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June 19, 2001

Sun's Missing Neutrinos: Hidden in Plain Sight


A drawing of the neutrino detector, built 1-mile underground and immersed in water within a cavity 110 feet deep.

Lawrence Berkeley National Laboratory
The bottom of the neutrino detector.

Sudbury Neutrino Observatory
The detector's acrylic vessel was filled with heavy water, fitted with photo sensors and encased in concrete.

After three decades of searching, physicists have tracked down subatomic particles that have eluded them for 30 years. The particles, it turns out, were right there all the while but had hidden themselves as if by magic.

"We've solved a 30-year-old puzzle of the missing neutrinos of the Sun," said Dr. Arthur B. McDonald, director of the Sudbury Neutrino Observatory, near Sudbury, Ontario. In doing so, though, the researchers have answered questions about neutrino behavior and the fate of the universe.

Neutrinos are ghostly particles, one of the fundamental building blocks of the universe, like quarks, electrons and photons. Billions of them, produced by fusion reactions within the Sun, fly through every person every second. Minuscule and devoid of electric charge, though, they pass unnoticed. In fact, they are practically undetectable.

In 1968, Dr. John Bahcall, an astrophysicist at the Institute for Advanced Study at Princeton, N.J., calculated that the rate of neutrinos from the Sun passing through one square inch of area should be about 30 million a second.

Experiments beginning in the 1970's counted much lower rates; more than half of the expected neutrinos were never seen. Dr. Bahcall's predictions, refined over the years, remained unchanged.

Yesterday, scientists at the Sudbury Neutrino Observatory announced the first experimental evidence that provides a solution.

During the neutrinos' 93-million- mile journey from the Sun to the Earth, the researchers said, about two-thirds change into other varieties that are more difficult to detect.

The total number of neutrinos from the Sun, they conclude, is about 35 million per square inch per second. "The agreement is pretty good between the predictions and the measurement," said Dr. Joshua R. Klein, a professor of physics at the University of Pennsylvania who coordinated the analysis of the data.

Dr. Bahcall was ecstatic.

"I feel very much like the way I expect that these prisoners that are sentenced for life do when a D.N.A. test proves they're not guilty," Dr. Bahcall said. "For 33 years, people have called into question my calculations on the Sun."

The new finding "shows the calculations were correct," he said. "I feel like dancing," he added.

Less happy is Dr. David O. Caldwell, an emeritus professor of physics at the University of California at Santa Barbara. "My personal reaction is one of great disappointment," Dr. Caldwell said. "I was hoping for a rather different result." The data ran contrary to his hopes of finding a new kind of neutrino.

Still, he said, the Sudbury results look "quite solid."

Neutrinos come in three types (physicists call them flavors): electron neutrinos, muon neutrinos and tau neutrinos, named according to the subatomic particles they usually associate with. Muon and tau particles are heavier particles that otherwise act like electrons. The neutrinos produced by the Sun are all electron neutrinos.

Spotting the rare occasions when a neutrino collides with another particle requires large quantities of material for the neutrinos collide with.

The detector in the Sudbury Neutrino Observatory consists of a 40- foot-wide acrylic sphere containing 1,000 tons of heavy water, in which the two hydrogen atoms of the water molecules have been replaced with deuterium atoms, a heavier version of hydrogen. The sphere is submerged within a 10-story cavity that was carved out of a nickel mine 11/4 miles underground and filled with 40,000 tons of ordinary water.

Occasionally, an electron neutrino will slam into one of the deuterium atoms in the heavy water, splitting it into a proton and a neutron. Detectors around the sphere of heavy water are able to spot the debris. The other two types of neutrinos cannot break up deuterium. The scientists have seen 1,169 such collisions since the experiment began in 1999.

The researchers compared their results with earlier neutrino counts from the Super-Kamiokande neutrino experiment in Japan, which primarily detects collisions between electron neutrinos and electrons. But muon and tau neutrinos can also occasionally bounce off electrons.

If all the neutrinos reaching Earth from the Sun were of the electron variety, then the neutrino rates measured by Super-Kamiokande and Sudbury should match up. But Super- Kamiokande detected more. Since the Sun produces only electron neutrinos the production of muon and tau neutrinos require higher-energy events, like matter falling into black holes or an exploding star that means some of them must change into muon or tau neutrinos.

"It's the first direct evidence for the changing of solar neutrinos from electron type to another type," Dr. Klein said. Most physicists had considered neutrino morphing to be the most likely explanation for the missing neutrinos.

Dr. Caldwell's theory was that the electron neutrinos were changing into "sterile" neutrinos that did not interact with ordinary matter. "It looks like they've done a very thorough job," he said. "It then is a real question if there is any room for a sterile neutrino. I don't see much hope for it right now."

According to the equations of particle physics, for this transformation of flavors to occur, at least one of the neutrino types must possess a smidgeon of mass. Coupled with earlier experimental results, the researchers conclude that each of the three neutrino flavors weigh, at most, one- 60,000th as much as an electron.

But the universe is filled with more neutrinos than any other known type of particle, and some physicists have wondered whether the collective gravitation pull of neutrinos may be strong enough to stop the outward expansion of the universe and pull it back into a big crunch.

At the upper limit, the mass of neutrinos may be substantial, perhaps as much as the rest of ordinary matter, but far short of the amount needed to collapse the universe.

Most of the mass of the universe is believed to be in "dark matter," a still unknown form of matter.

For particle physicists, the neutrino data is one more piece that they will need to incorporate into a future unified theory of physics that describes the behavior of all particles and forces. The current standard model does not predict the masses of neutrinos, but its equations are simpler if neutrinos have no mass.

Now scientists know the behavior of neutrinos is not simple. Neutrinos are "very schizophrenic," Dr. Bahcall said. Still unknown, for example, is whether the electron neutrinos are changing into muon neutrinos or tau neutrinos or both.

"I expect to be up late at night trying to answer that question," Dr. Bahcall said. "It's pointing us in a way in the right way, we hope to make a better theory, a more encompassing theory."

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