Model-Independent Constraints on Non-Unitary Neutrino Mixing from High-Precision Long-Baseline Experiments.

Our knowledge on the active three-neutrino mixing angles ($\theta_{12}$, $\theta_{13}$, and $\theta_{23}$) and the Dirac CP phase $\delta_{CP}$ is becoming accurate day-by-day enabling us to test the unitarity of the PMNS matrix with high precision. Future long-baseline experiments will play an important role in this direction. 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/JD+KD having one detector in Japan (T2HK/JD) and a second detector in Korea (KD). We estimate the sensitivities of these setups to place model-independent constraints on various NUNM parameters. We demonstrate the possible correlations between NUNM parameters and oscillations parameters $\theta_{23}$ and $\delta_{CP}$. Our numerical results, supported by approximate analytical expressions of oscillation probabilities in matter, reveal that JD+KD has better sensitivities for $|\alpha_{21}|$ and $\alpha_{22}$ than DUNE, due to its larger statistics in appearance channel and less systematic uncertainties in disappearance channel, respectively. For $|\alpha_{31}|$, $|\alpha_{32}|$, and $\alpha_{33}$, DUNE gives better constraints than JD+KD, due to its larger matter effect and wider neutrino energy spectrum. For $\alpha_{11}$, both DUNE and JD+KD give similar bounds. We also show how much the bounds on the NUNM parameters can be improved by combining the prospective data from DUNE and JD+KD setups. Due to zero-distance effects, the near detectors in both DUNE and JD+KD, can further improve the limits on $|\alpha_{21}|$ considerably, while giving slightly better constraints for $\alpha_{11}$ and $\alpha_{22}$. Finally, we observe that $\nu_\tau$ events in DUNE can improve the constraints on $|\alpha_{32}|$ and $\alpha_{33}$.