First exclusion of CP conservation in neutrino oscillation at 90% confidence level:
The observation of muon to electron-neutrino oscillation opens the possibility of using neutrino oscillation to study CP violation by comparing neutrino flavour transformations between neutrinos and anti-neutrinos. CP violation has been observed in quarks but not yet measured in leptons (particles, including neutrinos, that do not undergo strong interactions). The size of quark CP violation is not enough to explain the matter anti-matter asymmetry of the universe, and as such, leptons' role may be significant. As reported in Physical Review Letters (2017) as an Editor's Suggestion, T2K performed a combined analysis of neutrino and anti-neutrino oscillations and notably excluded CP conservation at the 90% confidence level.