Parameter degeneracy and reactor experiments

Degeneracies of the neutrino oscillation parameters are explained using the $\sin^22\theta_{13}$--$s^2_{23}$ plane. Measurements of $\sin^22\theta_{13}$ by reactor experiments are free from the parameter degeneracies which occur in accelerator appearance experiments, and reactor experiments play a role complementary to accelerator experiments. It is 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.