First Oscillation Spectrum Measurement at JUNO

A 53 km baseline for spectral oscillometry

JUNO’s 53 km distance from the Yangjiang and Taishan nuclear complexes (combined 35 GW thermal power) places the detector near the first maximum of the slow $\Delta m_{21}^2$ oscillation. At this baseline the reactor antineutrino spectrum is modulated by two simultaneous frequencies:

• A slow envelope oscillation at $\Delta m_{21}^2\approx 7.4\times 10^{-5}$ eV² — the “solar” frequency
• A fast modulation at $\Delta m_{31}^2\approx 2.5\times 10^{-3}$ eV² — the “atmospheric” frequency

The interference pattern between these two frequencies in the electron-antineutrino survival probability $P({\bar{\nu }}_e\to {\bar{\nu }}_e)=1-{\sin }^{2}2{\theta }_{12}{\cos }^4{\theta }_{13}{\sin }^2({\Delta }_{21})-{\sin }^{2}2{\theta }_{13}\left[{\cos }^2{\theta }_{12}{\sin }^2({\Delta }_{31})+{\sin }^2{\theta }_{12}{\sin }^2({\Delta }_{32})\right]$ depends on the sign of $\Delta m_{31}^2$ through the relative phase of the two fast modulations. Unlike matter-effect determinations, this dependence does not involve the Earth’s density or the CP-violating phase, giving a clean, model-independent route to the ordering.

Detector performance

The measurement demands stringent energy resolution — 3% at 1 MeV — achieved through:

• 20 kilotons of linear-alkylbenzene liquid scintillator of extreme optical purity
• 17,612 large-area (20-inch) photomultipliers + 25,600 small-area (3-inch) PMTs giving 78% photo-coverage
• Calibration system with 13 radioactive sources spanning 0.3–9 MeV
• Real-time water Cherenkov veto for cosmic muons

The 2025 paper documents that the as-built detector achieves the design resolution within the measured calibration uncertainty.

The 2025 data

The first-data sample, accumulated over approximately one year of running, comprises ~80,000 inverse-beta-decay candidates (delayed-coincidence positron + 2.2 MeV neutron capture gamma, in KamLAND-like fashion). The observed spectrum shows the anticipated oscillatory structure and is consistent with three-flavor oscillation parameters from global fits.

Statistical precision is not yet sufficient to discriminate between normal and inverted ordering; the design expectation is 3–4σ discrimination after approximately 6 years of data.

Companion experiment: TAO

A 2.8-ton gadolinium-loaded scintillator detector, TAO, sits at 30 m from one of the Taishan reactors. TAO provides a high-statistics, high-resolution reference spectrum of reactor antineutrinos at the source, independent of oscillation modeling. Combined with JUNO, it cancels flux-modeling uncertainties in the mass-ordering analysis.

Physics reach

Beyond the ordering, JUNO will deliver:

• $\Delta m_{21}^2$ and ${\sin }^2{\theta }_{12}$ to 0.5% precision — more than an order of magnitude better than current values
• Precision $\Delta m_{31}^2$ from the fast spectral modulation
• Solar-neutrino measurements below 1 MeV, complementing Borexino and SNO+
• Atmospheric-neutrino oscillation parameters through ~$10^4$ events per year
• Geoneutrino measurements with high statistics
• Supernova-neutrino sensitivity for galactic events (~5,000 events at 10 kpc)
• Diffuse supernova background searches

Significance

The 2025 first-oscillation-spectrum milestone demonstrates that the mass-ordering measurement through spectroscopy at a reactor baseline is within reach. Combined with long-baseline accelerator measurements at DUNE and Hyper-K, atmospheric matter-effect measurements at IceCube-Upgrade and ORCA, and cosmological mass-sum constraints from CMB-S4 and large surveys, the mass ordering is expected to be resolved definitively by the late 2020s.