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

Abstract

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 $\sin^2 2θ_{13}>0.01$, and nevertheless $ν_e$ appearance signal cannot be observed in the JPARC experiment, we conclude that the CP phase $δ$ becomes a value around $135^{\circ}$ $(45^{\circ})$ for $Δm^2_{31}>0$ $(Δm^2_{31}<0)$ without depending the uncertainties of solar and atmospheric parameters.