Constraining Unitarity of Three-Flavor Neutrino Mixing Matrix from Next-Generation Long-Baseline Experiments

The measurement of the standard three-flavor neutrino mixing matrix elements with very high precision makes it inevitable to test its unitarity property. So, in this work, we study the ability of the next generation long-baseline experiments DUNE and T2HKK to constrain various parameters relevant for non-unitary neutrino mixing (NUNM) in a complete model-independent fashion. We also discuss the possible correlation between the standard oscillation parameters \(\theta _{23}\) , \(\delta _{\textrm{CP}}\) , and various NUNM parameters. We observe that T2HKK has better sensitivities for NUNM parameters \(|\alpha _{21}|\) and \(\alpha _{22}\) as compared to DUNE because of its large statistics in the appearance channel and less systematic uncertainties in the disappearance channel. For \(|\alpha _{31}|\) , \(|\alpha _{32}|\) , and \(\alpha _{33}\) , DUNE performs better due to its larger matter effect and wide band beam. The expected limit on \(\alpha _{11}\) is similar from both the experiments. We also discuss the complementarity of the two experimental setups and improvement in the constraints when their prospective data are combined. We also observe that the near detectors can significantly constrain \(\alpha _{11}\) , \(|\alpha _{21}|\) , and \(\alpha _{22}\) because of the zero-distance effect. Finally, we demonstrate how appearance \(\nu _\tau \) channel in DUNE can improve the constraints on \(|\alpha _{32}|\) and \(\alpha _{33}\) .