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

A possibility to measure ${\sin }^{2}2{\theta }_{13}$ using reactor neutrinos is examined in detail. It is shown that the sensitivity ${\sin }^{2}2{\theta }_{13}>0.02$ 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 ${\sin }^{2}2{\theta }_{13}$ and ${\cos }^{2}2{\theta }_{23}$ are relatively large.