Physics potential of neutrino oscillation experiment with a far detector in Oki Island along the T2K baseline

摘要

Oki Island is located between Japan and Korea along the Tokai-To-Kamioka (T2K) baseline. The distance from J-PARC to Oki Island is about 653km, which is twice that of the T2K experiment (L = 295km). When the off-axis angle of the neutrino beam from J-PARC is 3.0◦ (2.0◦) at Super-Kamiokande (SK), the off-axis beam (OAB) with 1.4◦ (0.6◦) reaches at Oki Island. We examine physics case of placing a far detector in Oki Island during the T2K experimental period. We estimate the matter density profile along the Tokai-to-Oki baseline by using recent seismological measurements. For a detector of 100 kton fiducial volume and 2.5 × 1021 POT (protons on target) exposure for both ν μ and ( {{overline{nu}}_{mu }} ) beams, we find that the mass hierarchy pattern can be distinguished at 3 σ level if sin2 2θ RCT ≡ 4|U e3|2(1 − |U e3 |2) ≳ 0.09, by observing the electron-like CCQE (Charged-Current Quasi Elastic) events. The CP phase in the Maki-Nakagawa-Sakata lepton flavor mixing matrix, δ MNS, can be constrained with ±20◦. As a reference, we repeat the same analysis by placing the same detector in Korea at L = 1000 km and OAB=0.5◦ (T2KK) and also by placing it at the SK site (T2K122). The Tokai-to-Kamioka-OKI (T2KO) sensitivity to the mass hierarchy is about 1/3 (in ( varDelta chi_{min}^2 )) of T2KK, while the sensitivity to the phase δ MNS is similar between T2KO and T2KK. The T2K122 option has almost no sensitivity to the mass hierarchy, and cannot measure the CP phase except when δ MNS ~ −90◦ (90◦) for the normal (inverted) hierarchy.