Reactor Measurement of $\theta_{13}$ and Its Complementarity to Long-Baseline Experiments

A possibility to measure $\sin^22\theta_{13}$ using reactor neutrinos is examined in detail. It is shown that the sensitivity $\sin^22\theta_{13}>0.02$ can be reached with 20 ton-year data by placing identical CHOOZ-like detectors at near and far distances from a giant nuclear power plant whose total thermal energy is 24.3 ${\text{GW}_{\text{th}}}$. It is emphasized that this measurement is free from the parameter degeneracies which occur in accelerator appearance experiments, and therefore the reactor measurement plays a role complementary to accelerator experiments. It is also shown that the reactor measurement may be able to resolve the degeneracy in $\theta_{23}$ if $\sin^22\theta_{13}$ and $\cos^22\theta_{23}$ are relatively large.