In 2013, with just 8.4% of the currently approved exposure, νμ
appearance was discovered by T2K [1
]. Along with the νe
disappearance at reactor experiments, this observation confirmed the non-zero θ13
and thus opened the door to explore CP violation effect, which arises from an irreducible phase δCP
in the standard mixing matrix. T2K
, two largest long-baseline neutrino experiments in the world, started to search for this effect. Hyper-K
, the next generation of long-baseline neutrino experiments also put this search as the main target.