June 5, 1998

Mass Found in Elusive Particle; Universe May Never Be the Same

Investigation of the neutrino have not been limited to the Super-Kamiokande collaboration.

The leader of the collaboration's University of Hawaii Group, for example, Dr. John Learned, has also worked on an underwater detection system in the Pacific Ocean off the Hawaiian coast (which ran out of money before completion) and a project at the South Pole where a neutrino detector had been buried under thousands of feet of ice.

Another approach to penetrating the neutrino secrets involves the use of particle accelerators capable of producing intense beams of neutrinos. In two experiments currently being prepared, one in Japan and the other at Fermi National Accelerator Laboratory in Illinois, beams of neutrinos will be directed through the Earth toward detectors several hundred miles away. The goal will be to observe changes the neutrinos undergo in transit, both in numbers and types. Physicists expect the experiment to confirm the existence of neutrino oscillations like those seen in the Super-Kamiokande detector.

Although the neutrinos are now known to have some mass, most physicists agree that the mass must be very small. The Super-Kamiokande experiments suggest that the difference between the masses of muon neutrinos and other types of neutrinos is only about 0.07 electron volts (a measure of particle mass). This does not yield a value of the masses themselves, only of the difference between those of muon neutrinos and other types.

Although the mass of the neutrino of any flavor must be small, Totsuka said, it may be several electron-volts, and if so, the overall gravitational effect on the universe would perhaps be significant. It has been estimated that at any given moment, every teaspoon worth of volume of space throughout the universe contains an average of 300 neutrinos, so their aggregate number is staggering.

During the last 68 years, a legion of distinguished physicists has devoted inquiries and careers to the puzzling neutrino, which was given its name by the great Italian-American scientist Enrico Fermi. Fermi quickly came to believe in the particle's existence, even though it was not proved in his lifetime, and named it "neutrino," which means "little neutral one" in Italian.

Representatives of dozens of neutrino experiments meet once every two years to exchange ideas at conferences like the one under way here. Present are representatives of teams that have installed neutrino detectors on the bottom of Lake Baikal in Siberia, under the Aegean Sea off the Greek coast, inside the Gran Sasso tunnel under the Alps, under the ice covering the South Pole, and in many other places.

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