Exploring neutrino mixing with low energy superbeams

We explore as clearly as possible the features of neutrino oscillation whichare relevant for measurements of the CP violating Kobayashi-Maskawa phase deltaand the sign of \Delta m^2_{13}. We focus on the so called low-energy optionand discuss principles for optimizing experimental parameters to measure thesetwo quantities simultaneously. Toward the goal, we first formulate a method forobtaining a bird-eye view of the phenomenon of neutrino oscillation byintroducing a new powerful tool called the ``CP trajectory diagram inbi-probability space''. It allows us to represent pictorially the three effectsseparately in a single diagram; effect from genuine CP violation due to the sindelta term, effect from the CP conserving cos delta term, and the fake CPviolating effect due to earth matter. By using the CP trajectory diagram weobserve that there is a two-fold ambiguity in the determination of delta whichis related with the sign of Delta m^2_{13}. We then address the question ofwhat are the promising options for conceptual design of experiments at lowenergies which looks for CP violation and at the same time would resolve thetwo-fold ambiguity. We point out that a version with distance of about 700 km,with a megaton class water Cherenkov detector gives an optimal design whichallows simultaneous determination of delta and the sign of Delta m^2_{13} {\itin situ}. We also point out that there is a possibility that the similar {\itin situ} measurement of both quantities can be done at the Phase II of JHFexperiment with much shorter baseline, under the assumption of nature's kindsetting of delta to the region of sin{delta} \cdot Delta m^2_{13} < 0. Atechnique of running at high (sim 1 GeV) and low (sim 0.5 GeV) beam energies isproposed as a method for better identification of delta.Comment: 31 pages, 8 figures, in JHEP styl