Constraints on Non-Unitary Neutrino Mixing from Next-generation Long-Baseline Experiments

Our knowledge about 3$nu$ mixing angles and the CP-phase are becoming more accurate day-by-day. In this high-precision era of neutrino oscillation, it is inevitable to investigate the unitarity of the 3$nu$ mixing matrix. In this work, we study the impact of possible non-unitary neutrino mixing (NUNM) in the context of next-generation long-baseline experiments DUNE and T2HKK having one detector in Japan and
a second detector in Korea. We estimate the sensitivities of these setups to place direct, model-independent, and competitive constraints on various NUNM parameters. Also, we study the correlation of these NUNM parameters with $theta_{23}$ and $delta_{CP}$.
Our results using only far detector data, reveal that T2HKK has better sensitivities to the NUNM parameters $|alpha_{21}|$ and $alpha_{22}$ compared to DUNE as the former has larger statistics in the appearance channel and less systematic uncertainties in the disappearance channel. For $|alpha_{31}|$, $|alpha_{32}|$, and $alpha_{33}$, DUNE can establish better limits compared to T2HKK due to its larger matter effects. We also show how the presence of near detectors in these setups would modify the limits on these NUNM parameters. Finally, we observe that the $nu_{tau}$
appearance sample in DUNE can improve the constraints on $|alpha_{32}|$ and $alpha_{33}$.