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Scientists Discover 'Missing' Neutrinos
Canadian Mine Yields Major Find

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By Guy Gugliotta
Washington Post Staff Writer
Sunday, April 21, 2002; Page A11

More than a mile deep within the bowels of a Canadian nickel mine, scientists for the first time have counted all the solar neutrinos that are hitting the Earth, researchers announced yesterday.

Neutrinos are elusive, subatomic particles so small that thousands of them pass unimpeded through every human being every second.

The research, at the Sudbury Neutrino Observatory, near Sudbury, Ontario, demonstrated conclusively that there is no "solar neutrino deficit," scientists said. Neutrinos produced by the sun are reaching the Earth instead of mysteriously vanishing en route, as some scientists had theorized.

"Previous experiments have only seen one-third to one-half of what there is," said the University of Pennsylvania's Eugene Beier, one of an army of physicists involved in the experiment. "For the first time, we've been able to measure all of the neutrinos at once. You've got it all."

The new neutrino research has important implications for two of the enduring quandaries of physics. The Standard Model proposes a massless neutrino as part of its description of how nature works at its most basic level, but yesterday's announcement provided fresh confirmation that neutrinos have mass. Now, Beier said, "the Standard Model will need new parameters."

At the same time, even though neutrinos have a tiny mass -- only as much as one ten-millionth of an electron -- they may contribute to an understanding of "dark matter," the view shared by astronomers that 90 percent of the matter in the universe is in an unknown, invisible form.

Yet neutrinos' contribution is not large, Beier said. "Neutrinos probably have enough mass to be comparable to the stars in the universe. That may seem like a lot, but the stars don't really make up a lot of the mass."

Results of the Sudbury research were presented yesterday in Albuquerque at a joint meeting of the American Physical Society and the American Astronomical Society. A paper with 178 co-authors has been submitted for publication in Physical Review Letters.

Scientists have known of the existence of neutrinos since 1959, and subsequently confirmed the existence of three neutrino forms, or "flavors" -- electron neutrinos, muon neutrinos and tau neutrinos. Besides solar fusion, cosmic rays in the Earth's atmosphere also create neutrinos.

Neutrinos are so small that they can pass almost unimpeded through significant thicknesses of virtually any material on Earth. When scientists tried to measure them, they found many fewer than calculations based on solar fusion would suggest. This was the origin of the deficit.

The neutrino has begun surrendering its secrets in recent years, however, as scientists brought on-line special underground laboratories that block access to all but the most persistent neutrinos, whose activities can be measured and photographed as they smack into water-filled receptacles.

In the late 1990s scientists confirmed that atmospheric neutrinos "oscillate," or change flavors, as they journey from the sun to the Earth. Physicists agree that oscillation would not be possible unless neutrinos had mass.

In 1999, a team of scientists from Canada, the United States and the United Kingdom completed construction and calibration of the Sudbury observatory, built inside Canada's Creighton Nickel Mine.

The lab is the equivalent of a 10-story building constructed 6,800 feet below the Earth's surface. It contains a spherical tank 12 meters in diameter filled with 1,000 metric tons of heavy water -- water composed of heavy isotopes of hydrogen. The tank is monitored by about 10,000 light sensors.

Last year, in its first report since the observatory began operation, the research team showed that the "deficit" existed not because the neutrinos were mysteriously disappearing, but simply because muon and tau neutrinos could not be reliably counted.

But that experiment was based on tabulations comparing Sudbury data that measured only electron neutrinos with data from another underground lab that was measuring electron neutrinos and a bit of something else -- presumably muons and taus.

Then, however, the Sudbury scientists dug deeper into their data. By screening out residual radioactivity and other unwanted interference, they isolated the reaction they wanted.

"When the neutrino enters the tank, it can collide with a heavy water nucleus and knock a neutron loose," explained University of Washington physicist Hamish Robertson, another member of the team. "The neutron will bounce around, hit another nucleus and combine to form a tritium nucleus [another type of heavy water] and give off a gamma ray at the same time."

By counting the gamma rays, scientists know how many neutrinos there are, because "any flavor of neutrino will cause this to occur," Robertson said. "This is a very direct and obvious way to measure all the neutrinos in a single operation."

2002 The Washington Post Company



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