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

TL;DR

This paper addresses whether lepton non-unitarity, arising from gauge-singlet fermions, can be probed by coherent elastic neutrino-nucleus scattering (CE$ν$NS) and neutrino-electron scattering using next-generation Germanium reactor detectors. It develops a theoretical framework separating heavy neutral lepton (seesaw) and light sterile (3+$1$) regimes, and forecasts sensitivities for an upscaled CONUS+-like experiment at a 20 m distance from a 3.5 GW reactor. The results show that CE$ν$NS is the dominant channel in both limits: heavy mediators can be constrained up to TeV scales (with improvements from systematics and external oscillation data) and light sterile mixing can be probed down to small $\sin^2 2\theta_{14}$ values, though much of the SB-excluded space remains outside reach. Overall, the work highlights the strong potential of precision CE$ν$NS reactor experiments to test the structure of the lepton sector, while emphasizing reactor-flux uncertainties as a key limiting factor and the value of combining CE$ν$NS with oscillation information for global constraints.

Abstract

Coherent elastic neutrino-nucleus scattering (CE$ν$NS) has been experimentally confirmed using neutrinos from pion decay at rest, solar neutrinos and reactor antineutrinos. Future CE$ν$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$ν$NS and elastic neutrino--electron scattering (E$νe$S), and we present the corresponding sensitivity projections for a future CE$ν$NS reactor experiment obtained by upscaling the CONUS+ experiment, which reported the first observation of reactor CE$ν$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$ν$NS to test the structure of the lepton sector and to search for physics beyond the Standard Model.

Figures (14)

• Figure 1: Feynman diagram of CE$\nu$NS. Modifications due to lepton non-unitarity introduce corrections at the neutrino interaction vertices.
• Figure 2: Feynman diagrams for E$\nu e$S. In this case, there exist two diagrams with modifications depending on the mediator present in the interaction.
• Figure 3: Left: Prefactor $(2\alpha_{11}^{2} - \alpha_{22}^{2})$ present in Eqs. \ref{['eq:Neventshadron']} and \ref{['eq:Neventslepton']}. Depending on the parameter configuration, the expected CE$\nu$NS / E$\nu$eS signal can be smaller or larger than the SM expectation indicated by the black line. Right: Exemplary CE$\nu$NS spectra for the given alpha combinations, which are chosen to receive a bisection and doubling of the events. A flux of $\phi\sim 1.5\cdot10^{13}$/cm$^{2}$/s is assumed ($L=20$ m, $P_{\mathrm{th}}=3.5$ GW).
• Figure 4: Left: Oscillation probability of Eqs. \ref{['eq:oscillation_probability_cenns']} and \ref{['eq:oscillation_probability_eves']} present in the CE$\nu$NS and E$\nu$eS cross section for fixed $L/E_{\nu}$. Right: Expected CE$\nu$NS spectrum of a light sterile neutrino for the given parameter combination compared to the SM case. Again, a flux of $\phi\sim 1.5\cdot10^{13}$/cm$^{2}$/s is assumed ($L=20$ m, $P_{\mathrm{th}}=3.5$ GW).
• Figure 5: $\Delta \chi^{2}$ profiles of the individual alpha parameters - the other one being fixed to unity - for the assumed experimental configuration. We show three threshold configurations together with three assumptions on the experimental exposure,, which are indicated by different colors and line styles. Detailed profiles for the individual thresholds also combined with knowledge from oscillation experiments are illustrated in figure \ref{['fig:alpha_profiles_detailed']} in the appendix.