[Paper Review] Evidence of Coherent Elastic Neutrino-Nucleus Scattering with COHERENT's Germanium Array

S. Adamski, Myunggeun Ahn+86 more|arXiv (Cornell University)|Jun 19, 2024

Neutrino Physics Research4 citations

TL;DR

This paper reports the first direct observation of coherent elastic neutrino-nucleus scattering (CEvNS) on germanium nuclei using the COHERENT germanium detector array at the Spallation Neutron Source. With a 3.9σ significance and 20.6⁺⁷.¹₋₆.₃ signal-like events measured over background, the result is consistent with the Standard Model prediction within 1.95σ, marking a milestone in low-energy neutrino physics and enabling future precision tests of neutrino interactions and new physics beyond the Standard Model.

ABSTRACT

We report the first detection of coherent elastic neutrino-nucleus scattering (CEvNS) on natural germanium, measured at the Spallation Neutron Source at Oak Ridge National Laboratory. The Ge-Mini detector of the COHERENT collaboration employs large-mass, low-noise, high-purity germanium spectrometers, enabling excellent energy resolution, and an analysis threshold of 1.5 keV electron-equivalent ionization energy. We observe a on-beam excess of 20.6 +7.1 -6.3 counts with a total exposure of 10.22 GWhkg and we reject the no-CEvNS hypothesis with 3.9 sigma significance. The result agrees with the predicted standard model of particle physics signal rate within 2 sigma.

Motivation & Objective

• To detect coherent elastic neutrino-nucleus scattering (CEvNS) on germanium nuclei, a process predicted by the Standard Model but previously unobserved in this target material.
• To measure the CEvNS cross-section on germanium with high statistical significance using a low-threshold, high-sensitivity germanium detector array.
• To validate the theoretical prediction of CEvNS in a new target isotope (germanium-76) and assess systematic uncertainties in neutrino flux and detector response.
• To establish a foundation for future precision measurements of neutrino properties and searches for new physics beyond the Standard Model using low-energy neutrino detectors.
• To demonstrate the feasibility of using low-threshold germanium detectors for coherent neutrino scattering experiments, especially in noise-limited regimes.

Proposed method

• The COHERENT collaboration used a high-purity germanium detector array with an energy threshold of 1.5 keVₑₑ to detect low-energy nuclear recoils from CEvNS.
• Data were collected during on-beam and off-beam periods to separate signal from steady-state backgrounds using a simultaneous fit to both datasets.
• A likelihood-based analysis was performed over a 2D space of energy (1.5–8.5 keVₑₑ) and time (0–40 μs relative to beam trigger), with background PDFs constrained by off-beam data.
• The analysis excluded the 8.5–11.0 keVₑₑ region to avoid cosmogenic background lines, and applied a full 40 μs acceptance window to maximize background constraint.
• Systematic uncertainties were evaluated, with the dominant contribution (10.3%) arising from neutrino flux uncertainty at the Spallation Neutron Source.
• A one-sided χ² test confirmed the rejection of the null hypothesis (zero CEvNS signal) at 3.9σ, validated via toy Monte Carlo simulations.

Experimental results

Research questions

• RQ1 Is coherent elastic neutrino-nucleus scattering (CEvNS) observable on germanium nuclei at low energy deposits?
• RQ2 What is the statistical significance of the CEvNS signal observed in the germanium detector array?
• RQ3 How well does the measured CEvNS event rate agree with the Standard Model prediction for the given exposure and detector configuration?
• RQ4 What are the dominant systematic uncertainties affecting the measurement, and can they be reduced in future experiments?
• RQ5 Can pulse-shape analysis further improve sensitivity by rejecting surface events and extending the energy threshold?

Key findings

• The first direct observation of CEvNS on germanium nuclei was achieved with a significance of 3.9σ, rejecting the null hypothesis of no signal.
• A total of 20.6⁺⁷.¹₋₆.₃ CEvNS-signal-like events were measured over steady-state background, with a beam-related neutron contribution of 0.67 ± 0.34 events.
• The measured signal is consistent with the Standard Model prediction of 20.6⁺⁷.¹₋₆.₃ events within 1.95σ, considering all systematic uncertainties.
• The goodness-of-fit (reduced χ²) was 1.84 with a p-value of 0.40, indicating a statistically acceptable fit to the data.
• Systematic uncertainties total 10.3%, primarily due to uncertainty in the neutrino flux at the Spallation Neutron Source, with ongoing efforts to reduce this via independent flux measurements.
• Future upgrades, including increased exposure from the SNS Proton Power Upgrade to 2.0 MW, will significantly reduce statistical uncertainty and enhance sensitivity to new physics.