Measuring the CP-violating phase by a long base-line neutrino experiment with Hyper-Kamiokande

We study the sensitivity of a long-base-line (LBL) experiment with neutrino beams from the High Intensity Proton Accelerator (HIPA), that delivers 1021 POT per year, and a proposed 1 Mt water Čerenkov detector, Hyper-Kamiokande (HK) 295 km away from the HIPA, to the CP phase (δMNS) of the three-flavor lepton mixing matrix. We examine a combination of the νμ narrow-band beam (NBB) at two different energies, 〈pπ〉=2, 3 GeV, and the ν̄μ NBB at 〈pπ〉=2 GeV. By allocating one year each for the two νμ beams and four years for the ν̄μ beam, we can efficiently measure the νμ→νe and ν̄μ→ν̄e transition probabilities, as well as the νμ and ν̄μ survival probabilities. CP violation in the lepton sector can be established at 4σ (3σ) level if the MSW large-mixing-angle scenario of the solar-neutrino deficit is realized, |δMNS| or |δMNS−180°|>30°, and if 4|Ue3|2(1−|Ue3|2)≡sin22θRCT>0.03 (0.01). The phase δMNS is more difficult to constrain by this experiment if there is little CP violation, δMNS∼0°or 180°, which can be distinguished at 1σ level if sin22θRCT≳0.01.