Sterile Neutrinos
Sterile neutrinos are hypothetical particles that are believed to 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 carry an isospin charge of ±+1/2 under the weak interaction. It typically refers to neutrinos with right-handed chirality, which may be added to the Standard Model. 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 or heavy neutral leptons. The existence of right-handed neutrinos is theoretically well-motivated because all other known fermions have been observed with both left and right chirality. They could also explain in a natural way the small active neutrino masses inferred from neutrino oscillation.
