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

Our knowledge on the active 3\(\nu\) mixing angles (\(\theta_{12}\), \(\theta_{13}\), and \(\theta_{23}\)) and the CP phase \(\delta_{\mathrm{CP}}\) is becoming accurate day-by-day enabling us to test the unitarity of the leptonic mixing matrix with utmost precision. Future high-precision long-baseline experiments are going to 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 direct, model-independent, and competitive constraints on various NUNM parameters. We demonstrate the possible correlations between the NUNM parameters, \(\theta_{23}\), and \(\delta_{\mathrm{CP}}\). Our numerical results obtained using only far detector data and supported by simple approximate analytical expressions of the oscillation probabilities in matter, reveal that JD+KD has better sensitivities for \(|\alpha_{21}|\) and \(\alpha_{22}\) as compared to DUNE, due to its larger statistics in the appearance channel and less systematic uncertainties in the disappearance channel, respectively. For \(|\alpha_{31}|\), \(|\alpha_{32}|\), and \(\alpha_{33}\), DUNE gives better constraints as compared to 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. We find that due to zero-distance effects, the near detectors alone can also constrain \(\alpha_{11}\), \(|\alpha_{21}|\), and \(\alpha_{22}\) in both these setups. Finally, we observe that the \(\nu_\tau\) appearance sample in DUNE can improve the constraints on \(|\alpha_{32}|\) and \(\alpha_{33}\).