Prospects of very long baseline neutrino oscillation experiments with the KEK-JAERI high intensity proton accelerator

We study the physics potential of very long baseline neutrino-oscillation experiments with a high intensity proton accelerator which will be completed by the year 2007 in Tokai-village, as a joint project of KEK and JAERI (Japan Atomic Energy Research Institute). The 50 GeV proton synchrotron at J-PARC (Japan Proton Accelerator Research Complex) will deliver neutrino beams in the range of a few GeV with an intensity about two orders of magnitude higher than the present KEK beam for the K2K experiment. As a sequel to the proposed J-PARC–to–Super-Kamiokande experiment, we study the impact of experiments with a 100 kton-level detector and a baseline length of a few-thousand km. The pulsed narrow-band ν_μ beams allow us to measure the ν_μ→ν_e transition probability and the ν_μ survival probability through counting experiments at a large water-Čerenkov detector. We study the sensitivity of such experiments to the neutrino mass hierarchy, the mass-squared differences, the three angles, and one CP phase of the three-generation lepton-flavor-mixing matrix. We find that experiments at a distance between 1000 and 2000 km can determine the sign of the larger mass-squared difference (m₃²-m₁²) if the mixing between ν_e and ν₃ (the heaviest-or-lightest neutrino) is not too small: 4|U_e3|²(1-|U_e3|²)≳0.03. The CP phase can be constrained if the |U_e3| element is sufficiently large: 4|U_e3|²(1-|U_e3|²)≳0.06, and if the smaller mass-squared difference (m₂²-m₁²) and the U_e2 element are in the preferred range of the large-mixing-angle solution of the solar-neutrino deficit. The magnitude |m₃²-m₁²| and the matrix element U_μ3 can be precisely measured, but we find little sensitivity to m₂²-m₁² and the matrix element U_e2.