Heavy neutrino mixing in the T2HK, the T2HKK and an extension of the T2HK with a detector at Oki Islands

We study the discovery potential for the mixing of heavy isospin-singlet neutrinos in extensions of the Tokai-to-Kamioka (T2K) experiment, the Tokai-to-Hyper-Kamiokande (T2HK), the Tokai-to-Hyper-Kamiokande-to-Korea (T2HKK) with a Korea detector with ≅ 1000 km baseline length and 1 ^circle off-axis angle, and a plan of adding a small detector at Oki Islands to the T2HK. We further pursue the possibility of measuring the neutrino mass hierarchy and the standard CP-violating phase δ_CP in the presence of heavy neutrino mixing by fitting data with the standard oscillation parameters only. We show that the sensitivity to heavy neutrino mixing is highly dependent on δ_CP and new CP-violating phases in the heavy neutrino mixing matrix, and deteriorates considerably when these phases conspire to suppress interference between the standard oscillation amplitude and an amplitude arising from heavy neutrino mixing, at the first oscillation peak. Although this suppression is avoided by the use of a beam with smaller off-axis angle, the T2HKK and the T2HK+small Oki detector do not show improvement over the T2HK. As for the mass hierarchy measurement, the wrong mass hierarchy is possibly favored in the T2HK because heavy neutrino mixing can mimic matter effects. In contrast, the T2HKK and the T2HK+small Oki detector are capable of correctly measuring the mass hierarchy despite heavy neutrino mixing, as measurements with different baselines resolve degeneracy between heavy neutrino mixing and matter effects. Notably, adding a small detector at Oki to the T2HK drastically ameliorates the sensitivity, which is the central appeal of this paper. As for the δ_CP measurement, there can be a sizable discrepancy between the true δ_CP and the value obtained by fitting data with the standard oscillation parameters only, which can be comparable to 1σ resolution of the δ_CP measurement. Hence, if a hint of heavy neutrino mixing is discovered, it is necessary to incorporate the effects of heavy neutrino mixing to measure δ_CP correctly. (orig.)