![]() ![]() 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). ![]() (See Chirality (physics)#Chirality and helicity for the difference.)Ĭhirality is a fundamental property of particles and is relativistically invariant: It is the same regardless of the particle's speed and mass in every inertial reference frame. ![]() Yet all neutrinos have been observed with left-handed chirality, and all antineutrinos right-handed. This unexpected mass explains neutrinos with right-handed helicity and antineutrinos with left-handed helicity: Since they do not move at the speed of light, their helicity is not relativistic invariant (it is possible to move faster than them and observe the opposite helicity). Recent experiments such as neutrino oscillation, however, have shown that neutrinos have a non-zero mass, which is not predicted by the Standard Model and suggests new, unknown physics. These are the only helicities (and chiralities) allowed in the Standard Model of particle interactions particles with the contrary helicities are explicitly excluded from the formulas. In the massless limit, it means that only one of two possible chiralities is observed for either particle. See also: Neutrino: Chirality and Neutrino oscillationĮxperimental results show that all produced and observed neutrinos have left-handed helicities (spin antiparallel to momentum), and all antineutrinos have right-handed helicities, within the margin of error. However, results of the MicroBooNE experiment showed no evidence of sterile neutrinos in October 2021. In May 2018, physicists of the MiniBooNE experiment reported a stronger neutrino oscillation signal than expected, a possible hint of sterile neutrinos. They may, however, be responsible for a number of unexplained phenomena in physical cosmology and astrophysics, including dark matter, baryogenesis or hypothetical dark radiation. If they are heavier, the only directly observable consequence of their existence would be the observed active neutrino masses. 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. The search for sterile neutrinos is an active area of particle physics. This is in contrast to the number of active neutrino types required to ensure the electroweak interaction is free of anomalies, which must be exactly 3: the number of charged leptons and quark generations. To comply with theories of leptogenesis and dark matter, there must be at least 3 flavors of sterile neutrinos (if they exist). The mass of the right-handed neutrinos themselves is unknown and could have any value between 10 15 GeV and less than 1 eV. They could also explain in a natural way the small active neutrino masses inferred from neutrino oscillation. The existence of right-handed neutrinos is theoretically well-motivated, because the known active neutrinos are left-handed and all other known fermions have been observed with both left and right chirality. 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). ![]() The term typically refers to neutrinos with right-handed chirality (see right-handed neutrino), which may be inserted into the Standard Model. The term sterile neutrino is used to distinguish them from the known, ordinary active neutrinos in the Standard Model, which carry an isospin charge of ± + 1 / 2 and engage in the weak interaction. Sterile neutrinos (or inert neutrinos) are hypothetical particles (neutral leptons – neutrinos) that are believed to interact only via gravity and not via any of the other fundamental interactions of the Standard Model. Gravity other potential unknown interactions Hypothetical particle that interacts only via gravity Sterile neutrino, right-handed neutrino Composition ![]()
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