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By Guy Gugliotta
Washington Post Staff Writer
Tuesday, June 19, 2001; Page A02

Every second of every day, tens of thousands of ghostly subatomic particles called neutrinos -- tiny offspring of nuclear fusion in the core of the sun -- harmlessly flash through a person's body.

For 30 years physicists have known -- or thought they knew -- how many neutrinos ought to be reaching Earth, but every measuring device told the same story -- there were many fewer arrivals than expected. Either physicists had the wrong idea about solar fusion, or something was happening to the neutrinos en route.

Yesterday, an international team of physicists, working in a lab sunk more than a mile deep in a Canadian nickel mine, presented the first concrete evidence that the "solar neutrino deficit" occurs because while three types of neutrinos reach the Earth, only one -- until now -- could reliably be counted.

"We have measured the total number of neutrinos coming from the sun, and [the measurement] agrees with predictions made 30 years ago," said research physicist and team member Joshua Klein of the University of Pennsylvania. "It's the oldest, longest-lived puzzle in particle physics."

The findings, reported in the journal Physical Review Letters by 180 authors from the United States, Canada and the United Kingdom, have important implications for both the physics of the very small and the physics of the very large.

By showing that neutrinos toggle back and forth on their journey to Earth among three forms, or "flavors," the experiment provided fresh confirmation that neutrinos have mass. Scientists agree that such "oscillation" can occur only if a substance has mass.

This concept has thrown a sizable monkey wrench into the "Standard Model" of physics, which proposes a massless neutrino as part of its description of how nature works at its most basic level. The model will have to be modified significantly to accommodate the new data.

At the same time,, scientists had thought that a neutrino with mass, albeit only about one ten-millionth that of an electron, would help explain "dark matter" -- the view shared by astronomers that 90 percent of the mass in the cosmos is in an unknown, invisible form.

But while neutrinos can be found everywhere in the universe, the new experiment "is not going to solve the problem," said team member Richard Hahn, a senior chemist at Brookhaven National Laboratory. Instead, the new findings confirmed earlier research that the particles can account for only a small fraction of the universe's invisible mass. Scientists will have to search elsewhere for their dark matter.

The concept of a neutrino was first proposed in 1931 to explain a mysterious energy loss that occurred in some types of radioactive decay. The particle was actually discovered in 1959, and by 1976 scientists had confirmed three flavors -- the electron neutrino, the muon neutrino and the tau neutrino.

Solar fusion, the source of the largest share of the Earth's detectable neutrinos, gives off only electron neutrinos, Hahn explained, but far fewer of these particles were showing up than were expected. Many physicists theorized -- but could not prove -- that the electrons were changing to muons or tau during the trip.

"Think of it as a mix of two basic colors, like red or blue," Hahn said. "You get three shades of purple, but all three are intrinsic to the basic mix." Thus, a single neutrino embodies all three flavors.

Studying neutrinos, which come to Earth in sunshine or cosmic rays, has proved difficult because they are so small. They can pass through nearly anything and are almost impossible to detect, especially when they have to be distinguished from other incoming particles.

Following the experience of other scientific groups, the U.S.-Canada-U.K. team built a neutrino observatory 6,800 feet deep in the underground tunnels of the Creighton Nickel Mine outside Sudbury, Ontario. The massive stone shield blocks the entrance of all but the most persistent neutrinos.

The observatory's detector is a spherical vessel of acrylic plastic 12 meters in diameter filled with 1,000 tons of heavy water -- water composed of heavy isotopes of hydrogen or oxygen, or both. Almost 10,000 light sensors detect and photograph tiny flashes of light thrown off when neutrinos are stopped or scattered in the heavy water.

Klein said the heavy water was critical because the hydrogen nucleus in the water molecule has a neutron. When hit with an electron neutrino, the nucleus emits an electron and the neutron changes to a proton.

In this way, the researchers for the first time were able to count electron neutrinos, and determine that they comprised 35 percent of the neutrinos expected to reach the Earth.

Using other methods that captured a mix of electrons, muons and taus, the team showed there were indeed as many neutrinos coming to Earth as were leaving the sun. The deficit occurred because a huge percentage of muons and taus had gone undetected.

Construction of the neutrino observatory started 11 years ago, but calibration of the instruments and data collection began only in November 1999. Between then and January of this year, the team recorded 1,000 neutrino "events" in 240 days -- just over four per day.

To get to the lab atop the heavy water sphere, Hahn said scientists suited up with hard hats and lights and rode the elevator more than a mile down with the Canadian nickel miners working at the site. Next came a 1.5-mile hike along a passageway to the lab entrance.

"The whole lab is a clean room," Klein added. "You had to shower up and change clothes, and put on a hair net and a mask before you could go to work. It was about as clean as a hospital emergency room."