Study of the expected performance of the T2K experiment on muon neutrino to electron neutrino oscillation using data from K2K experiment

T2K is a neutrino oscillation experiment that will use the intense 2.5 degrees off-axis νμ beam produced at J-PARC (Japan). The far detector is Super-Kamiokande (SK), the 50 kt water Cherenkov detector located 295 km from J-PARC. The goal is to search for νe appearance, which will bring new information on θ13. The main background for νe appearance comes from intrinsic beam νe events (∼ 55%), and from mis-identified neutral current π0 events (∼ 45%); near detectors are needed to measure these background components before oscillation. A detector complex (2KM) including a water Cherenkov detector, located ∼ 1.8 km away from the source is under active study. This distance is advantageous because the neutrino spectrum is only a few percent different from that of SK, thereby reducing extrapolation systematics. In order to match SK performance, the water Cherenkov detector was designed with ∼ 5600 8-inch photo-multiplier tubes, after studies based on full simulation tuned to K2K data. The water Cherenkov reconstruction algorithms, mainly particle identification and e/π0 separation, were also studied at 2KM. Studies of νe appearance in the water Cherenkov detector show that using simple scaling extrapolation we conservatively predict 23.0 ± 8.0% (stat + syst) background events at SK for 5. 1021 p.o.t., in excellent agreement with the 23.8 background events obtained from an independent simulation of SK. The 2KM detector can achieve background subtraction to better than 10% accuracy, sufficient for T2K phase I. Detailed sensitivity studies, including all the relevant sources of systematics, show that the 2KM detector improves the sensitivity to sin2(2θ13) down to ∼ 1.4. 10-2 at 90% CL. (author)