A Not-So-Ghostly Particle

Are neutrinos helping to pull the universe together?

They are the ghosts of particle physics, so tiny and elusive that a billion could pass through a bar of lead without hitting anything en route. Traveling at the speed of light, neutrinos carry no charge and apparently no mass. Even after they were detected in 1956, they remained curious, spectral bits of energy. As one of the scientists who discovered them, Frederick Reines, explains, they seemed to have "no frills and no complications."

Last week Reines astonished colleagues with word that neutrinos may not be so simple after all. Returning to the same nuclear facility in South Carolina where he had performed his detection feat, he found that the erstwhile ghosts do indeed seem to have substance. Not much even on the nuclear scale, perhaps only one ten-thousandth of the mass of the electron, but big enough to stir the world of physics. If his results are right, they may help explain the sun's puzzling behavior and perhaps hint at the universe's ultimate fate.

In his initial work, Reines, 62, a researcher at the University of California in Irvine, had measured the flow of neutrinos, or more precisely anti-neutrinos (their antimatter twins), in the streams of particles coming from a nuclear reactor at the Government's Savannah River weapons-grade plutonium plant. This time, Reines, joined by Physicists Henry W. Sobel and Elaine Pasierb, faced a more complicated task. By now neutrinos were believed to come in at least three varieties, or "flavors": one associated with electrons, the two others with larger particles called muons and taus.

To catch these particles, the physicists placed a sealed container of deuterium, or heavy water, 11.2 meters (37 ft.) from the reactor. Immersed in the liquid were ten helium-filled tubes wired to an external oscilloscope. The detection apparatus was shielded in lead and cadmium cylinders and a foot-thick "pot" to block everything but neutrinos. As the particles barreled through the heavy water, some scored bull's-eye hits on the nuclei of its hydrogen atoms, which contain an extra neutron. These collisions produced other particles, including more neutrons that struck the helium-filled tubes and registered on the oscilloscope.

To their surprise, the scientists counted fewer than half the electron neutrinos anticipated. Reines speculates that they had changed flavors, or oscillated, turning into muon or tau neutrinos during their short journey. If such a switch had occurred, the neutrinos must have mass (since physics dictates that one kind of particle must have mass to turn into another). Said Reines: "The simple view [of the massless neutrino] does not square with experimental facts."

Some physicists greeted Reines' findings with skepticism. Said Theorist Chris Quigg of the Fermi National Accelerator Laboratory outside Chicago: "It's a brute force, heroic effort rather than an elegant experiment with lots of internal checks." But none denied that if Reines' case can be confirmed, it will have wide-ranging implications.

Massive, oscillating neutrinos could help theoreticians develop a more accurate picture of matter and perhaps find an underlying unity in the forces governing it. Says Nobel Laureate Sheldon Glashow: "With the discovery, we are at the root of what matter is made of and what the rules are by which elementary constituents are held together." The Reines theory could also explain why earlier experiments had detected a fraction of the expected flood of neutrinos from the nuclear fires of the sun. This had caused some scientists to offer the chilling thought that the sun is cooling. Reines' explanation: the missing neutrinos may have changed flavor.

His work may also shed new light on the most cosmic of questions: Where is the "missing mass" holding the galaxies and even the universe together? It could be countless neutrinos (actually, scientists estimate the number to be 1087, or one followed by 87 zeros). Their collective gravity could perhaps sufficiently slow down the current expansion of the universe, which has been under way since the Big Bang some 20 billion years ago. If the rush of galaxies away from one another could be reversed, they would eventually come together in a cataclysmic collapse.

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