Reactor measurement of θ12:: Principles, accuracies, and physics potentials -: art. no. 013005

We discuss reactor measurement of theta(12) which has a potential of reaching the ultimate sensitivity which surpasses all the methods so far proposed. The key is to place a detector at an appropriate baseline distance from the reactor neutrino source to have an oscillation maximum at around a peak energy of the event spectrum in the absence of oscillation. By a detailed statistical analysis the optimal distance is estimated to be similar or equal to(50-70) kmx[8x10(-5) eV(2)/Deltam(21)(2)], which is determined by maximizing the oscillation effect in the event number distribution and minimizing geo-neutrino background contamination. To estimate possible uncertainty caused by surrounding nuclear reactors in distance of similar to100 km, we examine a concrete example of a detector located at Mt. Komagatake, 54 km away from the Kashiwazaki-Kariwa nuclear power plant in Japan, the most powerful reactor complex in the world. The effect turns out to be small. Under a reasonable assumption of systematic error of 4% in the experiment, we find that sin(2)theta(12) can be determined to the accuracy of similar or equal to2%(similar or equal to3%), at 68.27% CL for 1 degree of freedom, for 60 GW(th).kton.yr (20 GW(th).kton.yr) operation. We also discuss implications of such an accurate measurement of theta(12).