Physics potential of T2KK: An extension of the T2K neutrino oscillation experiment with a far detector in Korea

We study physics potential of placing a far detector in the east coast of Korea, where the off-axis beam from J-PARC at Tokai village for the T2K project has significant intensity at a few GeV range. In particular we examine the capability of determining the mass hierarchy pattern and the CP phase of the lepton-flavor-mixing matrix when a 100 kt water Cerenkov detector is placed at various locations in Korea for the off-axis beam (OAB) of 2.5 deg. and 3 deg. at the Super-Kamiokande site. The best results are found for a combination of 3 deg. OAB at SK (L = 295 km) and 0.5 deg. OAB at L = 1000km, where the mass hierarchy pattern can be determined at 3-\sigma level for sin^2 2\theta_{RCT} \simgt 0.05 (0.06) when the hierarchy is normal (inverted), after 5 years of running (5 \times 10^{21}POT). We also find that the leptonic CP phase, \delta_{MNS}, can be constrained uniquely, without invoking anti-neutrino beams, as long as the mass hierarchy pattern is determined. Those results are obtained by assuming that the charged current quasi-elastic events can be separated from the other backgrounds with high efficiency, the neutrino energy can be reconstructed with a hundred MeV uncertainty, and the earth matter density along the baseline can be determined with 3% accuracy.