The K2K, an international collaboration of physicists from Japan, USA, Korea, Russia (INR RAS), Canada, Italy, Spain, France and Switzerland, has published the new result.

A neutrino beam generated at KEK was directed through the Earth to the Super-Kamiokande (SK) underground neutrino detector, located about 250km away.

Using data collected through February, 2004, K2K has observed 107 beam-induced neutrino interactions in SK. In the absence of the phenomenon called neutrino oscillations, which implies that neutrinos have mass, the expected number of such events would be 151 showing appearant deficit of the observed data.

The K2K data are consistent with the oscillation effects previously reported by Super-Kamiokande, using data from naturally-produced atmospheric neutrinos. K2K also reported the first significant evidence for the energy dependence of the oscillation effect, which is an expected consequence of the oscillation phenomenon.

Taking into account measurements of the beam obtained from "near" detectors on the KEK site, the probability that the observed data are consistent with the hypothesis of no oscillations (hence, massless neutrinos) is negligible (5 * 10-5).

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