Revisiting T2KK and T2KO physics potential and $\hat{I}{½}_{\hat{I}}¼$ - $\bar{\hat{I}}{½}_{\hat{I}}¼$ beam ratio

Abstract

We revisit the sensitivity study of the Tokai-to-Kamioka-and-Korea (T2KK) and Tokai-to-Kamioka-and-Oki (T2KO) proposals where a water Cerenkov 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 ($\hat{I}{´}_{CP}$). We systematically study the running ratio of the $\hat{I}{½}_{\hat{I}}¼$ and $\bar{\hat{I}}{½}_{\hat{I}}¼$ focusing beams with dedicated background estimation for the $\hat{I}{½}_e$ appearance and $\hat{I}{½}_{\hat{I}}¼$ disappearance signals, especially improving treatment of the neutral current $\ddot{I}{€}^0$ backgrounds. Using a $\hat{I}{½}_{\hat{I}}¼$ - $\bar{\hat{I}}{½}_{\hat{I}}¼$ beam ratio between 3 : 2 and 2.5 : 2.5 (in unit of $10^{21}$POT with the proton energy of 40 GeV), the mass hierarchy determination with the median sensitivity of 3 - 5 $\ddot{I}ƒ$ by the T2KK and 1 - 4 $\ddot{I}ƒ$ by the T2KO experiment are expected when ${\sin }^2\hat{I}{¸}_{23}=0.5$, depending on the mass hierarchy pattern and CP phase. These sensitivities are enhanced (reduced) by $30\%$ - $40\%$ in $\hat{I}”\ddot{I}{‡}^2$ when ${\sin }^2\hat{I}{¸}_{23}=0.6(0.4)$. The CP phase is measured with the uncertainty of $20^{\circ }$ - $50^{\circ }$ by the T2KK and T2KO using the $\hat{I}{½}_{\hat{I}}¼$ - $\bar{\hat{I}}{½}_{\hat{I}}¼$ focusing beam ratio between 3.5 : 1.5 and 1.5 : 3.5. These findings indicate that inclusion of the $\bar{\hat{I}}{½}_{\hat{I}}¼$ focusing beam improves the sensitivities of the T2KK and T2KO experiments to both the mass hierarchy determination and leptonic CP phase measurement simultaneously with the preferred beam ratio being between 3 : 2 - 2.5 : 2.5 ($\times 10^{21}$POT).

33 pages, 16 figures, 5 tables