Testing lepton non-unitarity with the next generation of (Germanium-based) CE$\nu$NS reactor experiments

TL;DR

This paper explores how future CE$ u$NS reactor experiments can detect deviations from lepton unitarity, potentially revealing new physics beyond the Standard Model through precision measurements of neutrino interactions.

Contribution

It investigates the impact of heavy and light sterile neutrinos on CE$ u$NS and elastic neutrino-electron scattering, providing sensitivity projections for future experiments.

Findings

CE$ u$NS experiments can probe TeV-scale new physics.

Deviations from unitarity affect neutrino scattering signals.

Future experiments have strong potential to test lepton sector structure.

Abstract

Coherent elastic neutrino-nucleus scattering (CE$\nu$NS) has been experimentally confirmed using neutrinos from pion decay at rest, solar neutrinos and reactor antineutrinos. Future CE$\nu$NS experiments will foreseeable 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 $3\times3$ 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 \textit{seesaw limit}, where the new fermions are heavy and kinematically inaccessible, leading to effective deviations from unitarity in the active sector; and the \textit{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 CE$\nu$NS and elastic neutrino--electron scattering (E$\nu e$S), and we present the corresponding sensitivity projections for a future CE$\nu$NS reactor experiment obtained by upscaling the CONUS+ experiment, which reported the first observation of reactor CE$\nu$NS. 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 CE$\nu$NS to test the structure of the lepton sector and to search for physics beyond the Standard Model.