Latest results from K2K
 
Neutrino mass and mixings are one of the keys to access beyond the standard model. The K2K experiment probes them through the neutrino oscillation experiment, which is suggested by the atmospheric neutrino observation by Super-Kamiokande in Kamioka using the neutrino beam generated by KEK 12 GeV-PS.
 
First, flux and spectrum of neutrino beam are measured by the near detector located in KEK site just after the neutrino production. After the 250km of flight, neutrinos are measured by the far detector, Super-Kamiokande, and are compared with expected values based on the near detector measurement.
 
K2K experiment started in 1999. The Super-Kamiokande detector was rebuilt with about half of the original photomultipliers after the accident in 2001. Total of 112 events were observed by Feb. 2004, for which 156+12/-10 events are expected if there is no neutrino oscillation. Figure 1 shows clear difference between expected and observed in the reconstructed energy spectrum which proved the existence of the neutrino oscillation. The probability to justify that neutrino oscillation does not exit according to our experiment ressult (the deficit of events and spectrum distortion) is estimated to be 0.003%. K2K confirmed the atmospheric neutrino oscillation.
 
We plan to improve the data quality further and to measure the unknown oscillation parameters with the next long baseline experiment, T2K, utilizing the neutrino source from J-PARC
 

The author of this article Associate Professor Kenji Kaneyuki is a member of K2K and Super-Kamiokande collaborations. He is also the member of next generation long baseline experiment, T2K collaboration.