Reactor measurement of ${\theta }_{13}$ and its complementarity to long-baseline experiments

The possibility of measuring ${\sin }^{2}2{\theta }_{13}$ using reactor neutrinos is examined in detail. It is shown that the sensitivity ${\sin }^{2}2{\theta }_{13}>0.02\mathrm{}$ can be reached with 40 ton yr data by placing identical CHOOZ-like detectors at near and far distances from a giant nuclear power plant whose total thermal energy is $24.3{\mathrm{GW}}_{\mathrm{th}}.$ It is emphasized that this measurement is free from the parameter degeneracies that occur in accelerator appearance experiments, and therefore the reactor measurement is 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.