neutrino

History

The neutrino was predicted by Wolfgang Pauli in 1930 to explain the mysterious energy loss in beta decay (when a nucleus emits an electron). They were first detected in 1956 in the 'Poltergeist' experiment at a nuclear reactor.

How it works

Occasionally a neutrino collides with an atomic nucleus, turning it into another element. Detectors are built on this: huge tanks of water where such collisions produce weak light flashes, which are registered by sensitive instruments.

History

The neutrino was predicted by Wolfgang Pauli in 1930 to explain the mysterious energy loss in beta decay (when a nucleus emits an electron). They were first detected in 1956 in the 'Poltergeist' experiment at a nuclear reactor.

How it works

Occasionally a neutrino collides with an atomic nucleus, turning it into another element. Detectors are built on this: huge tanks of water where such collisions produce weak light flashes, which are registered by sensitive instruments.

History

In 1930, Wolfgang Pauli in a letter to 'radioactive ladies and gentlemen' postulated the existence of a light neutral particle to save the law of energy conservation in beta decay. Enrico Fermi in 1934 built the theory of weak interaction and introduced the term 'neutrino'. The first direct detection (electron antineutrinos from a reactor) was achieved by Clyde Cowan and Frederick Reines in 1956 in the 'Poltergeist' experiment. Later, the discovery of muon (1962) and tau (2000) neutrinos completed the picture. Experiments at Super-Kamiokande (1998) and SNO (2001) proved neutrino oscillations, which became the first direct indication of physics beyond the Standard Model.

How it works

Neutrinos are emitted in weak interactions, where W- or Z-bosons transform one particle into another. For example, in beta decay of a neutron (n → p + e⁻ + ν̄ₑ), an antineutrino is born. Due to the extremely small probability of interaction, neutrinos are registered in massive detectors: a neutrino strikes a nucleus or an electron, creating a charged lepton that emits Cherenkov or scintillation light. Oscillations arise because flavor states are superpositions of mass states. As the wave function propagates, it accumulates different phases for different mass components, leading to periodic changes in the probability of detecting a particular flavor — this effect is described by the oscillation formula.

Open problems & fun facts

What is the absolute mass scale of neutrinos and what is their hierarchy (normal or inverted)?; Are neutrinos Majorana particles (i.e., are they identical to their antiparticles)? This will be tested by neutrinoless double beta decay.; Is CP symmetry violated (asymmetry between particles and antiparticles) in the neutrino sector?; What is the exact role of neutrinos in supernova nucleosynthesis and in the formation of large-scale structure of the Universe?

💡 To have a 50% probability of interacting with matter, a neutrino would need to fly through a lead wall one light-year thick.