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

The CONUS+ collaboration reports the most precise measurement to date of coherent elastic neutrino-nucleus scattering from reactor antineutrinos, achieving a 10.2 $\sigma$ significance with 1555 observed events that align with Standard Model predictions and yield a consistent determination of the weak mixing angle.

Original authors: N. Ackermann (CONUS+ Collaboration), H. Bonet (CONUS+ Collaboration), C. Buck (CONUS+ Collaboration), J. Hakenmüller (CONUS+ Collaboration), G. Heusser (CONUS+ Collaboration), M. Lindner (CONUS+ Colla

Published 2026-10-28

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Original authors: 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), 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 licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Deep within the subatomic world, a fundamental force known as the weak interaction governs how particles transform and decay. For decades, physicists have studied this force by watching how neutrinos, ghostly particles that rarely interact with matter, bounce off atomic nuclei. A specific type of collision, called coherent elastic neutrino-nucleus scattering, was predicted in 1974 but remained elusive for over forty years. In this process, a neutrino strikes an entire atomic nucleus as a single unit rather than hitting individual protons or neutrons, causing the nucleus to recoil with a tiny amount of energy. Because the nucleus is so much heavier than the neutrino, this recoil is incredibly faint, requiring detectors sensitive enough to feel the whisper of a single atomic kick. Understanding this interaction is crucial because it serves as a precise test of the Standard Model, the rulebook of particle physics, and offers a new way to look for physics that might exist beyond our current theories.

At the Leibstadt nuclear power plant in Switzerland, a team of researchers has now captured this elusive signal with unprecedented clarity. Using a specialized array of ultra-pure germanium crystals, the CONUS+ collaboration has recorded the most significant observation of reactor neutrinos bouncing off atomic nuclei to date. The experiment is situated just twenty meters from the heart of a massive reactor core, a source that floods the area with a steady stream of antineutrinos. When these particles strike the germanium atoms in the detectors, they cause the nuclei to recoil. However, because the energy transferred is so small, the resulting signal is easily drowned out by background noise from cosmic rays and natural radioactivity. To solve this, the team buried their detectors in a compact shield made of lead and borated polyethylene, effectively creating a quiet room deep underground where only the faintest whispers of neutrino interactions could be heard.

The researchers operated these detectors for a period equivalent to 1,296 kilogram-days of reactor operation, a massive exposure that allowed them to gather a vast amount of data. By carefully analyzing the energy signatures recorded during this time, they identified 1,555 events that matched the specific pattern expected from neutrino scattering. This number stands in remarkable agreement with the theoretical prediction of 1,701 events, with the difference falling well within the margin of statistical uncertainty. The statistical confidence in this result is extraordinarily high, reaching a level of 10.2 standard deviations, which means the chance of this result being a random fluke is virtually zero. This confirmation is a major step forward, transforming the detection of this phenomenon from a tentative first sighting into a robust, high-precision measurement.

Beyond simply counting the events, the team used these observations to measure a fundamental property of nature known as the weak mixing angle. This value describes how the weak force behaves at different energy levels and is a key parameter in the Standard Model. By analyzing the data at the low energy scale provided by the reactor, the researchers determined this angle to be 0.225, a value that aligns closely with the theoretical expectation of 0.239. This agreement suggests that our current understanding of particle physics holds true even at these low energy scales, where the effects of the weak force are subtle. The measurement also helps to rule out certain alternative theories that predicted a much higher rate of interactions, such as those suggested by a previous experiment that reported a significantly larger signal than what is observed here.

The success of this second run of the CONUS+ experiment relies heavily on the improvements made to the detector array. The team replaced smaller crystals with larger ones, increasing the total active mass by about fifty percent, which directly boosted the number of detectable events. They also refined their methods for calibrating the energy scale, ensuring that the tiny signals from the recoiling nuclei were measured with greater accuracy. While the measurement is still limited by the sheer number of events collected, the team estimates that just one more year of data could reduce the statistical uncertainty significantly. Future upgrades, including better techniques to distinguish neutrino signals from background noise, promise to push the precision even further. This work marks a transition from the initial discovery of reactor neutrino scattering to a new era of precision measurements, opening a fresh window into the fundamental forces that shape our universe.

Technical Summary: High Significance Measurement of Reactor Coherent Elastic Neutrino-Nucleus Scattering on Germanium by CONUS+

Problem and Context
Coherent elastic neutrino-nucleus scattering (CE $\nu$ NS), a Standard Model (SM) process predicted in 1974, involves the scattering of neutrinos off an entire nucleus, enhancing the cross-section and enabling detection with compact, kilogram-scale detectors. While the first experimental detection was achieved by the COHERENT Collaboration using a spallation neutron source, and evidence has been reported for solar neutrinos, reactor antineutrinos offer a complementary source with lower energies ( $E_\nu \lesssim 10$ MeV). Previous efforts to detect reactor CE $\nu$ NS have been limited by statistical uncertainties and energy scale calibration near the detector threshold. The CONUS+ experiment, operating at the Leibstadt nuclear power plant (KKL), previously reported a 3.7 $\sigma$ detection in its first run. The challenge addressed in this work is to achieve a high-significance, precision measurement of reactor CE $\nu$ NS on germanium to test the SM and constrain the weak mixing angle at low momentum transfer.

Methodology
The experiment utilizes high-purity germanium (HPGe) detectors located 20.7 meters from the 3.6 GW $_{th}$ reactor core at KKL, Switzerland.

Detector Upgrade: For Run 2, the detector array was upgraded from four 1 kg p-type point-contact diodes to a configuration including two new 2.42 kg diodes (C6 and C7) and the original 1 kg detector (C3). The total active mass is $(5.60^{+0.03}_{-0.04})$ kg.
Shielding and Background: The detectors are housed in a compact shield of layered lead and borated polyethylene, with an active muon veto and continuous radon-free air flushing. This setup suppresses background by four orders of magnitude.
Data Acquisition: Data were collected from February 2025 to July 2026, yielding an exposure of 1296 kg $\cdot$ days reactor-on and 86 kg $\cdot$ days reactor-off. The energy threshold for the new detectors is approximately 140–160 eV $_{ee}$ (electron-equivalent), with a noise Full Width at Half Maximum (FWHM) of ~50 eV $_{ee}$ .
Analysis Strategy: A blind likelihood analysis was employed. The CE $\nu$ NS signal, concentrated just above the energy threshold (Region of Interest: threshold to 800 eV $_{ee}$ ), was extracted via a binned Poisson likelihood fit performed simultaneously on reactor-on and reactor-off spectra for all three detectors.
Background Modeling: The background model includes seven components: muon- and neutron-induced events, cosmogenic activation (crystals and copper), radon, inert gases from the reactor, and $^{210}$ Pb in the shield. These were simulated using the Geant4-based MaGe framework and constrained by data above 0.4 keV $_{ee}$ .
Signal Prediction: The expected SM signal was calculated using the Lindhard model for quenching ( $k = 0.162 \pm 0.004$ ) and updated reactor thermal power and fission fractions. The prediction uncertainty was reduced to 11.6%, primarily through improved energy-scale calibration at the threshold.

Key Contributions and Results

Event Count and Significance: The analysis yields $1555 \pm 152$ CE $\nu$ NS events (combining statistical and systematic uncertainties). This corresponds to a statistical significance of 10.2 $\sigma$ , representing the most precise measurement of reactor CE $\nu$ NS to date. With this sample size, CONUS+ Run 2 provides the largest sample of detected CE $\nu$ NS interactions to date.
Agreement with Standard Model: The measured signal is in agreement with the SM expectation of $1701 \pm 198$ events within 0.6 standard deviations, with a ratio of $N_s/N_{SM} = 0.91 \pm 0.14$ .
Weak Mixing Angle: The measurement allows for a determination of the weak mixing angle at a momentum transfer of $Q \approx 10$ MeV. The result is $\sin^2 \theta_W = 0.225 \pm 0.022$ , which is consistent with the SM prediction of 0.2387 at low momentum transfer within 0.6 standard deviations.
Systematic Improvements: The total relative uncertainty on the signal count is 9.8% (7.5% statistical, 6.3% systematic). The dominant systematic uncertainties arise from the background model (3.2%) and trigger efficiency (2.6%). The uncertainty on the signal prediction is now dominated by the energy scale calibration (8.0%) and quenching factor (7.5%).
Comparison with Other Experiments: The CONUS+ Run 2 result provides the largest sample of detected CE $\nu$ NS interactions to date. It reaches a precision comparable to recent high-precision COHERENT measurements on germanium and is the most precise measurement at reactor antineutrino energies. The result is consistent with previous upper limits from other reactor experiments and contradicts the higher rate reported by Dresden-II when interpreted with a quenching factor exceeding the Lindhard model.

Significance
The paper claims that this result marks a transition in reactor CE $\nu$ NS physics from initial detection to precision measurements. With a significance exceeding 10 $\sigma$ , the experiment confirms the SM prediction for CE $\nu$ NS on germanium with high confidence. The determination of the weak mixing angle at the lowest momentum transfer probed by CE $\nu$ NS to date provides a valuable low-energy test of the SM. The authors note that while the measurement is currently statistics-dominated, further data taking and improvements in background reduction and calibration (specifically for the quenching factor and energy scale) will further reduce uncertainties, enabling tighter constraints on physics beyond the Standard Model, such as neutrino electromagnetic properties and non-standard interactions.

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