Why the universe is primarily comprised of 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 (Parity), with all matter exchanged with antimatter (Charge). The observed difference in the properties of matter and anti-matter, known as CP violation, has been observed in interactions involving quarks for many years. Such an effect has not been observed in neutrino interactions to date. Observation of this effect in the neutrino sector will open up extra avenues of inquiry into the question of matter domination in the universe.
The T2K experiment has been studying the properties of neutrinos through flavour oscillations, a process by which one type of neutrino can change to another type as the neutrino propagates through space. If CP violation occurs in neutrinos, it will manifest itself as a difference in the oscillation probabilities of neutrinos and anti-neutrinos. It does this by firing a pure beam of muon neutrinos from an accelerator at the Japan Proton Accelerator Research Complex (J-PARC) located in Tokai village, Ibaraki prefecture, on the east coast of Japan, towards the gigantic Super-Kamiokande underground detector in Kamioka, near the west coast of Japan, 295 kilometers (185 miles) away from Tokai. T2K searches for muon (anti)neutrinos changing flavour to electron (anti)neutrinos during their trip.
Whether the probability to see an electron neutrino at Super-Kamiokande exceeds, or not, the probability to observe an electron antineutrino depends on the value of a parameter called the CP violating phase, δCP, introduced by Kobayashi and Maskawa. The CP violating phase can take any value from –180o to +180o, and if it deviates from 0o and ±180o then CP violation occurs. It has been measured to be around 68o for quarks. T2K’s goal is to measure the neutrino CP phase for the first time.
T2K has recently released the results of its latest analysis. The T2K data excludes CP conservation at the 95% confidence level. The 90% CL allowed region for the CP violating phase, δCP, is [–161o; –48o], with the best fit point being –105o. This is significantly different from the value observed in the quark sector. This 2017 result is based on a total data set of 2.25x1021 protons on target (POT), which is 28% of the POT exposure that T2K is set to receive. If there were no neutrino-antineutrino asymmetry, the chance of observing an asymmetry as large as what T2K observed, due to random statistical fluctuations, is about 1 in 20. To explore and solidify this intriguing hint the T2K collaboration need more neutrino and anti-neutrino data. The full T2K exposure of 7.8x1021 POT is expected to come by ~2021.