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

TL;DR

This paper investigates how future high-precision long-baseline neutrino experiments can set model-independent constraints on non-unitary neutrino mixing parameters, improving our understanding of potential deviations from standard neutrino oscillation models.

Contribution

It provides a comprehensive analysis of the sensitivities of DUNE and T2HKK/JD+KD experiments to non-unitary neutrino mixing parameters, including correlations and combined constraints.

Findings

JD+KD has better sensitivity for certain NUNM parameters due to larger statistics and lower systematics.

DUNE provides stronger constraints on other NUNM parameters because of its matter effects and energy spectrum.

Near detectors can independently constrain some NUNM parameters due to zero-distance effects.

Abstract

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 $\alpha_{33}$.