Re-evaluation of the T2KK physics potential with simulations including backgrounds

Abstract

The Tokai-to-Kamioka-and-Korea (T2KK) neutrino oscillation experiment under examination can have a high sensitivity to determine the neutrino mass hierarchy for a combination of relatively large ( ∼ 3.0°) off-axis angle beam at Super-Kamiokande (SK) and small ( ∼ 0.5°) off-axis angle at L ∼ 1,000 km in Korea. We elaborate previous studies by taking into account smearing of reconstructed neutrino energy due to finite resolution of electron or muon energies, nuclear Fermi motion and resonance production, as well as the neutral current π0 production background to the νμ → νe oscillation signal. It is found that the mass hierarchy pattern can still be determined at 3σ level if sin22θRCT ≡ 4|Ue3|2(1 − |Ue3|2) ≳ 0.08(0.09) when the hierarchy is normal (inverted) with 5 × 1021} POT (protons on target) exposure, or 5 years of the T2K experiment, if a 100 kton water Čerenkov detector is placed in Korea. The π0 backgrounds deteriorate the capability of the mass hierarchy determination, whereas the events from CC nuclear resonance productions contribute positively to the hierarchy discrimination power. We also find that the π0 backgrounds seriously affect the CP phase measurement. Although δMNS can still be constrained with an accuracy of ∼ ±45° (±60°) at 1σ level for the normal (inverted) hierarchy with the above exposure if sin22θRCT ≳ 0.04, CP violation can no longer be established at 3σ level even for δMNS = ±90° and sin22θRCT = 0.1. About four times higher exposure will be needed to measure δMNS with ±30° accuracy.