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Prospect of the NUCLEUS Experiment at Chooz for Coherent Elastic Neutrino-Nucleus Scattering and New Physics Searches

H. Abele, G. Angloher, B. Arnold, M. Atzori Corona, A. Bento, E. Bossio, F. Buchsteiner, J. Burkhart, F. Cappella, M. Cappelli, N. Casali, R. Cerulli, A. Cruciani, G. Del Castello, M. del Gallo Roccagiovine, S. Dorer, A. Erhart, M. Friedl, S. Fichtinger, V. M. Ghete, M. Giammei, C. Goupy, J. Hakenmüller, D. Hauff, F. Jeanneau, E. Jericha, M. Kaznacheeva, H. Kluck, A. Langenkämper, T. Lasserre, D. Lhuillier, M. Mancuso, R. Martin, B. Mauri, A. Mazzolari, L. McCallin, H. Neyrial, C. Nones, L. Oberauer, L. Peters, F. Petricca, W. Potzel, F. Pröbst, F. Pucci, F. Reindl, M. Romagnoni, J. Rothe, N. Schermer, J. Schieck, S. Schönert, C. Schwertner, L. Scola, G. Soum-Sidikov, L. Stodolsky, A. Schröder, R. Strauss, R. Thalmeier, C. Tomei, L. Valla, M. Vignati, M. Vivier, A. Wallach, P. Wasser, A. Wex, L. Wienke

Abstract

The NUCLEUS experiment aims to measure coherent elastic neutrino-nucleus scattering (CE$ν$NS) at unprecedentedly low nuclear recoil energies using gram-scale cryogenic calorimeters operated at the Chooz nuclear power plant in France. Access to recoil energies at the $\mathcal{O}(10~\mathrm{eV})$ scale enables CE$ν$NS studies at extremely low momentum transfer and provides enhanced sensitivity to new physics. In this work, we present sensitivity projections for the upcoming NUCLEUS technical and physics runs, incorporating a data-driven treatment of the low-energy excess (LEE) observed during commissioning. We develop a likelihood framework that exploits reactor-power variation to disentangle signal and background in a low signal-to-background regime and to assess the impact of the dominant systematic uncertainties. For the Technical Run with a 7 g CaWO$_4$ target, we find competitive sensitivity to several scenarios beyond the Standard Model, which do not require a CE$ν$NS observation. For the Physics Run, assuming complete suppression of the LEE, we project a 4.7 $σ$ observation of CE$ν$NS with a statistical precision of about 20 % in 1 year, enabling a determination of the weak mixing angle at the lowest momentum transfer probed to date with CE$ν$NS and leading CE$ν$NS-based constraints on the neutrino charge radius and new mediator models.

Prospect of the NUCLEUS Experiment at Chooz for Coherent Elastic Neutrino-Nucleus Scattering and New Physics Searches

Abstract

The NUCLEUS experiment aims to measure coherent elastic neutrino-nucleus scattering (CENS) at unprecedentedly low nuclear recoil energies using gram-scale cryogenic calorimeters operated at the Chooz nuclear power plant in France. Access to recoil energies at the scale enables CENS studies at extremely low momentum transfer and provides enhanced sensitivity to new physics. In this work, we present sensitivity projections for the upcoming NUCLEUS technical and physics runs, incorporating a data-driven treatment of the low-energy excess (LEE) observed during commissioning. We develop a likelihood framework that exploits reactor-power variation to disentangle signal and background in a low signal-to-background regime and to assess the impact of the dominant systematic uncertainties. For the Technical Run with a 7 g CaWO target, we find competitive sensitivity to several scenarios beyond the Standard Model, which do not require a CENS observation. For the Physics Run, assuming complete suppression of the LEE, we project a 4.7 observation of CENS with a statistical precision of about 20 % in 1 year, enabling a determination of the weak mixing angle at the lowest momentum transfer probed to date with CENS and leading CENS-based constraints on the neutrino charge radius and new mediator models.

Paper Structure

This paper contains 19 sections, 19 equations, 12 figures, 1 table.

Table of Contents

  1. Introduction
  2. CE$\nu$NS Theory Framework
  3. Standard Model CE$\nu$NS Cross Section
  4. Beyond the Standard Model Scenarios
  5. Sensitivity Strategy
  6. Signal Prediction and Background Model
  7. CE$\nu$NS Signal Prediction
  8. Technical Run
  9. Physics Run (LEE-free)
  10. Reactor-Power Variation Analysis
  11. Results
  12. Sensitivity to Standard Model CE$\nu$NS
  13. Sensitivity To Physics Beyond the Standard Model
  14. Impact of Increased Target Mass
  15. Conclusions
  16. ...and 4 more sections

Figures (12)

  • Figure 1: Expected event rate in the NUCLEUS $\mathrm{CaWO_4}$ target at Chooz as a function of the observed energy for different physics scenarios. The green line shows the SM CE$\nu$NS prediction, while the dashed and dot-dashed curves correspond to a representative value of neutrino magnetic moment (magenta), modified neutrino charge radius (blue), and the presence of a new light universal mediator (red). The theoretical predictions are qualitatively compared with the LEE background measured during the NUCLEUS commissioning run at TUM NUCLEUS:2025ymr (dark gray line), and the particle background contribution estimated for the NUCLEUS detectors at VNS from Geant4 (G4) simulations Abele:2025yca (light gray line). Vertical lines mark recoil-energy thresholds of 20 eV and 35 eV.
  • Figure 2: Top: Assumed time evolution of the effective reactor thermal power (Eq. \ref{['eq:eff-power-dist']}) at the Chooz VNS over one year, reflecting realistic operational conditions. The two reactor cores are treated as a single effective source at an effective distance of $L_{\mathrm{eff}} \simeq 58.8$ m. Middle: Representative Monte Carlo pseudo-experiment for the Technical Run configuration, generated assuming a CE$\nu$NS signal normalized to 35 times the SM prediction for illustrative purposes. Time distribution of events over one year, showing the signal and background components separately, together with their expected PDFs. The signal follows the reactor-power variation, while the background reflects the time decay of the LEE. Bottom: Recoil-energy spectrum of the same pseudo-experiment, displaying the signal and background contributions and their corresponding energy PDFs.
  • Figure 3: Projected sensitivity of the NUCLEUS experiment to the weak mixing angle $\sin^2\theta_W(Q)_{\mathrm{\overline{MS}}}$ as a function of the momentum transfer $|\vec{q}|\doteq\rm Q$. The solid line represents the latest theoretical determination at zero momentum transfer ParticleDataGroup:2024cfk. The green point indicate the expected precision for the Physics Run, assuming complete suppression of the LEE. This is compared to the precision from atomic parity violation (APV) on cesium ParticleDataGroup:2024cfk and existing measurements from CE$\nu$NS experiments, specifically: CONUS+ Alpizar-Venegas:2025wor, COHERENT CsI COHERENT:2021xmm, Ar Cadeddu:2020lky, Ge AtzoriCorona:2025xgj, a combined analysis of XENONnT XENON:2024ijk and PandaX PandaX:2024muv data DeRomeri:2024iaw and the determination from LUX-ZEPLIN LZ:2025igz.
  • Figure 4: Projected sensitivity of the NUCLEUS experiment to the diagonal electronic neutrino charge radius $\langle r^2_{\nu_e}\rangle$. The green point indicate the expected precision for the Physics Run, assuming complete suppression of the LEE, compared to the precision from CONUS+ AtzoriCorona:2025ygn and COHERENT CsI+Ar AtzoriCorona:2024rtv, and a global fit of existing neutrino data AtzoriCorona:2025xwr.
  • Figure 5: Projected sensitivity of the NUCLEUS experiment to a universal light mediator model (left) and to a flavor preserving NSI scenario (right). The green regions indicate the parameter space that can be probed by NUCLEUS at 90% CL, with curves corresponding to the Technical Run (only for the light mediator case) and Physics Run (filled region), assuming complete suppression of the LEE for the latter. (left) The sensitivity to the light mediator model is compared to existing constraints at the 90% CL from CONUS+ DeRomeri:2025csuAtzoriCorona:2025ygn, COHERENT CsI+Ar DeRomeri:2022twg, CONNIE CONNIE:2019xid, XENONnT electron recoil ($\nu$ES) A:2022acy and CE$\nu$NS data Blanco-Mas:2024ale. (right) We compare the sensitivity to NSI with the results from COHERENT CsI COHERENT:2021xmm, TEXONO TEXONO:2025sub, CONUS+ DeRomeri:2025csu and XENONnT AristizabalSierra:2024nwf at the 90% CL.
  • ...and 7 more figures