Neutrinos

A neutrino is an elementary particle with no electric charge that interacts extremely weakly with matter. Neutrinos belong to the lepton family, have spin one-half, and occur in three flavours associated with the electron, muon and tau. Their distinctive combination of electrical neutrality, weak interactions and very small masses makes them difficult to detect.

Particle identity and quantum properties

Neutrinos are matter particles, not force carriers. Their properties distinguish them from charged leptons, quarks and the bosons that mediate interactions.

• Their spin of one-half makes them fermions, so they obey the Pauli exclusion principle, unlike integer-spin bosons.
• They have zero electric charge and carry no colour charge, unlike electrically charged leptons and strongly interacting quarks.
• The three active flavours are the electron neutrino, muon neutrino and tau neutrino; each is associated with its corresponding charged lepton in weak interactions.
• Whether neutrinos are distinct from their antiparticles, as Dirac particles are, or are their own antiparticles, as Majorana particles are, remains unresolved.

Weak interactions and penetration through matter

Neutrinos interact with ordinary matter through the weak interaction, rather than through the electromagnetic or strong interactions that dominate many other particle processes. Consequently, they can pass through enormous amounts of matter with a very low probability of interacting.

• In charged-current interactions, mediated by W bosons, a neutrino can produce its corresponding charged lepton when sufficient energy is available.
• In neutral-current interactions, mediated by the Z boson, a neutrino can exchange energy and momentum while remaining a neutrino.
• Detection therefore relies on observing the secondary particles or other signals produced in rare neutrino interactions, rather than an electrically charged track left by the neutrino itself.

Small masses and flavour oscillations

Neutrino oscillation means that a neutrino produced with one flavour can later be detected with another. This happens because flavour states are quantum mixtures of states with definite masses, which evolve differently during propagation.

• Oscillations establish that neutrino masses are not all equal and that at least some neutrinos have non-zero mass.
• Oscillation measurements determine differences between squared masses, not the absolute mass scale; neutrino masses are much smaller than those of charged leptons.
• The minimal Standard Model treats neutrinos as massless, so explaining their observed masses requires extending that minimal formulation.