Physics potential of Tokai-to-Kamioka-and-Korea neutrino oscillation experiment

The Tokai-to-Kamioka (T2K) neutrino oscillation experiment will start in 2009.In T2K. the center of the neutrino beam from J-PARC at Tokai will go throughunderground beneath Super-Kamiokande (SK), reach the sea level in the Japan(east) sea near Korean shore. When the beam center is 2.5°(3.0°) downward SK,the neutrino beam with an off-axis angle greater than 1.0°(0.5°) can be observedin Korea. We study the physics potential of the Tokai-to-Kamioka-and-Korea(T2KK) experiment when an additional 100 kt-level Water Čerenkov detector isplaced in Korea during the T2K experimental period. We find that the mattereffect, which arises from the coherent interaction of Ve (or ‾Ve) off the electrons inthe earth matter, is a powerful tool to determine the sign of (m23−m21), and theCP phase, σMNS, of the lepton flavor mixing matrix. The best combination of theoff-axis angles at each detector and the length for the Tokai-to-Korea baseline isfound to be 3.0°at SK ( L = 295 km) and 0.5° at L =1000km. If we choose thiscombination, and if the densities along the baselines are 2.8 g/cm3 for Tokai-to-Kamioka baseline and 3 g / cm3 for Tokai-to-Korea baseline with 3% uncertainties,the mass hierarchy pattern can be determined at 3-σ level for sin2 2θRCT＞～ 0.09,and the CP phase can be constrained with ± 30°error for sin2 2θRCT0.06, after5 years of running (5 × 1021 POT). When we combine the result of the futurereactor neutrino ercperiments, such as Double-CHOOZ, RENO, DAYA-BAY, thecapability of determining the mass hierarchy pattern in T2KK is enhanced, and thehierarchy pattern can be constrained at 3-σ level for sin22θRCT＞～ 0.055. Becauseof the importance of the matter effect, we study the matter profile along thebaselines of the T2KK experiment by using the recent geophysical measurements.The average matter density along the Tokai-to-Korea baselines is found to be 2.96,3.01, and 3.03 g/cm3 for the baseline length of L = 1000, 1100, and 1200 km,respectively, with about 6% uncertainty. Only the real part of the first Fouriermode gives non-negligible but very small contribution to the up vμ→ve oscillationprobability. We find that, even if we enhance the error of the average matterdensity from 3% to 6%, the capability of determining the neutrino mass hierarchypattern in T2KK does not deteriorate significantly because the measurement erroris dominated by statistics