Precise measurement of $\sin^22\theta_{13}$ using Japanese Reactors

After the KamLAND results, the remaining important targets in neutrino experiments are to measure still unknown 3 basic parameters; absolute neutrino mass scale, CP violation phase $\delta_{CP}$ and last mixing angle $\theta_{13}$. The angle $\theta_{13}$ among them is expected to be measured in near future by long baseline accelerator experiments and reactor experiments. In this paper, a realistic idea of high sensitivity reactor measurement of $\sin^22\theta_{13}$ is described. This experiment uses a giant nuclear power plant as the neutrino source and three identical detectors are used to cancel detector and neutrino flux uncertainties. The sensitivity reach on $\sin^22\theta_{13}$ is $0.017\sim0.026$ at $\Delta m^2_{13} \sim 3 \times 10^{-3}eV^2$, which is five to seven times better than the current upper limit measured by CHOOZ.