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(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(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).