The detector consists of an inner volume and an outer volume which contain 32,000 tons and 18,000 tons of pure water respectively. The outer detector is used to veto entering cosmic ray muons and is used as a buffer to keep radiation emitted by the surrounding rock and walls from entering the inner volume. The inner detector has 11,200 photomultiplier tubes (PMTs) attached to the bottom, top and sides facing inward. The PMTs collect the pale blue light called Cerenkov light which is emitted by particles travelling fast as light in the water. By measuring the direction and intensity of this light,information about particle interactions such as neutrino interactions or proton decay can be determined. Compared to Kamiokande, Super-Kamiokande has ten times the volume and twice the density of PMTs. Construction of the detector was completed in 1995 and observation began on April 1, 1996.

Purposes of Research

1. Cosmic Ray Physics

It is believed soon after the big bang, when the universe was at a very high temperature, many neutrinos were produced. In addition, when a star explodes as a supernova, many neutrinos are emitted. Neutrinos are also copiously produced in nuclear reactions in the core of the sun. Also cosmic rays which come into the earth's atmosphere and interact with oxygen or nitrogen nuclei produce neutrinos. Purposes of the research are to elucidate the source of energy of the sun and detect the properties of the enigmatic neutrinos by observing these neutrinos with considerable precision.

There are 4 kinds of forces in nature: strong, weak, electromagnetic and gravitational forces by which particles can interact. The electroweak theory that unifies the electromagnetic force and the weak force has been already established. The next step of elementary particle physics is to establish the Grand Unified Theories which unify the strong, weak and electromagnetic forces. The Grand Unified Theories predict that the basic constituent of matter, protons (the atomic nucleus of hydrogen), decays. They also predict the existence of monopoles, elementary particles which weigh as much as bacteria. We are trying to study the Grand Unified Theories experimentally by seeking proton decay and monopoles.

It is said that the amount of dark matter, matter which cannot be observed directly by optical means, which exists in the universe is over 10 times more than abundant than ordinary matter like Hydrogen or Oxygen. We are trying to reveal what the dark matter is through the research of neutrinos.

Solar neutrinos are used to study the energy source of the sun, nuclear fusion reactions. Also, they are useful to understand neutrino properties since they pass through high densities and travel a long distance during the flight from the core of the sun to the earth. The observation of solar neutrinos was done for 20 years by R. Davis in the United States. The number of solar neutrinos was only 30 percent of the expectation according to his result. To investigate this disagreement by another method, Kamiokande, which had been constructed for the research of proton decay, was renovated for the research of solar neutrinos. Observation was started in 1987. At Super-Kamiokande, more precise observation was started based on the experience from Kamiokande.

Results of Research

Super-Kamiokande, because of its huge volume, has already observed 44,000 solar neutrinos in 300 days. This number is more than 7 times larger than the number observed at Kamiokande in over 2,000 days.

The number of neutrinos is only 37 percent of the theoretical expectation according to the observations at Super-Kamiokande. The lack of solar neutrinos which Davis had asserted for 20 years was true. Though both Davis' experiment and Kamiokande and Super-Kamiokande observed a discrepancy of solar neutrinos, the measured disagreements with theory were different. Thus, the disagreements cannot be explained by changing the theory of solar energy generation.

If we consider this result together with the results from other groups, SAG and and GALLEX, we find it is more likely the lack of neutrinos is caused by the oscillation of neutrinos rather than a problem in solar physics. A precise result from Super-Kamiokande is expected in the future.

If the problem of solar neutrinos would be caused by the oscillation of neutrinos, it is predicted that the number of solar neutrinos is different in the day and at night ; however, there is not much difference in intensity of solar neutrinos between the day and night.

Atmospheric neutrinos contain muon neutrinos and electron neutrinos. The ratio (R=muon neutrinos/electron neutrinos) is expected to be about 2. Super-Kamiokande measures this ratio precisely despite uncertainties in the reactions at low energies.

The reactions that atmospheric neutrinos cause in Kamiokande is the biggest background to the research of proton decay. Therefore it was necessary to analyze atmospheric neutrinos carefully. The analysis of the early data produced the amazing result that the ratio R was close to 1 not 2, in conflict with expectation. It was not certain whether or not this result was caused by neutrino oscillations. To investigate this discrepancy, more data and precise analysis were needed. Observation is comparatively easy because the energies of atmospheric neutrinos are 10〜100 times bigger than the energies of solar neutrinos.

An energy dependence of the ratio R was observed in the lower energy area which is 100 MeV (a hundred million electron volts) to 1.3 GeV (1.3 billion electron volts). The observed value of R in this energy region was 1.2 for both Kamiokande and Super-Kamiokande.

The ratio in the high energy area which was more than 1.3 GeV was only 60 percent of the theoretical expectation both of Kamiokande and Super-Kamiokande. (The value of observation is compared with the one of theory because it is easy for the ratio to be bigger than 2 in the high energy area). Moreover, in the zenith angle distribution of the ratio, the anomaly was especially remarkable in the upward direction.

These results can be explained by neutrino oscillations between muon neutrinos and tau neutrinos.

Proton Decay Research

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