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Back to EurekAlert! A Service of the American Association for the Advancement of Science

 

Tiny particles cause big stir

Neutrino mass



When a neutrino strikes the heavy water of the SNO detector, a faint cone of light spreads out from the point of impact to surrounding light sensors. Sensor signals are then analyzed to give the neutrino's direction of travel and precise point and time for the interaction.
The announcement on June 18, 2001 of the first physics results from the Sudbury Neutrino Observatory (SNO) in which it was shown that solar neutrinos have a mass and can "oscillate" or change their identity en route from the sun to the Earth made headlines in newspapers all over the globe. The discovery was even the subject of an entire broadcast of the popular ABC news show Nightline.

"We're quite pleased with the results," said Kevin Lesko, a physicist with Berkeley Lab's Nuclear Science Division (NSD) who leads the Neutrino Astrophysics Group, in an interview with a reporter from the Associated Press.

After more than a year's worth of data, much of which was analyzed at NERSC (the National Energy Research Scientific Computing Center), SNO results show that solar neutrinos have a tiny mass-by some estimates about 1/60,000th that of an electron. The results also show that neutrinos are oscillating in transit, changing in type or "flavor" from electron (the flavor produced in the sun) to muon or tau neutrinos.

These results run contrary to the predictions of the Standard Model of Particles and Fields which has successfully explained fundamental physics since the 1970s. They do, however, clear up a mystery that has vexed scientists for three decades. Previous experiments have detected about one-half to two-thirds of the solar neutrinos predicted based on current understanding of thermonuclear reactions. Our sun is thought to produce more than two hundred trillion trillion neutrinos every second.

"We can say with greater than 99-percent confidence that solar neutrinos are undergoing changes (from one flavor to another) on their way to Earth," Lesko told the San Francisco Chronicle. "It's an exciting discovery and it means that the Standard Model will require some alterations."

Solving the mystery of the "missing" solar neutrinos is one of the primary missions of the $60 million SNO facility and its collaboration of more than 100 scientists from 11 other laboratories and universities in the United States, Canada, and the United Kingdom. Lesko and his colleagues in the Neutrino Astrophysics Group are prominent participants in this collaboration. Those colleagues include senior NSD scientists Bob Stokstad and Rick Norman, plus Yuen-dat Chan, Xin Chen, Alysia Marino, Colin Okada, and Alan Poon. Lesko himself, like Berkeley Lab, has been involved with the SNO project from its earliest days back in 1989.

SNO can be thought of as a type of telescope, though it has little in common with the instruments most people associate with that word. Operating out of a nickel mine more than a mile underground near Sudbury, in the Candian providence of Ontario, SNO consists of an 18-meters-in-diameter, 58,000-pound stainless steel geodesic sphere suspended in a pool filled with 7,000 tons of purified water. Inside this sphere is an acrylic vessel filled with 1,000 metric tons of heavy water (deuterium oxide or D2O). Attached to the sphere are 9,456 ultra-sensitive light-sensors called photomultiplier tubes.

When neutrinos passing through the heavy water interact with deuterium nuclei, flashes of light, called Cerenkov radiation, are emitted. The photomultiplier tubes detect these light flashes and convert them into electronic signals that scientists can analyze. SNO is the first neutrino telescope sensitive enough to measure not only ordinary electron neutrinos, but also the much more rare muon and tau neutrinos. This unprecedented sensitivity was made possible by a design that maximizes SNO's light-collecting capabilities.

"It is vital for the success of any neutrino experiment that as many photons as possible be detected," Lesko has said. "Therefore, we had to squeeze as many photomultiplier tubes as possible onto the geodesic dome while maintaining an adequate layer of water shielding between the tubes and the cavity walls of the SNO site."

Members of Berkeley Lab's Engineering Division (ED) solved the design challenge with a tesselated sphere surface made up of several hundred panels that come in five different shapes, each of which is built up from repeating patterns of hexagons. The result was a honeycomb pattern covering 60 percent of the sphere with photomultiplier tubes that, thanks to a unique mounting system and a series of corrosion-resistant plastic skirts, are watertight and can be individually aimed.

NSD and ED scientists and engineers, under Lesko's leadership, also designed the stainless steel support sphere which, in 1993, was assembled at Donal Machine in Petaluma, California. After it was successfully tested, the sphere was disassembled and shipped to Sudbury.

In addition to resolving the solar neutrino deficit, the first results from SNO also weigh in on a deficit concerning the total amount of matter in the universe.

Perhaps as much as 95 percent of the matter of the universe, as inferred through gravitational affects, is missing. That it is, this matter is invisible or "dark" to us. Some scientists had speculated that because neutrinos are the second most common particles in the universe (after photons), if they had mass they might account for a substantial portion of this dark matter.

Not so, says Lesko. "The measurements by SNO, when combined with previous measurements, provide a limit on the difference in mass between electron, muon, and tau neutrinos. This is the last piece of information necessary to set a limit on the total mass of all three."

The total mass limit, Lesko says, puts the combined mass of all the neutrinos in the universe about equal to the combined mass of all the visible stars. That means neutrinos can account for only a small percentage of all the dark matter.

The next round of SNO measurements will involve the addition of salt to the heavy water in the acrylic vessel. This will enable members of the SNO collaboration to observe another neutrino reaction with deuterium that is highly sensitive to all three neutrino flavors. The new measurements will enable the collaboration to study the transformation of neutrino flavors with even greater sensitivity. They will also be able to study other properties of neutrinos from the sun and from supernovae. The SNO experiments are expected to run for at least five more years.

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