Reactor measurement of \theta_{12}; Secret of the power

We demonstrate enormous power of dedicated reactor experiment for \theta_{12} with a detector placed at around the first oscillation maximum, which we call "SADO". It allows determination of sin^2\theta_{12} to the accuracy of \simeq 2% at 1 \sigma CL, which surpasses all the method so far proposed. Unlike reactor \theta_{13} experiments, the requirement for the systematic error is very mild, \simeq 4%, which makes it an even more feasible experiment. If we place a detector at \sim 60 km away from the Kashiwazaki-Kariwa nuclear power plant, 0.5 kt yr exposure of SADO is equivalent to \sim 100 kt yr exposure of KamLAND assuming the same systematic error.