Exploring neutrino mixing with low energy superbeams

We explore the features of neutrino oscillation which are relevant for measurements of the leptonic CP violating phase δ and the sign of Δm²₁₃ in experiments with low-energy conventional superbeams. Toward the goal, we introduce a new powerful tool called the ``CP trajectory diagram in bi-probability space'' which allows us to represent pictorially the three effects, the effects of (a) genuine CP violation due to the sin δ term, (b) CP conserving cos δ term, and (c) fake CP violation due to earth matter, separately in a single diagram. By using the diagram, we observe that there is a two-fold ambiguity in the determination of δ which is related with the sign of Δm²₁₃. Possible ways of resolving the ambiguity are discussed. In particular, we point out that an in situ simultaneous measurement of δ and the sign of Δm²₁₃ can be carried out at distances of about 700 km, or at the Phase II of the JHF experiment provided that sin δ · Δm²₁₃ < 0, both with a megaton class water Cherenkov detector.