1998: Discovery that Neutrinos Have Small Mass

Detection of Neutrinos and the Super Kamiokande detector.

What is a Neutrino?

A neutrino is a subatomic particle with no charge. There are three flavors: Electron, Muon, and Tau. Each Neutrino type correlates with a charged particle. Neutrinos go near light speed and have no charge. They interact with the weak interaction and do not take part in electromagnetic interactions. This allows them to travel great distances without interacting with much matter. Neutrinos are produced during decay processes of radioactive material.

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The Super-Kamiokande detector is a large Cherenkov radiation detector. It consists of a huge tank of pure water with detectors lining the inside. When a charged particle goes through the water faster than the speed of light in water, Cherenkov radiation is emitted. This is used to detect neutrino interactions (and their different flavors). The Tau neutrino is the last of the flavors to be detected.

Neutrino Oscillation and Mass

Detectors detected only a certain percentage of certain flavors of neutrinos which they were expecting. This was explained by neutrino oscillation, which is when a neutrino changes flavors. This can only be done if neutrinos have nonzero mass. The Super-Kamiokande detector backed these findings up, and showed that indeed neutrinos must have nonzero mass. The announcement of the discovery of neutrino oscillations (and thus neutrino mass being non-zero) was in 1998.