Neutrinos: Minute Particles, Massive Results

Scientists at Boston University's physics department played a leading role in the discovery of the first evidence that neutrinos -- tiny electrically neutral particles -- have mass.

This finding, which contradicts the standard theory of particle physics, may have significant implications in the debate over whether the universe has enough mass to halt, or even reverse, the outward expansion that began with the "Big Bang." It may, in fact, lead to a unified explanation of the basic nature of the universe. Because of their small size and lack of electrical charge, neutrinos can pass through the entire earth without interacting with matter, making them extremely difficult to detect.

The experiment is being carried out at the massive Super-Kamiokande detector, buried 1 kilometer underground in the Japanese mountains 300 kilometers west of Tokyo. Forty meters in diameter, 40 meters high, and weighing 50,000 tons, the tank is filled with the world's purest water and is observed by 14,000 photomultiplier tubes to detect the faint flashes of light produced by the neutrinos when they interact in the water. The scientists use the information from the detector to count the neutrinos and classify them according to type.

The Super-Kamiokande experiment detected the expected number of electron neutrinos, but only half the number of muon-neutrinos expected among those that traveled the greater distance (through the earth from the opposite side of the globe). This led the scientists to conclude that the missing muon-neutrinos had "oscillated" - changed into undetectable tau-neutrinos or some other unknown type of neutrino as they traveled. According to a basic principle of quantum mechanics, this transformation can occur only if neutrinos possess mass.

The Super-Kamiokande experiment is based on techniques pioneered by Boston University and University of California teams at a detector located in Cleveland Ohio, which discovered neutrinos from the supernova in 1987. The Super-Kamiokande detector is seven times the size of the Ohio detector.

Funding for the project is provided by the Japanese Ministry of Education, Science, Sports, and Culture (Monbusho) and the United States Department of Energy.