First measurement of reactor neutrino oscillations at JUNO (2511.14593v1)

Abstract: Neutrino oscillations, a quantum effect manifesting at macroscopic scales, are governed by lepton flavor mixing angles and neutrino mass-squared differences that are fundamental parameters of particle physics, representing phenomena beyond the Standard Model. Precision measurements of these parameters are essential for testing the completeness of the three-flavor framework, determining the mass ordering of neutrinos, and probing possible new physics. The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kton liquid-scintillator detector located 52.5 km from multiple reactor cores, designed to resolve the interference pattern of reactor neutrinos with sub-percent precision. Here we report, using the first 59.1 days of data collected since detector completion in August 2025, the first simultaneous high-precision determination of two neutrino oscillation parameters, $\sin^2 θ{12} = 0.3092\,\pm\,0.0087$ and $Δm^2{21} = (7.50\,\pm\,0.12)\times10^{-5}\;{\rm eV}^2$ for the normal mass ordering scenario, improving the precision by a factor of 1.6 relative to the combination of all previous measurements. These results advance the basic understanding of neutrinos, validate the detector's design, and confirm JUNO's readiness for its primary goal of resolving the neutrino mass ordering with a larger dataset. The rapid achievement with a short exposure highlights JUNO's potential to push the frontiers of precision neutrino physics and paves the way for its broad scientific program.

• The paper reports the first precise measurement of reactor neutrino oscillations, determining sin²θ₁₂ = 0.3092 ± 0.0087 and Δm²₂₁ = (7.50 ± 0.12)×10⁻⁵ eV² with improved precision.
• JUNO utilizes a 20 kton liquid scintillator detector with over 17,000 large PMTs at a 52.5 km baseline, achieving a target energy resolution of 3% at 1 MeV and 69.9% efficiency.
• Extensive calibration and background suppression, including spatio-temporal cuts and control samples, underpin the robust extraction of oscillation parameters and test of three-flavor mixing.

First Measurement of Reactor Neutrino Oscillations at JUNO

Introduction

The Jiangmen Underground Neutrino Observatory (JUNO) presents its initial results on reactor neutrino oscillations, marking a significant milestone for precision tests of the three-flavor mixing paradigm and the determination of neutrino oscillation parameters. JUNO, a multipurpose 20 kton liquid scintillator detector, is sited at an optimal baseline of 52.5 km from two large reactor complexes. Its design enables sub-percent sensitivity to solar and atmospheric oscillation parameters, with the crucial goal of resolving the neutrino mass ordering and probing potential beyond-Standard-Model phenomena.

Experimental Setup and Calibration

JUNO's detector configuration comprises a central detector surrounded by a water pool and an external top tracker for cosmic muon tagging. The experimental site, shielded by substantial overburden, minimizes cosmogenic backgrounds. The central detector utilizes an acrylic vessel filled with high-purity liquid scintillator, instrumented with over 17,000 20-inch and 25,000 3-inch PMTs to achieve the target energy resolution of 3% at 1 MeV, a critical requirement for distinguishing oscillatory patterns pertinent to the mass ordering.

Extensive calibration campaigns involving gamma and neutron sources, as well as cosmogenic and radioactive backgrounds, were performed to control energy scale non-linearity and spatial non-uniformity. The resulting energy scale uncertainty is constrained to 0.5%, and the positron energy non-linearity is determined with 1% precision.

Figure 1: JUNO energy scale uncertainty and calibration dataset, showing combined temporal and spatial stability.

Data Analysis and Event Selection

The analysis utilizes 59.1 days of data, corresponding to 2379 identified inverse beta decay (IBD) antineutrino candidates after comprehensive background suppression. Signal identification relies on prompt-delayed coincidence between positron energy deposition and delayed neutron capture. Multiple selection criteria, including spatio-temporal cuts, fiducial volume restrictions, muon and spallation neutron vetoes, and multiplicity filtering, optimize the signal efficiency and minimize background contributions.

Figure 2: Measured energy spectrum of prompt IBD candidates, with the best-fit oscillation model overlayed and separated backgrounds.

The event selection achieves a total efficiency of 69.9% with 1.6% relative uncertainty. Dominant backgrounds are cosmogenic $^9$Li/$^8$He decays, geoneutrinos, and contributions from distant reactors and radon daughters. Dedicated control samples and Monte Carlo simulations constrain both rates and spectral shapes, facilitating robust background subtraction.

Figure 3: Analysis of $^9$Li/$^8$He cosmogenic background rate as a function of visible muon energy, before and after vetoes.

Spectral Fit and Oscillation Parameter Extraction

The observed prompt energy spectrum is fitted using a semi-model-independent approach, integrating reactor flux measurements from Daya Bay and detailed theoretical predictions. The analysis accounts for multiple systematic effects including detector response, reactor operations, baseline configurations, cross sections, and matter effects.

Confidence intervals for the solar oscillation parameters ($\sin^2\theta_{12}$ and $\Delta m^2_{21}$) are determined via a $\chi^2$ minimization procedure using the Combined Neyman-Pearson statistic. External constraints on $\theta_{13}$ and $\Delta m^2_{31}$ are incorporated from Daya Bay results. Matter effects in the Earth's crust are demonstrated to be negligible for JUNO's baseline.

Figure 4: Confidence intervals on $\sin^2\theta_{12}$ and $\Delta m^2_{21}$ from JUNO, KamLAND, SNO+, and SK+SNO for comparison.

JUNO reports $\sin^2 \theta_{12} = 0.3092 \pm 0.0087$ and $\Delta m^2_{21} = (7.50 \pm 0.12) \times 10^{-5}$ eV$^2$ for normal mass ordering. The measurement improves precision by a factor of 1.6 compared to combined previous experiments (KamLAND, SK+SNO), reaching 1.55% and 2.81% uncertainties respectively.

Figure 5: Survival probability of reactor neutrinos vs. baseline, highlighting JUNO's position at the solar oscillation maximum.

Background Control and Detector Performance

JUNO's background analysis leverages detailed spatial, temporal, and energy-based vetos. Energy scale non-linearity calibration uses both monoenergetic sources and continuous cosmogenic spectra for positron and electron response modeling.

Figure 6: Calibration of energy scale non-linearity using gamma and cosmogenic beta sources, with fit models.

Spatial uniformity of reconstructed neutrino candidates within the fiducial volume is demonstrated across multiple reconstruction algorithms, affirming the robustness of vertex determination.

Figure 7: Spatial distribution of reactor neutrino candidates within the fiducial volume illustrating uniformity and background suppression.

Time-dependent rates precisely track reactor operation, confirming both flux predictions and detector stability.

Figure 8: Temporal distribution of reactor neutrino candidate rates correlated with reactor power changes.

Discussion and Implications

The simultaneous measurement of $\sin^2\theta_{12}$ and $\Delta m^2_{21}$ with unrivaled precision significantly advances the global knowledge of lepton mixing and solar-sector parameters. The experiment provides stringent tests of three-flavor unitarity, informs future searches for leptonic CP violation, and bolsters neutrinoless double-beta decay analyses where solar parameter uncertainties directly impact the effective Majorana mass reach. The sub-percent calibration and background modeling validate both the detector design and analytical pipeline for future mass ordering determination, sterile neutrino searches, and non-standard interaction probes.

The outcomes establish JUNO as the world's leading reactor neutrino facility for oscillation parameter measurement and self-consistent cross-validation with solar and accelerator experiments. The high-statistics, low-background environment—the product of meticulous engineering—offers substantial opportunities for astroparticle physics including geoneutrino studies, supernova detection, and dark matter searches.

Figure 9: Comparison of nuisance parameter post-fit pulls across three independent analysis chains, highlighting methodological consistency and systematic control.

Conclusion

The JUNO collaboration presents the first and most precise simultaneous determination of the solar oscillation parameters from reactor neutrinos, with $\sin^2 \theta_{12}$ and $\Delta m^2_{21}$ measured to 2.81% and 1.55% respectively (2511.14593). The rapid achievement in less than two months of exposure demonstrates both the detector's operational excellence and analytical maturity. These results establish a robust foundation for next-generation neutrino oscillation research, including mass ordering resolution and fundamental tests of neutrino flavor physics. The JUNO experiment is poised to advance particle physics and cosmology through ongoing improvements in analysis methodology, calibration, and extended data acquisition.