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

A possibility to measure $sin^22theta_{13}$ using reactor neutrinos is examined in detail. It is shown that the sensitivity $sin^22theta_{13}>0.02$ 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 $sin^22theta_{13}$ and $cos^22theta_{23}$ are relatively large.