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A Weighty Discovery in Particle Physics

In June 1998, an international team of Japanese and U.S. physicists unveiled strong evidence that elusive subatomic particles known as neutrinos have mass (or weight). These findings run counter to the standard model of particle 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 theorized totals, while the number of muon-neutrinos was significantly lower. This indicated that they were disappearing into another state, or “flavor,” such as an undetected tau-neutrino, or possibly another type.

Theorists expected that two-thirds of the neutrinos detected would be of muon “flavor,” and one-third would be electron-neutrinos, but the experiment resulted in too few muon-neutrinos.

The scientists concluded that the only explanation that made sense, given the data, is that muon-neutrinos were oscillating, or changing from one type of neutrino to another, which could occur only if they have mass.

The new findings may help astrophysicists trying to find the missing matter in the universe. Some estimates conclude that perhaps 90 percent of the universe's mass is “missing,” much of it assumed to be invisible dark matter that emits no light and is therefore not visible with current observing equipment. The missing mass may eventually be found to exist in the form of neutrinos.


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