Scientists detect neutrino beam traversing 250 km.
The beam was the first observed to traverse such a distance, which separates the High Energy Accelerator Research Organization (KEK) facility in Tsukuba, about 40 km northeast of Tokyo, and the Super-Kamiokande detector in the town of Kamioka, Gifu Prefecture, team members told reporters at the Education Ministry.
The experiment, dubbed K2K, marked a vast increase on the longest previously observed distance traveled by subatomic particles -- 1 km -- as well as the first step toward verifying the results of experiments, conducted in water last year, that suggested neutrinos possess mass.
If the findings are verified in additional experiments over the next three years or more, they could overturn the standard theory of particle physics that neutrinos have zero mass.
The K2K experiments involve the firing of a neutrino beam from the KEK proton synchroton accelerator to a near detector in the KEK lab and eventually to the Super-Kamiokande detector, a 50,000-ton tank of purified water 1,000 meters inside Kamioka Mine in the central Japan prefecture.
By comparing the neutrinos recorded in the near and far detectors, the scientists can observe neutrino oscillation, in which the particles change from muon-neutrinos to tau-neutrinos -- a phenomenon that requires mass.
Neutrinos also exist as electron-neutrinos.
The team, which began its experiments June 4, achieved the breakthrough at 6:42 p.m. on June 19 when more than 13,000 photomultiplier tubes lining the Super- Kamiokande tank were able to detect faint flashes of light from neutrino interactions.
It was concluded that the neutrino particles, which traveled the distance in a straight line at the speed of light, were artificially generated by the KEK accelerator because they were detected 0.00083 second after the beam was fired from Tsukuba.
The probability that the particles observed came from atmospheric neutrino interactions is small, at approximately 0.01%, the scientists said.
The team is planning to conduct 300 additional experiments over the next three years. If the number of muon neutrinos observed in the Super-Kamiokande detector is smaller than the number right after firing, it can be concluded that the particles oscillated into tau neutrinos, which would support the hypothesis that neutrinos have mass, they said.
In June last year, the Super-Kamiokande research group, led by the University of Tokyo's Institute for Cosmic Ray Research, found evidence showing for the first time that atmospheric neutrinos have mass.
The K2K experiments, which were devised to verify last year's results, are being conducted by 100 Japanese, U.S. and South Korean scientists with funding from Japan's Education Ministry, the U.S. Department of Energy and South Korea's Science and Technology Ministry, among others.