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Baryon Asymmetry of the Universe

Particles have their partners which have opposite “charge” called 'anti-particles'. For example, protons and neutrons, which constitute usual matter called 'Baryon', have their counter-parts, anti-baryons.

In the early universe, it is a natural assumption that baryons and anti-baryons had been produced by equal amount, however, antibaryons are barely observed in the universe and there is a remarkable imbalance of the baryon and anti-baryons. This ‘Baryon Asymmetry’ is one of the unsolved problems. There are several hypotheses proposed to explain the observed imbalance of the matter and anti-matter. Among of them, one of the possible mechanisms to explain the asymmetry is 'CP violation'.


C-symmetry is a symmetry under the exchange particle and anti-particle, which is called charge conjugation translation. If a phenomena in a system and corresponding phenomena in a charge conjugated system, the system has C-symmetry.

P-symmetry (Parity symmetry) is an symmetry under the mirror translation and CP-symmetry is a combination of C-symmetry and P-symmetry.

CP violation on Neutrinos

One interesting hypothesis explains that the baryon asymmetry is induced by the violation of the CP symmetry on neutrinos.

Partner of a neutrino under CP-symmetry is anti-neutrino and if CP-symmetry on neutrinos are violated, it is expected that the probabilities of some phenomena between neutrinos and anti-neutrinos are different.

Hyper-Kamiokande project is designed to investigate the difference of the neutrino oscillation probabilities between neutrinos and anti-neutrinos with intensive neutrino/anti-neutrino beam from J-PARC accelerator in Tokai village. All neutrino mixing parameters are already measured and have finite values. Fortunately, the values are relatively large and it makes it possible to investigate the CP violation of neutrinos. CP violation is expressed by an angle δ. The effect of CP violation is maximumly observed at δ=±90 degree. For that case, Hyper-Kamiokande is expected to discover non-zero size of the violation of the CP-symmetry at 8 sigma significance in 10 years of operation, and for 75% of the δ space, we expect to discover non-zero CP violation with 3 sigma significance.