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Testing lepton nonunitarity with the next generation of germanium-based CEνNS reactor experiments

Salvador Centelles Chuliá1,2,*, Manfred Lindner2,†, and Thomas Rink3,2,‡

  • *Contact author: salcen@ific.uv.es
  • †Contact author: lindner@mpi-hd.mpg.de
  • ‡Contact author: thomas.rink@kit.edu
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Phys. Rev. D 113, 096010 – Published 12 May, 2026

DOI: https://doi.org/10.1103/r2n6-5dlp

Abstract

Coherent elastic neutrino-nucleus scattering (CEνNS) has been experimentally confirmed using neutrinos from pion decay at rest, solar neutrinos, and reactor antineutrinos. Future CEνNS experiments will lead to precision measurements, which will be a powerful tool to search for new physics beyond the standard model. In this work, we investigate possible deviations from unitarity in the 3×3 leptonic mixing matrix that controls the propagation of active neutrinos. Such deviations may originate from the mixing with additional gauge singlet fermions, and depending on their mass scale and mixing, the resulting phenomenology can differ substantially. We explore two well-motivated regimes: the seesaw limit, where the new fermions are heavy and kinematically inaccessible, leading to effective deviations from unitarity in the active sector, and the light sterile limit, where they are light enough to be produced and participate in neutrino propagation and scattering processes. We show how these scenarios modify both CEνNS and elastic neutrino-electron scattering, and we present the corresponding sensitivity projections for a future CEνNS reactor experiment obtained by upscaling the CONUS+ experiment, which reported the first observation of reactor CEνNS. We identify the leading experimental systematics relevant for such an upscaling and demonstrate the resulting capability to probe TeV-scale new physics. Our results highlight the strong potential of CEνNS to test the structure of the lepton sector and to search for physics beyond the standard model.

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Phys. Rev. D 113, 096010– Published 12 May, 2026
  • Received 18 December 2025
  • Accepted 16 April 2026
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