Do neutrinos and antineutrinos have the same probability of switching "flavor", otherwise known as oscillating? A negative response would reveal a violation of Charge-Parity (CP) symmetry similar to that observed in 1964 for quarks. This CP violation is directly related to the enigmatic predominance of matter in today's universe, instead of the equal amounts of matter and antimatter that were produced during the Big Bang.

At the T2K experiment in Japan, scientists observe the oscillations of neutrinos or antineutrinos over a distance of 295 km. Between the close and remote detectors, the scientists have recorded 89 electron neutrinos and 7 electron antineutrinos, instead of the 67 and 9 expected in the absence of CP violation.

Combined with measurements of reactor neutrino oscillations, these new results reject the hypothesis that neutrinos and antineutrinos oscillate with the same probability at a 95% confidence level. This value would have to reach 99.7% in order to eliminate the hypothesis of a statistical fluctuation. This question should be answered within the next few years, as the 2021-26 campaign will include a more efficient near detector.