(Boston, Mass) -- A research team from the United States and Japan, organized by physicists at Boston University, the University of California-Irvine, and the University of Tokyo, has found 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" and may 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.
"This finding means that we need to take a look at the theoretical models of the structure of matter," says James Stone, professor of physics at Boston University and U.S. co-spokesman for the project. "It may shed light on some of our basic questions about the nature of the universe, including the question of the composition of 'dark matter' which has been proposed to account for the predicted total mass of the universe." The announcement was made today at "Neutrino '98", an international physics conference underway in Takayama, Japan.
Many neutrinos are created when high energy cosmic rays bombard the earth's upper atmosphere producing cascades of secondary particles that rain down upon the earth. For this study scientists used the massive Super-Kamiokande detector, buried 1,000 meters underground at the Kamioka Mining and Smelting Company Mine in Mozumi, Japan. The tank, 40 meters in diameter, 40 meters high, and weighing 50,000 tons, is filled with purified water.
More than 13,000 photomultiplier tubes, manufactured for the experiment by Hamamatsu Corporation, are mounted in the tank and used to detect faint flashes of light produced as a secondary effect -- like a sonic boom -- of neutrinos moving through the water. The scientists used the information from the detector to count the neutrinos and classify them according to type, either electron- or muon-neutrino.
Based on what we know about the particles produced by cosmic rays, twice as many muon-neutrinos as electron neutrinos should be detected, and this should be true regardless of the direction of the source of the neutrinos. The Super-Kamiokande experiment detected the expected number of electron neutrinos, but half the number of muon-neutrinos expected -- among those that travelled a greater distance (through the earth from the opposite side of the globe). The scientists concluded 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 the neutrinos possess mass. The experiment does not directly determine the masses of the neutrinos leading to the effect, but the rate of disappearance suggests that the difference in the masses of the oscillating types is very small. The primary results have a statistical signficance of more than 5 standard deviations. An independent measurement based on upward-going muons confirms the result at a level of more than 3 standard deviations.
The Super-Kamiokande experiment is based on techniques pioneered by the 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.
Costs for this project exceed $100 million, primarily provided by the Japanese Ministry of Education, Science, Sports, and Culture (Monbusho). Funding for the detector's outer most region was provided by the United States Department of Energy. About 100 physicists from 23 institutions are participating in the project.
Since the beginning of its operation in April, 1996, the Super-Kamiokande experiment has been the most sensitive in the world for monitoring neutrinos from various sources. Important results have also been found in the measurements of electron-neutrinos coming from the sun. The number detected is about 35% of the number predicted by the well established theoretical model of the sun's neutrino producing processes. There has also been an indication that the observed energy spectrum of those neutrinos is different from the predicted one. These observations may also be interpreted as the result of oscillations.