The Super-Kamiokande detector, half-filled with pure water. (Courtesy of the Institute for Cosmic Ray Research, the University of Tokyo.)

For many years, scientists believed that neutrinos, released with electrons when a nucleus undergoes beta decay, might be massless. But in 1995, after collecting three years of data, Los Alamos National Laboratory reported evidence for neutrino oscillations, or transformations of one type of neutrino into another. This finding was supported recently when an international team, including Americans, working at Japan's Super-Kamiokande detector (supported in part by the Office of Science) demonstrated that the muon type of neutrinos oscillate, probably changing into the tau type, as they pass through the Earth. Such oscillations can occur only if neutrinos have mass, albeit the smallest yet observed for elementary particles, and there is "mixing" among certain particles—phenomena that contradict the Standard Model, physicists' current theory of matter and the forces of nature. The difference in mass of the two types of neutrinos was measured based on energy and flight distance from neutrino production in the atmosphere by cosmic radiation to an underground instrument.

Scientific Impact: When verified, this discovery will force a revision in the Standard Model. Furthermore, scientists might have some help in answering a question posed by current models of the universe, which suggest that more matter is present than can be detected by observing electromagnetic radiation; neutrinos might constitute the missing "dark matter."

Social Impact: This discovery, because it may alter estimates of the total mass of the universe, has implications for human understanding of the origin and eventual fate of the universe. The measured neutrino mass is sufficient that the relic neutrinos made in huge numbers at the time of the Big Bang creation of the universe would account for much of its mass.

Reference: "Tau Neutrinos Favored over Sterile Neutrinos in Atmospheric Muon Neutrino Oscillations," The Super-Kamiokande Collaboration, Phys. Rev. Lett. 85: 3999-4003 (2000).

URL: http://www.phys.washington.edu/~superk/

Technical Contact: Dr. James Stone, james.stone@science.doe.gov

Press Contact: Jeff Sherwood, DOE Office of Public Affairs, 202-586-5806

SC-Funding Office: Office of High Energy and Nuclear Physics