The T2K (Tokai-to-Kamioka) experiment in Japan has observed three electron antineutrino events in a muon antineutrino beam. T2K creates and monitors a muon antineutrino beam at the J-PARC facility in Tokai village on the east coast of Japan, and sends it 295 km west to the Super-Kamiokande detector underneath Mt. Ikeno near Toyama, which was built, and is operated, by ICRR. This is the first antineutrino appearance search by the collaboration, which also observed muon antineutrino disappearance earlier this year. The appearance and disappearance of neutrino and antineutrinos flavours is called neutrino oscillation and occurs because of the quantum mechanical interference of neutrino masses and flavours.
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KEK INPS News Room “First look at antineutrino appearance from T2K”