Revisiting T2KK and T2KO physics potential and – beam ratio

Kaoru Hagiwara; Pyungwon Ko; Naotoshi Okamura; Yoshitaro Takaesu

doi:10.1140/epjc/s10052-017-4684-1

2024 Impact factor 4.8

Particles and Fields

Open Access

Eur. Phys. J. C (2017) 77: 138
https://doi.org/10.1140/epjc/s10052-017-4684-1

Regular Article - Theoretical Physics

Revisiting T2KK and T2KO physics potential and $\nu _\mu$ – ${\bar{\nu }}_\mu$ beam ratio

Kaoru Hagiwara¹^,2, Pyungwon Ko³, Naotoshi Okamura⁴ and Yoshitaro Takaesu⁵^*

¹ Theory Center, KEK, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
² Department of Accelerator Science, Sokendai, 1-1 Oho, Tsukuba, Ibaraki, 305-0801, Japan
³ School of Physics, KIAS, 85 Hoegi-ro, Dongdaemun-gu, Seoul, 130-722, Korea
⁴ Department of Radiological Sciences, International University of Health and Welfare, 2600-1 Kitakanemaru, Ohtawara, Tochigi, 324-8501, Japan
⁵ Department of Physics, University of Tokyo, Tokyo, 113-0033, Japan

^* e-mail: takaesu@hep-th.phys.s.u-tokyo.ac.jp

Received: 1 June 2016
Accepted: 8 February 2017
Published online: 1 March 2017

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 $\check{\mathrm{C}}$ 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 $\nu _\mu$ and ${\bar{\nu }}_\mu$ focusing beams with dedicated background estimation for the $\nu _e$ appearance and $\nu _\mu$ disappearance signals, especially improving treatment of the neutral-current $\pi ^0$ backgrounds. Using a $\nu _\mu$ – ${\bar{\nu }}_\mu$ 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 $\sin ^2 \theta _{23} = 0.5$ , depending on the mass-hierarchy pattern and CP phase. These sensitivities are enhanced (reduced) by 30– $40\%$ in $\Delta \chi ^2$ when $\sin ^2 \theta _{23} = 0.6\, (0.4)$ . The CP phase is measured with the uncertainty of $20^\circ$ – $50^\circ$ by the T2KK and T2KO using the $\nu _\mu$ – ${\bar{\nu }}_\mu$ focusing beam ratio between 3.5:1.5 and 1.5:3.5. These findings indicate that inclusion of the ${\bar{\nu }}_\mu$ 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 ( ${\times } 10^{21}$ POT).