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In June 1998, an international team of Japanese and US physicists unveiled strong evidence that elusive subatomic particles known as neutrinos have mass. These findings run counter to the standard model of particles physics – the basic theory about the structure of matter – which holds that these electrically neutral, weakly interacting particles have no mass. The discovery means that existing theoretical models of matter must now be revised to include neutrinos with mass.
Neutrinos occur in three states: electron, muon, and tau, with the names signifying what is produced when a neutrino collides with another particle. Observers do not see the neutrinos themselves, but can detect the creation of electrons and muons from faint flashes of light following a particle collision.
The physicists used the giant Super-Kamiokande – the world’s biggest neutrino detector buried deep underground in Mozumi, Japan. In the experiment, conducted in a 50,000-ton tank of purified water, neutrinos created when cosmic rays bombard Earth’s upper atmosphere were counted relative to the number expected to penetrate the cavern. The experimenters found that the number of electron-neutrinos detected was relatively constant with the theorized totals, while the number of muon-neutrinos was significantly lower. This indicated that they were disappearing into another state, of “flavor”, such as undetected tau-neutrino, or possibly another type.
