Neutrino Discovery‹

A Fact Sheet

The Super-Kamiokande detector is a 50,000-ton double-layered tank of ultra pure water observed by 11,146 photomultiplier tubes, each 20 inches in diameter. The equivalent of an acre of photocathode, it is the largest light detection area ever assembled by more than a factor of ten. Located in a specially carved out cavity in an old zinc mine 2,000 feet under Mount Ikena near Kamioka in the Japanese alps, the detector is reached by driving through a 2 km-long tunnel. The underground site also includes a huge reverse osmosis water filtration system, calibration electron accelerator, five trailers of electronics, the main control room, preparation areas, etc.

The Super-Kamiokande project has been collecting data since April 1, 1996. This discovery is based on data collected through January 15, 1998.
Energetic charged elementary particles traveling at close to the vacuum speed of light (300,000 km per second) exceed the speed of light in water. This results in the optical equivalent of a sonic boom, Cherenkov radiation, in which a flash is emitted in a 42-degree half-angle cone trailing the particle. This nanosecond directional burst of blue light is detected with photomulitpliers. Its pattern, timing and intensity allow physicists to determine the particle's direction, energy and identity.
Data are acquired at a high rate (about 100 triggers per second), partially processed and sent via fiber optics to the laboratory outside the mine, where they are archived and filtered into different analysis streams.
Most of the results discussed in the current paper are deduced from the cases (two-thirds of the time) when a neutrino produces either a single electron or a single muon. These interactions are recorded in the inner 22.5 kilotons of water about 5.5 times per day.

Super-Kamiokande Collaboration claims the discovery of neutrino mass and oscillations. The claim is based upon atmospheric neutrino data, which resolves an anomaly uncovered in 1985 and confirmed and elaborated by subsequent experiments. In its analysis of the present data base, the team observed a deficit of muon neutrinos coming from great distances and at lower energies; the functional behavior of this deficit indicates that muon neutrinos oscillate, thus they have mass.

Oscillations require neutrinos to have mass. Finding non-zero neutrino mass is big news for elementary particle physics, requiring revision of the Standard Model, which has fit all elementary particle data to date, but sets neutrino masses at zero.
The Super-Kamiokande team hopes the insight gained from the peculiar mixing observed between neutrinos spurs progress toward a unified theory that explains the generations or flavors and predicts particle masses.
The team also infers that the total mass of neutrinos in the universe must be significant--at a minimum amounting to a significant fraction (10 - 100 percent) of the baryonic mass of the universe and perhaps representing the dominant mass in the universe.
In any event, neutrinos cannot now be neglected in the bookkeeping of the mass of the universe. Indeed, some theoretical calculations indicate that neutrinos may have played a crucial role in the production of an excess of matter over anti-matter, and are thus intimately linked to our very existence.
Clearly this is the single most important finding about neutrinos since their discovery. Some experts call this result the single most important result of the decade in elementary particle physics.

The collaboration team includes about 100 physicists. from Japan and the United States.
The lead Japan group is from the University of Tokyo's Institute for Cosmic Ray Research, whose director, Professor Yoji Totsuka, is spokesman for the collaboration.
Other Japanese institutions are Gifu University, the High Energy Research Organization (KEK), Kobe University, Niigata University, Osaka University, Tohoku University, Tokai University and Tokyo Institute of Technology.
Major U.S. collaborators are from Boston University; University of California, Irvine; University of Hawaii; Louisiana State University; State University of New York at Stony Brook; and University of Washington. Other collaborators are from Brookhaven National Laboratory; California State University, Dominguez Hills; Los Alamos National Laboratory; University of Maryland and George Mason University.
U.S. team coordinators are Professors Hank Sobel, UC Irvine (head of the old Reines neutrino group), and Jim Stone of Boston University. U.S. collaborators include many veterans from the IMB experiment.