# Heavy neutrino mixing in the T2HK, the T2HKK and an extension of the T2HK with a detector at Oki Islands

## TL;DR

This paper investigates the potential to detect heavy neutrino mixing in extended T2K experiments, showing that adding a detector at Oki Islands improves sensitivity and allows for better measurement of neutrino properties.

## Contribution

It introduces a new extension of T2HK with an Oki Islands detector and analyzes its improved sensitivity to heavy neutrino mixing and neutrino oscillation parameters.

## Key findings

- Adding a detector at Oki Islands mitigates sensitivity deterioration.
- The extended setup improves measurement accuracy of neutrino mass hierarchy.
- Data with varying L/E ratios enhance parameter determination.

## Abstract

We study the discovery potential for the mixing of heavy isospin-singlet neutrinos in extensions of the Tokai-to-Kamioka (T2K) experiment: The Tokai-to-Hyper-Kamiokande (T2HK), the Tokai-to-Hyper-Kamiokande-to-Korea (T2HKK), and a plan of adding a new detector at Oki Islands to the T2HK. We parametrize the mixing of heavy neutrinos in terms of a non-unitary mixing matrix acting on active flavors. It is shown that in the T2HK and T2HKK, the sensitivity to heavy neutrino mixing deteriorates for some values of $CP$-violating phases in the standard and the non-unitary mixing matrices, but the deterioration is drastically mitigated if a detector at Oki Islands is added to the T2HK. We also consider the feasibility of measuring the mass hierarchy and the standard $CP$-violating phase $\delta_{CP}$ in the presence of heavy neutrino mixing, by fitting experimental data with the standard oscillation parameters only, i.e., under the assumption of unitary mixing. It is revealed that such measurement can be performed to some accuracy in the T2HKK and the extension of the T2HK with a detector at Oki Islands, owing to the fact that data with largely varying $L/E$ are collected in these experiments, while it cannot be in the T2HK.