Search for Electron Antineutrino Appearaance
In the T2K experiment, in which the High Energy Physics Laboratory of Osaka City University is participating, data has been collected in anti-neutrino mode since May 2014, and the disappearance phenomenon of muon anti-neutrinos has already been observed as of May 2015. Currently, data collection is ongoing to observe the appearance phenomenon of electron anti-neutrinos. During the first run in anti-neutrino mode from May 2014 to June 2015, data amounting to 10% of the goal was acquired, and three candidate events for electron anti-neutrinos were found within this data.
 Figure 1 : A candidate event of the electron anti-neutrino interaction observed at Super-Kamiokande. |
 Figure 2 : The distribution of the momentum and scattering angle of three observed electron candidates produced by electron anti-neutrino interaction. Three black dots show data, and the colored background histogram represents the expected distribution based on the neutrino mode results. |
Figure 1 shows one of these events observed in the Super-Kamiokande detector. In the T2K experiment, data was collected in neutrino mode from 2010 to 2013 to measure the rate at which muon neutrinos change into electron neutrinos due to neutrino oscillation. Applying this rate directly to anti-neutrinos, 3.8 electron anti-neutrino events, including 1.8 background events, were expected. Figure 2 shows the distribution of the momentum and scattering angle of electrons produced by the electron anti-neutrino interactions. The three black dots represent the data, while the colored histogram shows the expected distribution based on the results of the neutrino mode. With only three data points, it is still too early to draw any conclusions, but this is the first result in antineutrino mode in the T2K experiment.
If CP symmetry is violated in neutrinos, the appearance rate of electron anti-neutrinos may differ from the expected rate. The violation of CP symmetry is a significant idea to explain why the universe is mostly made up of matter with very little anti-matter, and it is believed that this might also be caused by neutrinos. Currently, data collection continues with increased beam intensity from the accelerator, and adding this data is expected to provide further insights.