New Index of CP Phase Effect and θ₁₃ Screening in Long Baseline Neutrino Experiments

We introduce a new index of the leptonic CP phase dependence $I_{\rm CP}$ and derive the maximal condition for this index in a simple and general form. $I_{\rm CP}\simeq 100%$ may be realized even in the JPARC experiment. In the case that the 1-3 mixing angle can be observed in the next generation reactor experiments, namely $\sin^2 2\theta_{13}>0.01$, and nevertheless $\nu_e$ appearance signal cannot be observed in the JPARC experiment, we conclude that the CP phase $\delta$ becomes a value around $135^{\circ}$ $(45^{\circ})$ for $\Delta m^2_{31}>0$ $(\Delta m^2_{31}<0)$ without depending the uncertainties of solar and atmospheric parameters.