Exploring Neutrino Mixing with Low Energy Superbeams

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Abstract
We explore as clearly as possible the features of neutrino oscillation which are relevant for measurements of the CP violating Kobayashi-Maskawa phase delta and the sign of \Delta m^2_{13}. We focus on the so called low-energy option and discuss principles for optimizing experimental parameters to measure these two quantities simultaneously. Toward the goal, we first formulate a method for obtaining a bird-eye view of the phenomenon of neutrino oscillation by introducing a new powerful tool called the ``CP trajectory diagram in bi-probability space''. It allows us to represent pictorially the three effects separately in a single diagram; effect from genuine CP violation due to the sin delta term, effect from the CP conserving cos delta term, and the fake CP violating effect due to earth matter. By using the CP trajectory diagram we observe that there is a two-fold ambiguity in the determination of delta which is related with the sign of Delta m^2_{13}. We then address the question of what are the promising options for conceptual design of experiments at low energies which looks for CP violation and at the same time would resolve the two-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 which allows simultaneous determination of delta and the sign of Delta m^2_{13} {\it in situ}. We also point out that there is a possibility that the similar {\it in situ} measurement of both quantities can be done at the Phase II of JHF experiment with much shorter baseline, under the assumption of nature's kind setting of delta to the region of sin{delta} \cdot Delta m^2_{13} < 0. A technique of running at high (sim 1 GeV) and low (sim 0.5 GeV) beam energies is proposed as a method for better identification of delta.

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