The evidence of the neutrino oscillations was discovered by the study of atmospheric neutrinos in 1998. In 1999, an accelerator based experiment was started to study this phenomena. This experiment is called the KEK to Kamioka long baseline neutrino oscillation experiment (K2K). The neutrinos are artificially generated by using the proton accelerator at KEK and observed in the Super-Kamiokande detector located 250km away from KEK.
This experiment was completed in Nov. 2004. As a result, 112 neutrino events are observed in Super-Kamiokande, while the expected number of events without oscillations is 158. The probability to observe the deficit without neutrino oscillations is estimated to be 0.0015% and this deficit confirms the prediction of the neutrino oscillation discovered by the observation of atmospheric neutrinos.