Revisiting T2KK and T2KO physics potential and [equation] – [equation] beam ratio

摘要

We revisit the sensitivity study of the Tokai-to-Kamioka-and-Korea (T2KK) and Tokai-to-Kamioka-and-Oki (T2KO) proposals where a water (check{mathrm{C}})erenkov? detector with the 100?kton fiducial volume is placed in Korea ((L = 1000)?km) and Oki island ((L = 653)?km) in Japan, respectively, in addition to the Super-Kamiokande for determination of the neutrino mass hierarchy and leptonic CP phase ((delta _{scriptscriptstyle mathrm{CP}})). We systematically study the running ratio of the ( u _mu ) and ({ar{ u }}_mu ) focusing beams with dedicated background estimation for the ( u _e) appearance and ( u _mu ) disappearance signals, especially improving treatment of the neutral-current (pi ^0) backgrounds. Using a ( u _mu )–({ar{ u }}_mu ) beam ratio between 3:2 and 2.5:2.5 (in units of (10^{21})POT with the proton energy of 40?GeV), the mass-hierarchy determination with the median sensitivity of 3–5 (sigma ) by the T2KK and 1–4 (sigma ) by the T2KO experiment are expected when (sin ^2 heta _{23} = 0.5), depending on the mass-hierarchy pattern and CP phase. These sensitivities are enhanced (reduced) by 30–(40%) in (Delta chi ^2) when (sin ^2 heta _{23} = 0.6, (0.4)). The CP phase is measured with the uncertainty of (20^circ )–(50^circ ) by the T2KK and T2KO using the ( u _mu )–({ar{ u }}_mu ) focusing beam ratio between 3.5:1.5 and 1.5:3.5. These findings indicate that inclusion of the ({ar{ u }}_mu ) focusing beam improves the sensitivities of the T2KK and T2KO experiments to both the mass-hierarchy determination and the leptonic CP phase measurement simultaneously with the preferred beam ratio being between 3:2–2.5:2.5 (({ imes } 10^{21})POT).