Current and Future Neutrino Oscillation Constraints on Leptonic Unitarity

TL;DR

This paper assesses current and future experimental constraints on the unitarity of the lepton mixing matrix, highlighting how upcoming experiments will significantly improve our understanding of neutrino mixing and potential deviations from unitarity.

Contribution

It provides a comprehensive projection of how next-generation neutrino experiments will enhance constraints on leptonic unitarity and explores the complementarity of various experimental approaches.

Findings

Future experiments will constrain all rows and columns of the mixing matrix to within 10%

The $e$-row will be measured with less than 1% uncertainty

Measurements of mixing matrix elements will improve by a factor of 3 on average

Abstract

The unitarity of the lepton mixing matrix is a critical assumption underlying the standard neutrino-mixing paradigm. However, many models seeking to explain the as-yet-unknown origin of neutrino masses predict deviations from unitarity in the mixing of the active neutrino states. Motivated by the prospect that future experiments may provide a precise measurement of the lepton mixing matrix, we revisit current constraints on unitarity violation from oscillation measurements and project how next-generation experiments will improve our current knowledge. With the next-generation data, the normalizations of all rows and columns of the lepton mixing matrix will be constrained to $\lesssim$10\% precision, with the $e$-row best measured at $\lesssim$1\% and the $\tau$-row worst measured at ${\sim}10\%$ precision. The measurements of the mixing matrix elements themselves will be improved on average by a factor of $3$. We highlight the complementarity of DUNE, T2HK, JUNO, and IceCube Upgrade for these improvements, as well as the importance of $\nu_\tau$ appearance measurements and sterile neutrino searches for tests of leptonic unitarity.