Yoichiro Suzuki

Director, Institute for Cosmic Ray Research, University of Tokyo



The large under-ground water Cherenkov detector, Super-Kamiokande (SK), is able to measure neutrino interactions over the 5 decades of the energy from 5 MeV to a few hundred TeV.
Interactions of the primary cosmic rays with the earth's atmosphere produce secondary particles which subsequently decay into neutrinos. The flux of those atmospheric neutrinos is supposed to be nearly up-down symmetric reflecting the uniform distribution of the primary cosmic rays. In 1998, SK obtained a strong evidence of neutrino oscillations; the deficit of the flux coming upward to the detector, were observed. The measured asymmetric zenith angle distributions well agreed with the expected one from the neutrino oscillation. This is the first evidence that the neutrino has a mass and mixing: the mass difference is ~0.002 eV² and the mixing angle is nearly maximal. This discovery suggests that the current standard theory of particle physics should be extended to include the finite neutrino masses, and indicates the existence of the large energy scale as yet explored.
In 2004, the experimental evidence on the atmospheric neutrino oscillation was strengthened by the observation of the oscillatory pattern in the L/E analysis, and the supporting results from the first man-made long base line neutrino experiment, the K2K experiment.
The solar neutrinos have been measured through neutrino-electron scattering in SK and therefore, the direction, time and energy have been able to measured. SK has clearly observed the 7% annual flux variation which is consistent with the expectation from the eccentricity of the earth's orbit around the sun. The non-observation of the day-night flux differences and the non-observation of the spectrum distortions have placed the strong constraint on the oscillation parameters. In 2000, "the small mixing angle solutions" were rejected and it was shown that the solar neutrino oscillation should have large mixing.
The conclusive evidence of the solar neutrino oscillation was obtained by comparing the precisely measured flux by neutrino-electron scattering in SK and the flux obtained by the charged current interaction of electron neutrinos in SNO in 2001. Neutrino-electron scattering has sensitivity to the electron neutrinos and also to the muon and tau neutrinos, but with a reduced sensitivity. The comparison of the two experiments has indicated that there is no electron neutrino component in the solar neutrino flux measured in SK.
The result of the fit by using all the solar neutrino experiments has selected " a large mixing angle solution" as the solar neutrino oscillation.
The Super-Kamiokande experiment has opened up a new field and we will continue to explore the neutrino physics.





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