New Index of CP Phase Effect and ${\theta }_{13}$ Screening in Long Baseline Neutrino Experiments

We introduce a new index of the leptonic CP phase dependence $I_{\mathrm{C}\mathrm{P}}$ and derive the maximal condition for this index in a simple and general form. $I_{\mathrm{C}\mathrm{P}}\simeq 100$ 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 $\mathrm{\Delta }{m}_{31}^2>0$ $(\mathrm{\Delta }{m}_{31}^2<0)$ without depending the uncertainties of solar and atmospheric parameters.