September, 2000
Last Update: March 2001

Present status of the K2K Long Baseline Neutrino Oscillation Experiment


The purpose of the K2K(KEK-to-Kamioka) Long Baseline Neutrino Oscillation Experiment is to confirm neutrino oscillation. The University of Tokyo's 50,000-ton water-Cherenkov detector, Super-Kamiokande, located at Kamioka in Gifu Prefecture, 250 km away from KEK, detects artificially produced muon neutrinos fired from KEK 12 GeV Proton Synchrotron. The phenomenon of neutrino oscillation means neutrinos to have non-zero mass. In June 1998, the Super-Kamiokande collaboration reported strong evidence for neutrino oscillation (muon neutrino to tau neutrino) in the atmospheric neutrino data taken with the Super- Kamiokande detector. If muon neutrinos oscillate into tau neutrinos on their way to Kamioka from KEK, the number of muon neutrinos observed in the Super-Kamiokande detector will be much smaller than the number expected without oscillation.

On June 19, 1999, the K2K Experiment observed its first neutrino event due to the KEK neutrino beam in the Super-Kamiokande detector. The data were taken in June and November, 1999, and in January, February, March, May, and June, 2000. Until the end of June, 2000, 28 neutrinos fired from KEK were detected inside the central 22,500 tons of the 50,000-ton water Cherenkov detector*. If these neutrinos came from KEK, the time of occurrence of each event in Super-Kamiokande had to coincide with the expected time, which is calculated from the time at which the neutrino beam was shot from KEK and its time-of-flight between KEK and Super-Kamiokande. The agreement between the observed and expected times were found to be better than a millionth of second, verifying that all the 28 events are really due to neutrinos fired form KEK.

On the other hand, 37.8+3.5-3.8 neutrino events were expected to be observed in the Super- Kamiokande detector if neutrinos do not oscillate. This number can be estimated from the number of neutrino events observed in the near neutrino detector located at KEK. The fact that the observed number of events in the Super-Kamiokande is smaller than expected means that null neutrino oscillation is disfavored at 90% probability, where the statistical and systematic errors of the data are taken into account. This result was reported for the first time on July 28, 2000, at the ICHEP 2000 Conference held in Osaka, Japan. Scientifically, neutrino oscillation can be concluded when the probability of supporting this hypothesis becomes higher than 99%. We aim at this goal by running the experiment for some more years.

K2K collaboration

* By using the central 22,500 tons of the Super- Kamiokande detector, we can make reliable observations with the smallest systematic errors. All the Super- Kamiokande observations including atmospheric neutrinos and solar neutrinos use the central 22,500 tons.

Fig.1 One of the Super-Kamiokande events caused by neutrinos fired from KEK
Fig.2 The time at which a neutrino is detected in Super-Kamiokande and the time at which the neutrino beam pulse is shot from KEK are both measured with use of GPS. The difference of these times, after further subtracting time of flight of the neutrinos between KEK and Super-Kamiokande, distribute between -0.2 and +1.3 microsecond. This is consistent with the pulse width (1.1 microsecond) of the neutrino beam from KEK, considering the resolution of time measurements.

For further information:

K2K Web pages

K2K Contact Persons

K2K Collaboration Spokesperson

Prof. K. Nishikawa
Dept. of Physics
Kyoto University
Kyoto, Japan
TEL: 075-753-3859 / 3820

KEK Representative

Prof. K. Nakamura
Tsukuba-shi, Ibaraki-ken
TEL: 0298-64-5435

Super-Kamiokande Spokesperson

Prof. Y. Totsuka,
Kamioka Observatory
ICRR, University of Tokyo
Kamioka, Gifu-ken
TEL: 0578-5-9600

ICRR representative

Prof. Y. Suzuki,
Kamioka Observatory
ICRR, University of Tokyo
Kamioka, Gifu-ken
TEL: 0578-5-9601

Co-spokespersons for US K2K collaboration

Prof. Chang Kee Jung
Dept. of Physics and Astronomy
The State University of New York at Stony Brook
Stony Brook, NY 11794-3800
TEL: 516-632-8108, 516-474-4563

Prof. Jeffrey Wilkes
Department of Physics
University of Washington
Seattle, WA 98195-1560
TEL: 206-543-4232

Spokesperson for Korean K2K collaboration

Prof. C.O. Kim
Department of Physics
Korea University
Seoul 136-701
Republic of Korea
TEL: 2-3290-4338, 2-964-9175