J-PARC Press Release: Electron neutrino oscillation detected at T2K

Tsukuba, Japan, June 15, 2011. 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 Super-Kamiokande detector, in Japan. © 2011, High Energy Accelerator Research Organization, KEK.

The T2K experiment is searching for the neutrino oscillation phenomena, where particular types of neutrinos transform into other types of neutrinos. These observations help 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, located 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 into electron neutrinos (electron neutrino appearance) is the primary goal of the T2K experiment. Observing the appearance of electron neutrinos will determine the direction of future neutrino physics research, as well as provide the most promising clue to the mystery of the matter-dominated universe, and thus researchers around the world pay 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 the 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 were identified out of these eighty-eight events as cleanly identifiable electron neutrino interactions.

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 of 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 a 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.

The Japan Proton Accelerator Research Complex (J-PARC). © 2011, High Energy Accelerator Research Organization, KEK.

T2K is a CERN recognized experiment (RE13). The CERN contributions to T2K include: the gifted UA1/NOMAD magnet and magnetic measurements; the construction, test and calibration of detectors for the ND280 near detectors; and the precise measurements of particle production by 30 GeV protons on carbon and on the T2K replica target, by the NA61/SHINE experiment.

by T2K Press Office