Tsukuba, February 24, 2010. The T2K experiment, whose primary purpose is to study neutrino interactions at a large distance from their source, has detected 6 electron neutrino candidate events based on the data collected before March 11, 2011. For the first time, it was possible to observe an indication that muon neutrinos are able to transform into electron neutrinos over a distance of 295 km through the quantum mechanical phenomena of neutrino flavor oscillations.

The T2K experiment aims to search for neutrino oscillation, phenomena in which a particular types of neutrinos transform into other types of neutrinos. These observations help to determine neutrino masses, as well elucidating the uncharted nature of neutrinos such as the relationship among three neutrino generations (types). T2K aims at the world’s best sensitivity by detecting neutrinos with the Super-Kamiokande detector in Kamioka in Gifu Prefecture, Japan, situated 295 km away from the Japan Proton Accelerator Research Complex (J-PARC) where the muon neutrinos are produced. In particular, observing oscillations from muon neutrinos to electron neutrinos (electron neutrino appearance) is the primary goal of the T2K experiment. Observing the appearance of electron neutrinos will determine the research direction of neutrino physics in the future, as well as provide the most promising clue to the mystery of the matter dominated universe, and thus researchers around the world draw special attention to and compete in such observations. The T2K experiment, claiming the world’s best sensitivity, has internationally attracted more than 500 researchers from 12 countries.

Based on the analysis of all data collected by the T2K experiment between January 2010 when it began full operation and March 11, 2011 when it was interrupted due to the great East Japan earthquake, 88 neutrino events were detected by the Super-Kamiokande. Six candidate events cleanly identifiable as electron neutrino interactions were identified out of these eighty-eight events.

When electron neutrinos interact with matter, electrons are produced. However, electrons are also observed with some probability in background events other than the electron neutrino appearance. In the current T2K experiment, 1.5 such background events were expected to be detected and thereby the probability of the existence of electron neutrino appearance is estimated to be 99.3%, suggesting the appearance of electron neutrinos for the first time.

The T2K experiment collected about 2% of the original goal of the total number of events to be collected before the great East Japan earthquake hit on March 11, 2011. After J-PARC resumes producing muon neutrinos, which is planned to happen by the end of 2011, the T2K experiment will continue striving to accumulate the target number of events to confirm electron neutrino appearance, as well as pursue the further understanding of this appearance by combining the neutrino measurements with measurements using anti-neutrinos, which is also the purpose of this experiment. Additionally, the researchers aim to search for CP violation in leptons to explore the origin of matter in the universe by upgrading the accelerators at J-PARC to much higher intensity and enhancing the performance of the detectors. Electron neutrino appearance is the key to detecting leptonic CP violation, and the current observation result indicates that the T2K experiment has made a significant step towards this future goal.

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