「First time observation of electron neutrino appearance by T2K experiment」


Neutrino oscillation is a quantum mechanical phenomenon whereby an elementary particle called neutrino can change its “flavor” (electron, muon, or tau flavor) during its flight. There are three modes to neutrino oscillation predicted by the Standard Model of particles, and while the first and the second modes have been extensively studied and determined by neutrino experiments such as Super-Kamiokande through observation of solar and atmospheric neutrinos, the third mode has yet to be established. One such method is to measure the oscillation of muon neutrinos produced in a particle accelerator into electron neutrinos.

By the data collected until April 2013, T2K (Tokai-to-Kamioka long baseline neutrino oscillation experiment) observed 28 electron neutrino candidates at the far detector Super-Kamiokande, more than would be expected (4.6 events) without the electron neutrino appearance phenomena. The probability that random fluctuations of background events alone would produce the observation is less than one in a trillion. Therefore, T2K shows definitive evidence for the appearance of electron neutrinos as a result of muon neutrino transitions into electron neutrinos in their flight.

The discovery of the electron neutrino appearance opens the possibility to explore a key to the mystery of the universe--why the universe is dominated by matter particles, not anti-matter particles. We now enter new era of precision measurement of the electron appearance probability toward the goal. The T2K experiment is expected to accumulate 10 times more data in the near future. In addition, the Hyper-Kamiokande detector holding 1 million ton water target being about 20 times larger than Super-Kamiokande is under development.


[Full text of press release]


* In 2011, First time in the world 「T2K Experiment observes indication of electron neutrino appearance phenomena」
* The T2K experiment
* About the T2K experiment(Kamioka)
* About the T2K experiment(KEK)
* High Energy Accelerator Research Organization
* Institute for Cosmic Ray Research, University of Tokyo
* J-PARC center