Neutrinos: 1998

Construction of Kamioka Underground Observatory, the predecessor of the present Kamioka Observatory, Institute for Cosmic Ray Research, University of Tokyo began in 1982 and was completed in April, 1983. The purpose of the observatory was to detect proton decay, one of the most fundamental questions of elementary particle physics.

The detector, named KamiokaNDE for Kamioka Nucleon Decay Experiment, was a tank which contained 3,000 tons of pure water and had about 1,000 photomultiplier tubes (PMTs) attached to the inner surface. The size of the tank was 16.0 m in height and 15.6 m in diameter. An upgrade of the detector was started in 1985 to allow the detector to observe solar neutrinos. As a result, the detector (KamiokaNDE-II) had become sensitive enough to detect neutrinos from SN 1987A, a supernova which was observed from in the Large Magellanic Cloud in February 1987. Solar neutrinos were observed in 1988 adding to the advancements in neutrino astronomy and neutrino astrophysics. The ability of the Kamiokande experiment to observe the direction of electrons produced in solar neutrino interactions allowed the experimenters to directly demonstrate for the first time that the sun was a source of neutrinos.

Despite its success in neutrino observation, Kamiokande did not detect proton decay, its first aim. Also, even higher sensitivity was needed to observe neutrinos with high statistical confidence. This led to the construction of Super-Kamiokande, with ten times more water volume and PMTs than Kamiokande. Super-Kamiokande started observation in 1996.

Super-Kamiokande Collaboration announced the first evidence of neutrino oscillations in 1998, consistent with the theory that the neutrino has non-zero mass. Until this, all observational evidences were consistent with neutrinos being massless, although theorists had speculated on the possibility of neutrinos having non-zero mass for many years.

Neutrino oscillation is a quantum mechanical phenomenon whereby a neutrino created with a specific lepton flavor (electron, muon or tau) can later be measured to have a different flavor. The probability of measuring a particular flavor for a neutrino varies periodically as it propagates. Neutrino oscillation is of theoretical and experimental interest as observation of the phenomenon implies that the neutrino has a non-zero mass, which is not part of the original Standard Model of particle physics.

We announce today at "Neutrino '98", the international physics conference underway in Takayama, Japan, that the Super-Kamiokande Experiment has found evidence for non-zero neutrino mass. Neutrinos are tiny, electrically neutral, sub-atomic particles. Papers related to the results were submitted to the scientific journals "Physical Review Letters" and "Physics Letters." The experiment yields results that are outside the standard theory of particle physics, which describes the fundamental constituents of matter and their interactions. Until now, there has been no firm evidence that neutrinos possess mass.

The new evidence is based upon studies of neutrinos which are created when cosmic rays, fast-moving particles from space, bombard the earth's upper atmosphere producing cascades of secondary particles which rain down upon the earth. Most of these neutrinos pass through the entire earth un-scathed. The Super-Kamiokande group uses a large, 50,000 ton tank of highly purified water, located about 1000 meters underground in the Kamioka Mining and Smelting Company Mozumi Mine. Faint flashes of light given off by the neutrino interactions in the tank are detected by more than 13,000 photomultiplier tubes that were manufactured for the experiment by Hamamatsu Corporation.

By classifying the neutrino interactions according to the type of neutrino involved (electron-neutrino or muon-neutrino) and counting their relative numbers as a function of the distance from their creation point, we conclude that the muon-neutrinos are "oscillating". Oscillation is the changing back and forth of a neutrino's type as it travels through space or matter. This can occur only if the neutrino possesses mass. The Super-Kamiokande result indicates that muon-neutrinos are disappearing into undetected tau-neutrinos or perhaps some other type of neutrino (e.g., sterile-neutrino). The experiment does not determine directly the masses of the neutrinos leading to this effect, but the rate of disappearance suggests that the difference in masses between the oscillating types is very small. The primary result that we are reporting has a statistical significance of more than 5 standard deviations. An independent measurement based on upward-going muons in the detector confirms the result at the level of more than 3 standard deviations.

The Super-Kamiokande Collaboration includes scientists from 23 institutions in Japan and the United States. Principal funding for the experiment is provided by the Japanese Ministry of Education, Science, Sports, and Culture (Mombusho) while funding for the detector's outer most region is provided by the United States Department of Energy. In addition to advancing our understanding of basic science, the collaboration has established a strong international partnership between the Japanese and American teams.

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. In our studies, we have found interesting results 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. In addition, we obtained an indication that the observed energy spectrum of those neutrinos is deformed from the the predicted one. Super-Kamiokande's observation of too few electron-neutrinos coming from the direction of the sun also may be interpreted as due to oscillations. We are continuing to study this exciting possibility.

Reflecting on the significance of the new finding, we note that massive neutrinos must now be incorporated into the theoretical models of the structure of matter and that astrophysists concerned with finding the 'missing or dark matter' in the universe, must now consider the neutrino as a serious candidate.

--The Super-Kamiokande Collaboration