Science / Medicine : Sun Conundrum : Mystery of Missing Neutrinos Is Worthy of Sherlock Holmes

Physicists say that they are drawing close to solving a mystery about the sun that has stumped them for more than 20 years.

The protagonists in this story are neutrinos, subatomic particles that have no charge and little or no mass and are very difficult to detect. Since 1968, scientists have been at a loss to explain why the sun seems to produce fewer than half the neutrinos it should.

Now the results of a joint Soviet-American experiment support a theory that the missing neutrinos exist but are not detected because they change character as they pass through the sun. Some physicists are also excited because this solution to the “solar neutrino problem” suggests a new form of particle physics with phenomena never before seen, said Peter Rosen, a physicist involved in the experiment.

These phenomena promise to provide clues to physics’ most fundamental questions. For instance, the question of whether neutrinos have mass is important for constructing a unified field theory that explains how all the forces in nature are related.

The fusion of hydrogen to helium drives the fires of the sun, and each such fusion reaction creates neutrinos. In theory, there should be no dearth of neutrinos streaming from the sun. Scientists calculate that there should be about 60 billion neutrinos passing through every square centimeter of the Earth’s surface every second, but the particles so rarely react with matter that the vast majority of them pass through our planet unhindered.

A few neutrinos out of the billions, though, can be found if the detector is large enough. In 1967, physicist Raymond Davis of the Brookhaven National Laboratory in Upton, N.Y., filled part of the Homestake Gold Mine in South Dakota with 100,000 gallons of cleaning fluid and started looking for neutrinos, which interact slightly with chlorine atoms in the fluid.

Davis found fewer than half of the neutrinos he should have according to solar theory, indicating that the sun was producing far fewer neutrinos than predicted. At the time, one colleague declared the results “socially unacceptable,” Davis said.

Over the last two decades other experiments in South Dakota and around the world have confirmed Davis’ initial, mystifying result. Scientists are forced to accept it, but they have found the possible explanations unpalatable.

One explanation noted that these experiments could detect only high-energy neutrinos, which are produced in a relatively rare fusion reaction involving boron-B in the core of the sun. Perhaps, they thought, this reaction is even rarer than previously believed, and that this would explain the detection of so few neutrinos. Physicists logged countless hours in an unsuccessful attempt to construct a model of the sun in which its core was cooler, and the boron-B reaction rarer, than in the standard model, Davis said.

That is how the mystery remained until last year, when a new type of neutrino detector was completed by Soviet and American scientists under a mountain in the Soviet Union. (Detectors are buried underground to screen out cosmic rays, enabling the detection of neutrinos, which pass through the Earth). At the heart of the Soviet-American germanium experiment (SAGE) is 30 tons of liquid gallium, a strange metal that is solid at room temperature but will melt in the hand. Gallium allows scientists to detect low-energy neutrinos, the type created by the fusion of two protons, the most common reaction in the sun.

Any shortage of low-energy neutrinos could not easily be explained by fiddling with a model of the sun, said physicist Peter Rosen of the University of Arlington. “You might be able to explain it if the core of the sun recently went out, but this is considered pretty much impossible,” Rosen said.

This year, the scientists working on SAGE ran their experiment for the first time and found not only a shortage of neutrinos--they found none. Like the dog that didn’t bark in a Sherlock Holmes mystery, the lack of signals from neutrinos may point toward a solution to the problem.

If the results from SAGE hold up, they would support a theory that the sun is producing just as many neutrinos as it should, but those neutrinos are changing “flavors” in the intensely energetic interior of the sun, Rosen said. The new flavors would not be detectable by SAGE.

According to this theory, dubbed the MSW theory after the initials of its proponents, the electron-type neutrinos change to muon or tau neutrinos, which are associated with particles that are almost never found outside particle accelerators. “What we’re seeing is probably the first indication of new particle physics--evidence of a new physical phenomenon previously theorized but never before observed,” Rosen said.

Another implication of this theory is that neutrinos would have to possess mass, requiring the modification of the most widely held theory about relationships between basic particles, Rosen said. This theory, called the Winberg-Salaam-Glashow model, describes the relationship between the electromagnetic force and the weak nuclear force. In the model, which garnered a Nobel prize for the three scientists who constructed it, neutrinos have no mass.

Other scientists involved in the SAGE project caution that these are only preliminary results. The detection of neutrinos is an extremely subtle art, said physicist John Wilkerson of Los Alamos National Laboratory in New Mexico.

Neutrinos are detected when they interact with the nucleus of a gallium atom and change it to an atom of germanium. Scientists expected to detect no more than 18 neutrinos over a five-month period, so spotting them required finding only a few atoms of germanium in 30 tons of gallium, Wilkerson noted.

According to tests done so far, SAGE is working fine, but scientists have yet to do a final calibration of the detector with a neutrino source in the laboratory, Wilkerson added. Physicists hope support for the results from SAGE will come from a similar gallium experiment, called GALLEX, now beginning at Italy’s Gran Sasso Laboratory.

Final confirmation of the MSW theory will have to wait until the completion of a joint U.S.-Canadian experiment called the Sudbury Neutrino Observatory (SNO), which is being built in Canada. By looking for neutrino interaction with deuterium atoms in 1,000 tons of heavy water, SNO scientists will be able to mark the passage of all “flavors” of neutri-nos--the electron, muon and tau types. If neutrinos do indeed change flavors, the total number of neutrinos counted at SNO should be exactly the number Davis thought he would find more than 20 years ago.

Until those results come in, many physicists are excited but cautious about the SAGE results. “It’s fine to be excited, especially for the theorist, because they need to work on something,” Wilkerson said. “The hint is there (about the MSW theory), but it’s going to take another year to prove it.”

SEARCHING FOR NEUTRINOS

What is a neutrino?

Once created in a nuclear-decay process, it is a stable neutral particle that has zero rest mass, travels at the speed of light, and interacts extremely weakly with matter. Of all the known particles in the universe, the neutrino may be the most unusual.

Composition of sun’s neutrinos Low-energy neutrinos: The most common in the sun. Created by the fusion of two protons. Each fusion reaction, which drives the fires of the sun, creates neutrinos. High-energy neutrinos: Created by a rare fusion reaction involving boron-B in the core of the sun. Travel speed of neutrinos: Neutrinos travel at the speed of light, 186,000 miles per second. It takes 8.3 minutes to travel the 93,000,000 miles from the sun to reach the Earth’s surface. In search of neutrinos: In 1967, Raymond Davis had 100,000 gallons of cleaning fluid put into the Homestake Gold Mine in South Dakota and started looking for neutrinos.

Chamber filled with cleaning fluid containing chlorine. Neutrinos pass through but only a few react with chlorine atoms. Neutrino bombardment: There are about 60 million neutrinos passing through one square centimeter of the Earth’s crust per second.

One in 10 billion of those that pass through the center of the Earth will encounter another particle, which makes the neutrino the most difficult particle to detect. (One cubic cm. actual size)

Source: Encyclopedia Americana