T2K presents first CP violation search results at ICHEP 2016


The international T2K Collaboration announced today their findings on the symmetry between neutrino and antineutrino oscillation. With nearly twice the antineutrino data in 2016 compared to their initial results using antineutrino data in 2015, T2K has performed a new analysis of all data, fitting both neutrino and antineutrino modes simultaneously. The announcement was made by Konosuke Iwamoto, of the University of Rochester, who presented the results at the 38th International Conference on High Energy Physics in Chicago (ICHEP). T2K’s new data continue the trends observed in 2015, which is a preference for maximal disappearance of muon neutrinos, as well as a discrepancy between the electron neutrino and electron antineutrino appearance rates.

Why the universe is dominated by matter today, instead of being comprised of equal parts matter and antimatter, is one of the most intriguing questions in all of science. One of the conditions required for the observed dominance of matter over antimatter to develop is the violation of Charge-Parity (CP) symmetry, which is the principle that the laws of physics should be the same if viewed upside-down in a mirror, with all matter exchanged with antimatter. If CP violation occurs in neutrinos, it will manifest itself as a difference in the oscillation probabilities of neutrinos and antineutrinos.

T2K’s observed electron antineutrino appearance event rate is lower than would be expected based on the electron neutrino appearance event rate, assuming that CP symmetry is conserved. T2K observes 32 electron neutrinos and 4 electron antineutrinos, when they expect around 23 neutrinos and 7 antineutrinos with no CP violation. When analyzed in a full framework of three neutrino and antineutrino flavors, and combined with measurements of electron antineutrino disappearance from reactor experiments, the T2K data favor maximal CP violation.

The actual T2K data yield a 90% confidence interval:
–0.99π < δCP < –0.12π for the normal mass ordering and
–0.67π < δCP < –0.24π for the inverted.
The best fit points lie near the maximally CP violating value δCP = –0.5π , with the CP conserving values (δCP =0 and δCP = π) lying outside of the T2K 90% confidence level interval.

This 2016 result is based on a total data set of 1.51x1021 protons on target (POT), which is 19% of the POT exposure that T2K is set to receive. The probability that this observation is a result of random statistical fluctuations that would mimic a neutrino-antineutrino asymmetry when none exists is about 1 in 20, motivating the need for more neutrino and antineutrino data to explore and solidify these intriguing results.

The full T2K exposure of 7.8x1021 POT is expected to come by ~2021, thanks to planned upgrades to the J-PARC MR accelerator and the neutrino beamline. Moreover, T2K is proposing a run extension that will lead to a full exposure of 20x1021 POT, with 3σ sensitivity to CP violation observation by ~2025, when the next generation experiments are expected to begin operations.

More information about the T2K experiment can be found on the T2K public website and on our institutes website.