What Is a Neutrino?

Wolfgang Pauli first postulated the neutrino in 1930 to explain missing energy in beta decay, and Enrico Fermi named it (Italian for "little neutral one"). Clyde Cowan and Frederick Reines experimentally confirmed neutrinos in 1956. Three "flavours" exist — electron, muon, and tau neutrinos — and they can oscillate between flavours as they travel, a discovery that proved they have a tiny but non-zero mass. About 100 trillion neutrinos from the Sun pass through every square metre of Earth every second, yet they so rarely interact with matter that a neutrino could travel through a light-year of solid lead with only about a 50% chance of being stopped. Massive detectors like IceCube (a cubic kilometre of Antarctic ice) and Super-Kamiokande (50,000 tonnes of ultra-pure water) are needed to catch even a handful.