished by its origin, mass scale, or interaction structure. In many models, a neutrin behaves as a gauge-singlet state that can mix with active neutrinos, leading to observable consequences in oscillation experiments and cosmology.

Properties commonly attributed to a neutrin include spin 1/2, absence of electric charge, and extremely weak interactions with ordinary matter. Depending on the model, the neutrin can be Dirac or Majorana in nature, and its mass is typically assumed to be well below the electroweak scale, though some scenarios place it in the keV to eV range. The neutrin may participate in weak interactions through mixing with active neutrinos or through new, so-called sterile channels, linking it to the broader framework of neutrino mass generation.

In cosmology and astrophysics, neutrin-like states are studied as potential contributors to the relic radiation, dark matter, or to explanations of neutrino anomalies. Experimental searches focus on precision beta decay spectra, neutrino oscillation data, and missing-energy signatures in colliders and meson decays. Detecting a neutrin would require observing deviations from the Standard Model predictions attributable to mixing, sterile interactions, or altered decay kinematics.

See also: neutrino, sterile neutrino, beyond-Standard-Model physics.