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Neutrinos exhibit multiple personalities

6 December 2002

By Bob Sanders, Media Relations

BERKELEY - Neutrinos have always seemed weird, zipping around at nearly the speed of light and passing through matter as if it were not even there. Now they're getting weirder. Results of a Japan-based experiment confirm previous suspicions that neutrinos change their identities as they wing through space, and that they have a small, but measurable, mass.

"Our results make the case for neutrino oscillation and mass seemingly inescapable," said Stuart Freedman, a co-spokesperson for the U.S. team that announced the results Friday, Dec. 6. Freedman is a nuclear physicist and professor of physics at UC Berkeley with a joint appointment at the Lawrence Berkeley National Laboratory (LBNL).

The results come from an experiment called KamLAND, or Kamioka Liquid scintillator Anti-Neutrino Detector, located in a mine cavern beneath the mountains of Japan's main island of Honshu, near the city of Toyama. The largest low-energy anti-neutrino detector ever built, KamLAND looked for neutrinos' mirror-image particles, called anti-neutrinos, emanating from 51 nuclear reactors in Japan plus 18 reactors in South Korea. During 145 days of operation, KamLAND detected slightly less than 2/3 of the anti-neutrinos predicted to come from these reactors.

Because of this discrepancy, the Standard Model of physics that predicted the expected anti-neutrino flux is in need of updating, the 92-member research team conclude. The necessary revisions must account for the fact that anti-neutrinos, and thus neutrinos, change flavor with time, oscillating among the three known types, electron, muon and tau. This means, too, that neutrinos have a slight mass, though the team was not able to estimate how large.

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