June 5, 1998 BOSTON, Mass.--A research team of scientists 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 sub-atomic 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 negligible size and lack of 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 new look at the theoretical models of the structure of matter," says James Stone, professor of physics at Boston University and U.S. co-spokesperson for the project. "It sheds light on our basic questions about the nature of the universe, including the question the validity of the standard model of particle physics." The announcement was made June 4 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--the size of a nine story building--and weighing 50,000 tons, is filled with the world's purist water.
More than 13,000 cameras are mounted in the tank and used to detect the 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 our knowledge of 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 Kamikande detected the expected number of electron-neutrinos--but only half the number of muon-neutrinos that were expected--among those that traveled a greater distance (through the earth from the opposite side of the globe). The scientists concluded that the missing muon-neutrinos had "oscillated" away, i.e., changed into undetectable tau-neutrinos or some other unknown type of neutrino as they traveled. According to Einstein, this transformation can occur only if the neutrinos possess mass. The experiment directly determines that the mass difference between the neutrinos is very small. The primary results have a statistical significance of more than 5 standard deviations. An independent measurement based on upward-going muons in the detector confirms the result.
The Super Kamiokande experiment is based on techniques pioneered by the Boston University and University of California-Irvine teams at a detector located in Cleveland, Ohio, which discovered neutrinos from a supernova in 1987, and the first indications of the oscillation of neutrinos a decade ago. The Super Kamiokande detector is seven times the size of the Ohio detector.
"The major problem in physics for the last quarter century has been the problem of mass--Where does it come from?" says Lawrence Sulak, an initiator of the first detector and principal investigator on the current project. "Until this observation neutrinos were thought to be massless. Now we know that we have not missed a fundamental symmetry of nature that forbids a neutrino mass. On the contrary, that neutrinos do have mass provides a critical clue in the unification of the particles and forces of nature."
Costs for this astrophysical observatory exceed $100 million, primarily provided by the Japanese Ministry of Education, Science, Sports, and Culture (Monbusho). Funding for the detector's outer 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. Neutrino oscillations 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.
Further information and images of the detector can be found at http://hep.bu.edu/~superk.
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