Real Gone Neutrinos

By J. Madeleine Nash/Chicago

Neutrinos are the phantoms of the subatomic world. They seem to have no mass, may travel at the speed of light and are virtually impossible to detect. According to the standard theories of physics, these exotic particles are produced by various nuclear reactions. Quadrillions of neutrinos from the sun bombard the earth every second, yet most of them pass right through the planet without causing so much as a ripple.

Since 1968 scientists have been monitoring huge detectors for signs of these fleeting visitors from the sun. But so far, the results have been both disappointing and intriguing: the experiments have detected far fewer neutrinos than solar models predicted. Scientists were especially baffled by a recent report from a Soviet-American research team that set up a detector to monitor neutrinos emitted by the fusion of hydrogen atoms, the sun's main reaction. After four months of operation near the Soviet town of Baksan, the experiment has yet to turn up a single solar neutrino.

The case of the missing solar neutrinos has stirred growing excitement in the physics world. There are three possibilities: the Baksan experiment is wildly wrong, scientists don't understand the sun as well as they thought they did, or scientists have underestimated the elusiveness of the neutrino. The answer to the mystery could have profound implications for physicists' understanding of the universe. Two eminent theorists, John Bahcall of the Institute for Advanced Study in Princeton, N.J., and Cornell University's Hans Bethe have co-authored a paper that elaborates on an intriguing solution to the puzzle: neutrinos escape detection by changing from one form into another. Says Bahcall: "Nature may be smarter than we thought."

Experimental evidence indicates that neutrinos come in three varieties: the electron neutrino, the muon neutrino and the tau neutrino. Solar fusion gives off the electron type. Bahcall and Bethe speculate that electron neutrinos change into the muon or tau versions somewhere between the sun and Earth. "It's as if they started out sweet," marvels Bethe, who won the Nobel Prize in 1967 for explaining how nuclear fusion powers the sun, "and then suddenly turned salty." Thus the Baksan experiment may have come up empty- handed because it was not designed to detect muon or tau neutrinos.

Finding any kind of neutrino is a neat trick. The Baksan detector consists of four tanks filled with 30 tons of the element gallium, which liquefies at about room temperature. If a solar neutrino of the right energy interacts with the material in the tanks, a feat of atomic alchemy will transmute some of the gallium into germanium, another metallic element.

First scientists must eliminate other sources of radiation that may trigger false signals in the gallium. (To shield the experiment from cosmic rays, the detectors are installed in an underground tunnel, beneath a mile of rock.) About the only thing harder than proving that solar neutrinos passed through the gallium-filled tanks is proving that they didn't.

If Bahcall and Bethe are right, neutrinos have long been misunderstood. For example, if one kind of neutrino can change into another, then these apparently massless particles must possess at least a tiny mass. That raises the possibility that the heaviest neutrinos might be weighty enough to account for the "dark matter" that is believed to make up 90% of the known universe. The composition of that matter is one of the great unanswered questions of physics. But before that theory can be pursued, the results of the Baksan experiment must be confirmed, so another gallium neutrino trap is starting up deep beneath the Italian Alps.