Leptonic CP violation search and the ambiguity of δm₃₁²

We consider a search for the CP-violating angle δ_CP in long baseline neutrino oscillation experiments. We show that the subleading δ_CP-dependent terms in the ν_μ→ν_e oscillation probability can be easily obscured by the ambiguity of the leading term which depends on |δm₃₁²|. It is thus necessary to determine the value of δm₃₁² with a sufficient accuracy. The ν_μ survival events, which can be accumulated simultaneously with the ν_e appearance events, can serve for this purpose owing to its large statistics. Therefore, the combined analysis of ν_e appearance and ν_μ survival events is crucial to provide a restrictive constraint on δ_CP. Taking a test experimental setup, we demonstrate in the δ_CP-δm₃₁² plane that the analysis of ν_e appearance events leads to less restrictive constraints on the value of δ_CP due to the ambiguity of δm₃₁² and that the combined analysis efficiently improves the constraints.