T2K Results Restrict Possible Values of Neutrino CP Phase – Do particles and antiparticles behave differently? A major step forward published in Nature –

April 16, 2020

Keio University

The T2K Collaboration

The T2K Collaboration has succeeded in excluding a region from -2 degrees to 165 degrees, which is nearly half of the possible range of -180 to 180 degrees for the neutrino CP phase, with a 99.7% confidence level. In the phenomenon of neutrino oscillation, where neutrinos change into different types of neutrinos as they travel through space, the CP phase determines the magnitude of the "difference in behavior between particles and antiparticles." The value of the CP phase, one of the quantities indicating the fundamental properties of neutrinos, was previously completely unknown, and this marks the first time in the world that a constraint has been successfully placed on it.

The T2K experiment, promoted by researchers including Associate Professor Yasuhiro Nishimura of the Department of Physics, Faculty of Science and Technology, Keio University, is an international collaboration with about 500 participants from 12 countries. It observes neutrinos generated at the J-PARC (Japan Proton Accelerator Research Complex) facility, which are detected 295 kilometers away at the Super-Kamiokande detector in Hida, Gifu Prefecture. The results of this research, using data acquired up to 2018, were published in the general academic journal "Nature" on April 16, 2020.

For more detailed information on this research and its results, please see below.

Institute for Cosmic Ray Research, The University of Tokyo

[Press Release] Significantly Restricting the Neutrino "CP Phase" - A Major Step Forward in Verifying the Difference in Behavior Between Particles and Antiparticles, Published in Nature - | ICRR | Institute for Cosmic Ray Research University of Tokyo (in Japanese)

[Press Release] T2K Results Restrict Possible Values of Neutrino CP Phase – Published in Nature, the results are a major step forward in the study of difference between matter and antimatter – | ICRR | Institute for Cosmic Ray Research University of Tokyo (in English)

High Energy Accelerator Research Organization (KEK)

https://www.kek.jp/ja/newsroom/2020/04/16/0000/ (in Japanese)

https://www.kek.jp/en/newsroom/2020/04/16/0000/ (in English)

For the full press release, please see below.