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. Principle 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
The Super-Kamiokande experiment, using a gigantic water filled detector, began operation over 2 years ago in April, 1996. Today, we announced that evidence has been found for non-zero neutrino mass (See attached, EVIDENCE FOR MASSIVE NEUTRINOS). This important physics result follows from our precise measurement of the composition of atmospheric neutrinos. We will continue our observations and will study the details which may clarify the role of the mysterious neutrinos in elementary particle physics and in the universe.
In addition, we will continue to study the characteristics of electron-neutrinos from the sun; we will maintain a vigilant watch for neutrino bursts arising from supernovae explosions within our galaxy; and we will search for proton decay to the longest lifetimes ever probed experimentally. We expect results from these studies to be as significant as the present finding of finite neutrino mass through our study of atmospheric neutrinos.
The sensitive measurements being made by Super-Kamiokande require continuous operation of the detector facility to obtain maximum efficiency for acquiring data. This is essential if important new physics results are to be achieved. Unfortunately, due to the financial difficulty of the Japanese government, the operating funds for Super-Kamiokande have been reduced by 15% for the present fiscal year. This is a serious budget cut for the experiment and may require us to cease operation for up to 2 months this year.
An additional 15% cut back is expected in the next fiscal year, resulting in a 30% reduction compared to the JFY97 budget. Under these circumstances, we will have no option but to stop the operation of the experiment and make no observations for a certain time, perhaps as long as 4 months. We regret that these cuts in government funds are being applied equally, across-the-board, to all institutions, without consideration of the relative scientific importance of the projects or the devastating impact that such cuts may have.
Super-Kamiokande has begun to produce physics results which will require changes in our standard pictures of particle physics and the universe. More results are expected with continued operation of the experiment. Budget cuts which stop the detector operation, jeopardize the strength of the international collaboration and could result in the loss of important observations, such as a rare supernova event. The U.S. collaborators are requesting that ICRR, The University of Tokyo, make every effort not to interrupt the experiment. We ask everyone to understand the importance of this international collaboration for obtaining new observations in basic physics research. We appeal for continuous operation support for Super-Kamiokande.
The Super-Kamiokande Collaboration.