Appendix

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.

2σ (thin lines) and 3σ (thick lines) sensitivity to the mass ordering by J-PARC upgrades and new detectors.

2σ (thin lines) and 3σ (thick lines) sensitivity to CP violation by J-PARC upgrades and new detectors.

95 percent C.L. (dotted lines) and 3σ (solid lines) sensitivity to the mass ordering assuming the normal mass hierarchy. For the inverted hierarchy, flip each curve around a vertical line through δ=π. All sensitivities assume 3 years of neutrino and 3 years of antineutrino running, corresponding to 15×10²⁰, 30×10²⁰, 60×10²⁰, and 120×10²⁰ protons on target for each neutrino type for NuMI, SNuMI, Project X at 120 GeV, and Project X at 60 GeV, respectively. Curves A-D use 120 GeV protons and E uses 60 GeV protons.

A) NOνA 15 kt detector with NuMI
B) NOνA 15 kt detector with SNuMI
C) NOνA 15 kt detector with Project X
D) Two 100 kt LAr detectors at fi rst (700 km) and second (810 km) oscillation maxima using Project X and the NuMI beamline.
E) One 100 kt LAr (equivalent to ~300 kt water Cerenkov) detector at 1300 km using a wide-band neutrino beam with Project X.

3σ sensitivity to CP-violation assuming the normal mass hierarchy. For the inverted hierarchy, flip each curve around a vertical line through δ=π. All sensitivities assume 3 years of neutrino and 3 years of antineutrino running, corresponding to 60×10²⁰ and 120×10²⁰ protons on target for each neutrino type for Project X at 120 GeV and 60 GeV, respectively.

A) Two 100 kt LAr detectors at fi rst (700 km) and second (810 km) oscillation maxima using Project X at 120 GeV and the NuMI beamline.
B) One 100 kt LAr (equivalent to ~300 kt water Cerenkov) detector at 1300 km using a wide-band neutrino beam with Project X at 60 GeV.