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 ≲10% precision, with the e-row best measured at ≲1% and the τ-row worst measured at ∼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 ντ appearance measurements and sterile neutrino searches for tests of leptonic unitarity.
Highlights
With the discovery that neutrinos oscillate came a new understanding of the standard model (SM) of particle physics — neutrinos have mass and leptons mix
We introduce the formalism we use throughout our analyses, which allows for the possibility that the leptonic mixing matrix (LMM) is not unitary
While χ2 tables are provided by the collaboration for the results presented in ref. [69], these assume three-neutrino mixing and a unitary leptonic mixing matrix
Summary
With the discovery that neutrinos oscillate came a new understanding of the standard model (SM) of particle physics — neutrinos have mass and leptons mix. In our companion paper [15], we explored this combination of current and future data to address the unitarity constraints and CP violation present in the LMM through unitarity triangles, an approach familiarized by studies of the quark mixing matrix. Appendix E offers some discussion regarding the LSND and MiniBooNE anomalies, whether they can be resolved in this framework, and how they may be tested in next-generation experiments
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