First Indication of Solar $^8$B Neutrinos via Coherent Elastic Neutrino-Nucleus Scattering with XENONnT

We present the first measurement of nuclear recoils from solar $^8$B neutrinos via coherent elastic neutrino-nucleus scattering with the XENONnT dark matter experiment. The central detector of XENONnT is a low-background, two-phase time projection chamber with a 5.9 t sensitive liquid xenon target. A blind analysis with an exposure of 3.51 t$\times$yr resulted in 37 observed events above 0.5 keV, with ($26.4^{+1.4}{-1.3}$) events expected from backgrounds. The background-only hypothesis is rejected with a statistical significance of 2.73 $σ$. The measured $^8$B solar neutrino flux of $(4.7{-2.3}^{+3.6})\times 10^6 \mathrm{cm}^{-2}\mathrm{s}^{-1}$ is consistent with results from the Sudbury Neutrino Observatory. The measured neutrino flux-weighted CE$ν$NS cross section on Xe of $(1.1^{+0.8}_{-0.5})\times10^{-39} \mathrm{cm}^2$ is consistent with the Standard Model prediction. This is the first direct measurement of nuclear recoils from solar neutrinos with a dark matter detector.

💡 Research Summary

The XENONnT collaboration reports the first observation of nuclear recoils induced by solar ⁸B neutrinos via coherent elastic neutrino‑nucleus scattering (CEνNS) using a liquid‑xenon time‑projection chamber originally built for dark‑matter searches. The detector contains a 5.9‑ton active xenon target observed with 494 photomultiplier tubes that record prompt scintillation (S1) and delayed electroluminescence (S2) signals. Two data‑taking periods, SR0 (108 days) and SR1 (208.5 days), provide a combined exposure of 3.51 t·yr after fiducial cuts, corresponding to 3.97 t·yr and 4.10 t·yr of target mass respectively.

The expected CEνNS signal from solar ⁸B neutrinos is calculated using the measured ⁸B flux from the Sudbury Neutrino Observatory, the neutrino energy spectrum, and the Standard Model CEνNS cross‑section on xenon. Most detectable recoils lie between 0.7 keV and 2.1 keV nuclear‑recoil energy, with 90 % of the signal in this range. Calibration of the low‑energy response is performed with 152 keV neutrons from an external ⁸⁸Y‑Be source, allowing a data‑driven model of light yield (Ly) and charge yield (Qy). The analysis lowers the S2 threshold from 200 PE (used in WIMP searches) to 120 PE and relaxes the S1 coincidence requirement from three‑fold to two‑fold, increasing the expected CEνNS rate to 3.7 events/(t·yr) for SR0 and 3.3 events/(t·yr) for SR1 – roughly a factor of 17 improvement over previous WIMP‑only analyses.

Event selection requires S1 signals with at least two photo‑electron hits, S2 signals between 120 PE and 500 PE, and consistency of the S2 top‑array pattern with the optical response of the detector. Multiple‑S2 events are rejected to suppress neutron backgrounds, and coincidences with the muon veto or neutron veto are removed. The dominant background is accidental coincidence (AC) of isolated S1 and S2 pulses, which, after all cuts, is estimated to contribute 26.4 ± 1.4 events. Additional backgrounds from surface ²¹⁰Pb contamination, radiogenic neutrons, and electronic recoils are modeled and constrained using side‑band data.

A profile‑likelihood fit simultaneously extracts the CEνNS signal strength and background normalizations. The background‑only hypothesis (signal strength μ = 0) is rejected at 2.73 σ (one‑sided), indicating a statistically significant excess of events consistent with solar ⁸B CEνNS. The measured ⁸B neutrino flux is (4.7 +3.6 −2.3) × 10⁶ cm⁻² s⁻¹, compatible within uncertainties with the SNO measurement of ≈5.2 × 10⁶ cm⁻² s⁻¹. The flux‑weighted CEνNS cross‑section on xenon is (1.1 +0.8 −0.5) × 10⁻³⁹ cm², in agreement with the Standard Model prediction of about 1.0 × 10⁻³⁹ cm².

This result constitutes three firsts: the first direct detection of astrophysical neutrino‑induced nuclear recoils, the first CEνNS measurement with a xenon target, and the first step toward the “neutrino fog” limit for dark‑matter experiments, where solar neutrinos become an irreducible background. The authors emphasize that with larger exposures and further reductions of the low‑energy threshold, future runs of XENONnT or next‑generation xenon detectors (e.g., DARWIN) could measure lower‑energy solar components (pp, CNO) and probe non‑standard neutrino interactions, nuclear form‑factor uncertainties, and provide a calibrated neutrino source for beyond‑Standard‑Model searches. The work demonstrates that dark‑matter detectors are powerful tools for neutrino physics, opening a new interdisciplinary avenue between astroparticle and nuclear physics.