Prospects of Very Long Base-Line Neutrino Oscillation Experiments with the KEK-JAERI High Intensity Proton Accelerator

We study physics potential of Very Long Base-Line (VLBL) Neutrino-Oscillation Experiments with the High Intensity Proton Accelerator (HIPA), which will be completed by the year 2007 in Tokai-village, Japan, as a joint project of KEK and JAERI (Japan Atomic Energy Research Institute). The HIPA 50 GeV proton beam will deliver neutrino beams of a few GeV range with the intensity about two orders of magnitude higher than the present KEK beam for K2K experiment. As a sequel to the proposed HIPA-to-Super-Kamiokande experiment, we study impacts of experiments with a 100 kton-level detector and the base-line length of a few-thousand km. The pulsed narrow-band nu_mu beams (NBB) allow us to measure the nu_mu to nu_e transition probability and the nu_mu survival probability through counting experiments at large water-Cerenkov detector. We study 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 1,000 and 2,000 km can determine the sign of the larger mass-squared difference (m_3^2-m_1^2) if the mixing between nu_e and nu_3 (the heaviest-or-lightest neutrino) is not too small; 2|U_{e3}|^2(1-|U_{e3}|^2) gsim 0.03. The CP phase can be constrained if the |U_{e3}| element is sufficiently large, 2|U_{e3}|^2(1-|U_{e3}|^2) gsim 0.06, and if the smaller mass-squared difference (m_2^2-m_1^2) and the U_{e2} element are in the prefered range of the large-mixing-angle solution of the solar-neutrino deficit. The magunitude | m_3^2-m_1^2| and the matrix element U_{mu 3} can be precisely measured, but we find little sensitivity to m_2^2-m_1^2 and the matrix element U_{e2}.