Physicists Find Evidence That Neutrinos Have Mass

June 05, 1998

A physics collaboration that includes a team from the University of Washington has unveiled evidence indicating that subatomic particles known as neutrinos have mass.

The findings counter assumptions in the Standard Model of particle physics, which has held that the electrically neutral, weakly interacting particles have no mass.

Physicists working deep underground in the Super-Kamiokande Observatory, about 125 miles northwest of Tokyo, have concluded that muon-neutrinos are oscillating, or changing from one type of neutrino to another, something that could occur only if they have mass.

The results were announced at Neutrino '98, an international physics conference being held in Takayama, Japan.

The discovery means that neutrinos with mass must now be included in theoretical models for the structure of matter, and are an indication of "a broken symmetry" in existing models, said UW Physics Research Professor Jeffrey Wilkes.

"Such deviations from simplicity in nature are usually a good sign that there is much more to be learned," he said.

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.

In the Super-Kam experiment, conducted in a 50,000-ton tank of highly purified water, neutrinos created when cosmic rays bombard the earth's upper atmosphere are counted relative to the number expected to penetrate the cavern. The number of electron-neutrinos detected is relatively constant with theorized totals, while the number of muon-neutrinos is significantly lower. That indicates they are disappearing into another state, or "flavor," such as an undetected tau-neutrino, or possibly another type, such as a sterile-neutrino.

Theorists expect that two-thirds of the neutrinos detected would be of the muon "flavor" and one-third would be electron-neutrinos, but the experiment is seeing too few muon-neutrinos, Wilkes said.

"Our conclusion is that the only explanation that makes sense, given our data, is neutrino oscillation," he said.

The Super-Kam team also said the findings should be considered seriously by astrophysicists who are trying to find the "missing" matter in the universe. Some estimates are that perhaps 90 percent of the universe's mass is "missing," much of it assumed to be dark matter that emits no light and therefore is not visible with current observing equipment.

Wilkes and Physics Professor Kenneth Young, along with post-doctoral researcher Larry Wai, graduate students Jeff George and Andy Stachyra, and research engineer Hans Berns, make up the UW contingent in the experiment, which began operating in April 1996 in a zinc mine about 3,250 feet beneath the surface. About half the 100 researchers involved in Super-Kam come from several U.S. institutions. The rest are from Japan, which is providing 90 percent of the funding.

The other U.S. institutions with significant involvement in the project are Boston University, the University of California at Irvine, the University of Hawaii, Louisiana State University, the University of Maryland and the State University of New York at Stoney Brook.

For more information, contact Wilkes at or (206) 543-4232.

University of Washington

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