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

KEK = Korporata Energjetike e Kosoves
ref. www.ear.eu.int

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