Appendix C: The international neutrino program
While Europe and Japan have accelerator-based programs in neutrino science, this section discusses the Japanese neutrino program.
The Japanese experiment T2K will be competitive with NOνA with respect to measurement of the mixing angle θ13, but NOνA is the only near-future experiment with possible sensitivity to the ordering of the states in the neutrino mass spectrum (the mass hierarchy).
Japan is considering a possible post-T2K program that would involve a 4 MW beam from J-PARC sending neutrinos and antineutrinos to a new megaton-scale detector near the present Super-Kamiokande detector. Japanese and Korean researchers are giving some thought to the possibility of splitting the large new detector into two parts, one of which would be placed in Korea, approximately 1000 km from the J-PARC neutrino source. The figures on page 37 show the ability of this program to determine the mass hierarchy, and to establish the presence of CP violation, after four years of running with neutrinos, and another four years with antineutrinos. The curves, from M. Ishitsuka, T. Kajita, H. Minakata, and H. Nunokawa, Phys. Rev. D72, 033003 (2005), assume that the actual mass ordering is normal; curves for inverted ordering are similar.
By virtue of its longer baseline, the Project-X program would have better sensitivity to the neutrino mass ordering than the prospective J-PARC program, even if part of its megaton-scale detector is placed in Korea and much better sensitivity to this ordering if there is no detector in Korea. The figures on page 37 compare the potential sensitivities of the J-PARC and Project-X programs. The sensitivity to CP violation would be comparable in the U.S. and Japanese programs. The figures for the Project-X program use assumptions identical to those in the NuSAG report, except for the beam power. Water-Cerenkov (300 kt) and liquid-argon (100 kt) detector technologies would achieve similar sensitivities.
Quite apart from their relative sensitivities, the Japanese and U.S. programs, when combined, would be much stronger than either one alone, because they would operate under different physical conditions. In the U.S. program, there could be a wide-energy-band beam directed at a single large detector, possibly using liquid-argon technology, 1300 km away. In the Japanese program, there would be a much lower-energy, and narrower-band beam directed at either a single large water-Cerenkov detector 300 km away, or possibly a split version of this detector, with part of it 300 km from the neutrino source and the rest in Korea, about 1000 km from the source. Thanks to these differences between the U.S. and Japanese programs, together they would provide a much better probe of the mysteries of the neutrino world than either one alone.