Artificial Neutrino Beam Detected After Passing Through 250km of Earth

Announced on Monday, June 28, 1999, 14:00 Japan Standard Time
(in USA, = 7 pm Sunday 6/27 Hawaii Standard Time, 10 pm Sunday 6/27 PDT, 1 am Monday 6/28 EDT)


The First K2K event at Super-Kamiokande:

Each PMT's color indicates the time of arrival of the light collected, and the size indicates the amount of charge. The crosses give the reconstructed neutrino interaction vertex, and the diamond marks the neutrino beam direction from the vertex. The right-upper figure is the outer anti detector(OD), which surrounds the inner detector. No response in the OD indicates the interaction occured inside of the inner detector.

Related Graphics:

On June 19, 1999, 6:42 PM, Japanese Standard Time, the K2K (KEK to Kamioka) Long Baseline Neutrino Oscillation Experiment observed its first neutrino event due to the KEK neutrino beam in the Super-Kamiokande detector, the first step towards the verification of the neutrino oscillation results announced by the Super-Kamiokande experiment in June last year. This is also the first demonstration that a particle that had been produced artificially and traversed 250km in Earth was detected. The event characteristics are consistent with a neutrino interaction in water. The time of the event is within approximately one micro-second of the expected event time. Both the direction and the time of the event are in the range of  expectation considering the detection resolution of the experiment. The probability that the event came from an atmospheric neutrino interaction is estimated to be 0.01%, or one part in ten thousand.

KEK is a Japanese national laboratory for High Energy Accelerator Research and  is located in Tsukuba City, near Tokyo on the east coast of the main island of Japan. Super-Kamiokande is a 50000 ton water Cherenkov detector, situated at the Kamioka Observatory, Institute for Cosmic Ray Research, the University of Tokyo, 250 km away from KEK.

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. That finding was a major discovery, with a far reaching impact in  elementary particle physics, cosmology and astrophysics. The phenomenon of neutrino oscillation, which requires neutrinos to have non-zero mass, will alter our view of the world of elementary particles. Consequently the Standard Model, the currently prevailing theory of the elementary particles, must be modified. In the Standard Model the neutrinos are assumed to have zero mass. The finding will also make the theories of the Grand Unification more viable and attractive, and make the universe heavier than we currently assume.

To confirm the above finding by the Super-Kamiokande experiment with an accelerator-produced neutrino beam, we proposed the K2K experiment, which consists of a neutrino beam line, a near detector complex inside the KEK laboratory, and a far detector (the Super-Kamiokande detector) at Kamioka, 250 km away from KEK.

In the K2K experiment, the neutrino beam generated by the KEK proton synchrotron accelerator is aimed at the near and far detectors, which are carefully aligned in a straight line. Then, by comparing the neutrino events recorded in these detectors, we can examine the neutrino oscillation phenomenon. For example, 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. It would appear that  muon neutrinos have "disappeared".

The K2K near detector complex consists of sophisticated particle detectors: a one kiloton water Cherenkov detector (a miniature Super-Kamiokande detector), a scintillating fiber tracker with water targets, a scintillator veto counter, a lead glass calorimeter and a muon range detector.

K2K near detector construction was completed in January, 1999, and neutrino beam line commissioning was started on January 27, 1999. On March 5, 1999, we started beam line tuning, and we are currently (June, 1999) taking data. Among all planned or proposed long baseline neutrino oscillation experiments, K2K is the first experiment to be online.

The K2K collaboration is an international consortium of institutions from Japan, Korea, and the United States. There are about 100 collaborating members from 20 institutions. KEK and ICRR, University of Tokyo, are the co-host institutions of the experiment.

The K2K experiment has been built and operated from funding by the Japanese Ministry of Education, Science, Sports and Culture, the United States Department of Energy, the Korea Science and Engineering Foundation, the Ministry of Science and Technology, the Korea Research Foundation.

For further information on this announcement or on the K2K experiment, please visit the following web sites and/or contact our representatives.

-The K2K Collaboration

June 28, 1999

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