High significance measurement of reactor coherent elastic neutrino-nucleus scattering on germanium by CONUS+

N. Ackermann; H. Bonet; C. Buck; J. Hakenmüller; G. Heusser; M. Lindner; W. Maneschg; M. Meier; S. Mertens; D. Piani; M. Rank; T. Rink; E. Sánchez García; Y. Shi; L. Schönfeld; I. Stalder; H. Strecker; J. Wendel; J. Woenckhaus

AI summary · arXiv: 2610.37318v1 · hep-ex

High significance measurement of reactor coherent elastic neutrino-nucleus scattering on germanium by CONUS+

N. Ackermann (CONUS+ Collaboration), H. Bonet (CONUS+ Collaboration), C. Buck (CONUS+ Collaboration), J. Hakenmüller (CONUS+ Collaboration), G. Heusser (CONUS+ Collaboration), M. Lindner (CONUS+ Collaboration), W. Maneschg (CONUS+ Collaboration), M. Meier (CONUS+ Collaboration),

and 11 more authors

S. Mertens (CONUS+ Collaboration), D. Piani (CONUS+ Collaboration), M. Rank (CONUS+ Collaboration), T. Rink (CONUS+ Collaboration), E. Sánchez García (CONUS+ Collaboration), Y. Shi (CONUS+ Collaboration), L. Schönfeld (CONUS+ Collaboration), I. Stalder (CONUS+ Collaboration), H. Strecker (CONUS+ Collaboration), J. Wendel (CONUS+ Collaboration), J. Woenckhaus (CONUS+ Collaboration)

Original paper Submitted to arXiv on 27 Oct. 2026 6 pages, 4 figures, 1 table. A companion paper with the detailed analysis is submitted simultaneously License: CC BY 4.0 Also in: fr

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The essentials

CONUS+ measured reactor CEνNS on germanium with 10.2σ significance, finding a rate consistent with the Standard Model and determining the weak mixing angle.

Question

Can reactor antineutrinos produce a statistically significant CEνNS signal in germanium, and how precisely can its rate and the weak mixing angle be measured? The study also assesses agreement with the Standard Model at low momentum transfer.

Method

The analysis used 1296 kg·days of reactor-on data and 86 kg·days of reactor-off data from three high-purity germanium detectors at the Leibstadt nuclear power plant. A blind, binned Poisson likelihood fit simultaneously analyzed the on/off spectra, with background components modeled using Geant4 and nuisance parameters constrained by Gaussian pull terms. The predicted signal was 1701 ± 198 events, using a Lindhard quenching model with k = 0.162 ± 0.004.

Findings

Result The combined fit measured 1555 ± 116 statistical ± 98 systematic CEνNS events, with a significance of 10.2σ.
See the passage in the paper
The combined fit yields $N_{s}=1555\pm 116\,\mathrm{(stat)}\pm 98\,\mathrm{(syst)}$ CE$\nu$NS events, corresponding to a significance of 10.2 $\sigma$, in agreement with the expected sensitivity, and Likelihood B yields a consistent result of $N_{s}=1562\pm 149$.
Result The observed event count is consistent with the Standard Model prediction, with a measured-to-predicted ratio of 0.91 ± 0.14.
See the passage in the paper
Compared with $N_{\mathrm{SM}}=1701\pm 198$, the measurement yields $N_{s}/N_{\mathrm{SM}}=0.91\pm 0.14$, combining both uncertainties in quadrature and neglecting their correlations.
Result The measurement determines the weak mixing angle as 0.225 ± 0.022 at momentum transfer of about 10 MeV, consistent with the stated Standard Model expectation.
See the passage in the paper
From the measured ratio $N_{s}/N_{\mathrm{SM}}$, including the uncertainties on the measurement and on the signal prediction, we obtain $\sin^{2}\theta_{W}=0.225\pm 0.022$, in agreement within 0.6 standard deviations with the SM expectation of $\sin^{2}\theta_{W}=0.2387$ at low momentum transfer [38].
Authors’ interpretation The authors conclude that run 2 advances reactor CEνNS from initial detection toward precision measurements.
See the passage in the paper
Constraints on physics beyond the SM previously derived from run 1, such as on neutrino electromagnetic properties, non-standard interactions, and light mediators [7], will substantially benefit from the run 2 precision, marking the transition of reactor CE$\nu$NS from first detection towards precision measurements.

Limitations

The CEνNS event count remains statistics-dominated, while uncertainty in the signal prediction limits the rate ratio and weak-mixing-angle determination.
See the passage in the paper
Unlike $N_{s}$, the ratio is systematics-dominated (13.2% vs. 7.5% statistical), arising mostly from the uncertainty on the signal prediction, which therefore limits the determination of the weak mixing angle and constraints on physics beyond the SM.
The fit shows a poor overall goodness-of-fit, with deviations concentrated mainly in C3 and the C6 reactor-off data.
See the passage in the paper
The combined fit has $\chi^{2}$/ndf = 1.37 for 381 effective degrees of freedom ($p=7.1\times 10^{-6}$, from pseudo-experiments). The deviations arise mainly in C3, which contributes about 11% of the expected signal, and in the reactor-off data of C6, while C7, with the largest expected signal (53%), is well described.

Why it matters

The authors argue that the larger exposure and improved detector performance deliver the most precise reactor CEνNS measurement to date and a low-momentum-transfer weak-mixing-angle determination. They describe the result as a step toward precision reactor CEνNS measurements and improved constraints on physics beyond the Standard Model.

Written by AI from the paper. Passages are checked word for word against the text we analysed; the rest may contain errors.

AI-generated keywords: CEvNS CONUS+ germanium detectors reactor antineutrinos weak mixing angle

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Comprehensive Summary
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Original abstract

The paper reports a new measurement of coherent elastic neutrino–nucleus scattering (CEvNS) by reactor antineutrinos interacting with germanium. It asks whether the CONUS+ experiment can measure this low-energy process precisely enough to establish a clear signal and test the Standard Model (SM). Reactor CEvNS is difficult to observe because it produces tiny nuclear recoils, only a fraction of which appears as detectable ionization. The signal lies close to the detector energy threshold, making calibration, recoil quenching, and background control particularly important. CONUS+ is located 20.7 metres from the 3.6-gigawatt-thermal Leibstadt nuclear reactor in Switzerland. During its second data-taking period, the experiment used three high-purity germanium detectors with a combined active mass of 5.60 kg, including two newly installed, larger detectors. The analysis included 1,296 kilogram-days of reactor-on data and 86 kilogram-days of reactor-off data, collected between February 2025 and July 2026. The signal region extended from each detector’s threshold—between 140 and 170 electron-equivalent eV—to 800 eV. The team calibrated the energy scale using lines from germanium produced inside the detectors. It modelled backgrounds with simulations constrained by measured spectra at higher energies. A blind analysis concealed the lowest-energy data until the analysis procedures had been tested. The predicted SM signal was 1,701 ± 198 events. This estimate accounts for reactor conditions, detector response, and the conversion of nuclear recoil energy into ionization using the Lindhard quenching model. The researchers extracted the signal by simultaneously fitting reactor-on and reactor-off energy spectra from all three detectors, including nuisance parameters to account for detector and background uncertainties. The combined fit yielded 1,555 ± 116 statistical ± 98 systematic CEvNS events, or 1,555 ± 152 in total. The result has a significance of 10.2 sigma, and the ratio of the measured signal to the SM prediction is 0.91 ± 0.14, consistent with the SM. The paper describes this as the most precise reactor CEvNS measurement to date and reports the largest detected CEvNS sample in a single study. Results from the individual detectors were also consistent with the prediction, although their significances varied: 7.2 sigma and 7.8 sigma for the two larger detectors, and 2.4 sigma for the smaller one. The authors also inferred the weak mixing angle at a momentum transfer of about 10 MeV, obtaining sin²θW = 0.225 ± 0.022. This is consistent within 0.6 standard deviations with the low-momentum SM value of 0.2387 and ranks among the lowest-momentum-transfer determinations from CEvNS. The total uncertainty on the extracted event count is 9.8% and is dominated by statistics. The ratio to the SM prediction has a larger uncertainty of 14%, because uncertainties in the predicted signal contribute substantially. Energy-scale calibration and quenching dominate the prediction’s uncertainty, while background modelling and trigger efficiency are the largest systematic contributions to the fitted event count. The fit shows some tension between the data and the background-plus-signal model, particularly for the smaller detector and for reactor-off data from one of the larger detectors. Several checks support the signal estimate, although a fit to the reactor-on-minus-reactor-off data yields a less significant result of 3.1 sigma, with a larger uncertainty. The authors conclude that CONUS+ has entered a precision-measurement phase. They note that improved statistics, background control, threshold calibration, and quenching measurements will be needed to sharpen tests of the SM and searches for new physics.

CONUS+ reports a high-significance neutrino measurement

Neutrinos are famously difficult to detect. They rarely interact with matter, and when they do, the resulting signal can be exceptionally small. A new study from the CONUS+ experiment reports a substantial step forward: a high-significance measurement of coherent elastic neutrino–nucleus scattering, or CEvNS, using antineutrinos from a nuclear reactor and germanium detectors.

The paper, titled “High significance measurement of reactor coherent elastic neutrino-nucleus scattering on germanium by CONUS+,” is written by N. Ackermann, H. Bonet, C. Buck, J. Hakenmüller, G. Heusser, M. Lindner et al.. Its result is consistent with the Standard Model and is described as the most precise reactor CEvNS measurement to date.

A tiny recoil, close to the threshold

In CEvNS, a neutrino scatters from an entire atomic nucleus rather than from one of its individual particles. The nucleus recoils, but at reactor energies that motion is minute. Only part of the recoil’s energy becomes detectable ionization in a germanium detector, and the signal sits near the detector’s energy threshold.

That makes the measurement unusually sensitive to details of the experiment. Researchers must understand the detector’s energy scale, how nuclear recoil energy translates into ionization, and how to distinguish a real neutrino signal from background events. A small error in any of these areas can matter when the signal is so close to the threshold.

Watching antineutrinos from a nearby reactor

CONUS+ operates 20.7 metres from the Leibstadt nuclear reactor in Switzerland, which produces 3.6 gigawatts of thermal power. In its second data-taking period, the experiment used three high-purity germanium detectors with a combined active mass of 5.60 kilograms. Two of those detectors were newly installed and larger than before.

The analysis covered 1,296 kilogram-days of data collected while the reactor was on and 86 kilogram-days collected while it was off. These data were gathered between February 2025 and July 2026. The signal region ran from each detector’s threshold—between 140 and 170 electron-equivalent eV—to 800 eV.

The team calibrated the energy scale using signals from germanium produced inside the detectors. It estimated backgrounds with simulations checked against measured spectra at higher energies. The analysis was also blinded: the lowest-energy data remained hidden while the researchers tested their methods, helping reduce the chance of shaping the analysis around an apparent signal.

A signal consistent with the Standard Model

After the procedures were set, the researchers fitted the reactor-on and reactor-off energy spectra from all three detectors together. The fit included nuisance parameters to account for uncertainties in the detectors and backgrounds. The predicted Standard Model signal was 1,701 ± 198 events. That prediction incorporates reactor conditions, detector response, and the conversion of nuclear recoil energy into ionization using the Lindhard quenching model.

The fit yielded 1,555 ± 116 statistical ± 98 systematic CEvNS events, or 1,555 ± 152 when the uncertainties are combined. This is a 10.2-sigma result: a strong signal by the standards of particle physics. The measured event count is 0.91 ± 0.14 times the Standard Model prediction, consistent with the theory.

Measurements from the individual detectors also agreed with the prediction, though with different levels of significance. The two larger detectors reached 7.2 sigma and 7.8 sigma, while the smaller detector reached 2.4 sigma. Taken together, the three detectors provided the largest CEvNS event sample detected in a single study, according to the paper.

What the measurement says—and what remains uncertain

The team also used the data to infer the weak mixing angle, a quantity that describes how particles interact through the weak force. At a momentum transfer of about 10 MeV, they obtained sin²θW = 0.225 ± 0.022. This is consistent within 0.6 standard deviations with the low-momentum Standard Model value of 0.2387, and ranks among the lowest-momentum-transfer determinations from CEvNS.

The total uncertainty on the extracted event count is 9.8%, with statistics contributing the most. The ratio to the predicted signal has a larger uncertainty, 14%, because uncertainty in the prediction itself also plays a substantial role. Energy-scale calibration and quenching are the main sources of uncertainty in the prediction; background modelling and trigger efficiency are the largest systematic contributions to the fitted event count.

The analysis is not without complications. The researchers report some tension between the data and the background-plus-signal model, especially for the smaller detector and for reactor-off data from one of the larger detectors. Several checks support the signal estimate, but a separate fit to the reactor-on-minus-reactor-off data produced a less significant result of 3.1 sigma and a larger uncertainty. These points make continued scrutiny and improved control of detector effects important.

The road to more precise tests

For CONUS+, the result marks a move toward precision measurements of reactor CEvNS. More data could reduce statistical uncertainty, while improved background control, threshold calibration, and measurements of nuclear-recoil quenching could make the interpretation more precise. Those improvements will matter not only for sharper tests of the Standard Model, but also for searches for possible new physics.

Created on 02 Nov. 2026

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Weekly letter Presented in "Particles, nuclei and gravitation this week" of 2 November 2026. Particles, nuclei and gravitation this week: Testing the hidden machinery of matter

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High significance measurement of reactor coherent elastic neutrino-nucleus scattering on germanium by CONUS+

The essentials

Question

Method

Findings

Limitations

Why it matters

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Definitions

CONUS+ reports a high-significance neutrino measurement

A tiny recoil, close to the threshold

Watching antineutrinos from a nearby reactor

A signal consistent with the Standard Model

What the measurement says—and what remains uncertain

The road to more precise tests

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