This is the fourth part of the 2030s Roadmap series. We turn to the experiment that will settle the neutrino mass ordering through reactor antineutrinos: JUNO.
What JUNO is
The Jiangmen Underground Neutrino Observatory (JUNO) is a 20-kiloton liquid scintillator detector located 700 m underground in Jiangmen, Guangdong province, China.
The central detector: a transparent acrylic sphere 35.4 m in diameter, holding 20 kilotons of linear alkylbenzene scintillator. Surrounding the sphere: 17,612 large 20-inch photomultiplier tubes plus 25,600 small 3-inch PMTs. Around all of that: 35 kilotons of ultrapure water serving as cosmic-ray veto, photomultipliered as well.
The geometry is precisely chosen for one purpose: detecting reactor antineutrinos from the Taishan and Yangjiang reactor complexes, both at distances of about 52.5 km from JUNO.
Why 52.5 km
The mass-ordering signal in reactor antineutrinos is a small, fast wiggle riding on top of a slow envelope. The wiggle has frequency proportional to $\Delta m^2_{31}$ or $\Delta m^2_{32}$ (the “atmospheric” splittings). The envelope is proportional to $\Delta m^2_{21}$ (the “solar” splitting).
At 52.5 km, both oscillation modes are at appropriate phase: $\Delta m^2_{21} L / 4E$ near maximum, and $\Delta m^2_{31} L / 4E$ producing the fast wiggle on top. The interference between the two patterns encodes the mass ordering.
The exact distance was chosen during JUNO’s design phase to optimize the mass-ordering sensitivity.
The 3% energy resolution challenge
Resolving the mass-ordering wiggle requires distinguishing energy bins of about 50 keV at MeV-scale antineutrino energies — corresponding to ~3% energy resolution at 1 MeV. This is roughly twice as good as KamLAND, the previous generation’s reference.
Every design choice in JUNO was driven by hitting 3%:
Large PMT coverage: ~75% photocathode coverage. This is much higher than typical liquid-scintillator experiments (~30% for KamLAND).
High-quality scintillator: Linear alkylbenzene with PPO and bis-MSB wavelength shifters. Attenuation length > 20 m at 430 nm.
Custom PMTs: 20-inch tubes from Hamamatsu and Chinese NNVT consortium. Better quantum efficiency, faster response.
Geometry: Spherical for symmetric light collection.
Calibration: Multi-modal in-situ calibration using radioactive sources at known positions.
The combination is the largest liquid-scintillator detector ever built with the most aggressive energy-resolution target ever attempted.
Status and timeline
2024-2025: Construction completion, filling, commissioning.
Late 2024/early 2025: Full physics data-taking begins.
2025-2028: Data accumulation, calibration refinement.
Around 2030: First 3σ mass-ordering result.
Mid-2030s: Combined with DUNE and Hyper-K, definitive 5σ result.
What JUNO will measure
Mass ordering: The primary scientific goal. Independent of matter effects (which JUNO has very little of). Independent of $\delta_{CP}$. Statistically robust by ~2030.
θ_12 precision: JUNO will measure the solar mixing angle at sub-percent precision — the world’s most precise. From the smooth envelope of the spectrum.
Δm²_21 precision: Similarly, world’s most precise. Currently known to ~1.5%; JUNO will push to ~0.3%.
|Δm²_31| (or 32) precision: From the fast wiggle. Currently known to ~1%; JUNO will push to ~0.3%.
Geoneutrinos: JUNO will detect antineutrinos from radioactive decay in Earth’s crust and mantle at the highest precision yet — measuring the contribution of geological radioactivity to Earth’s heat budget.
Solar neutrinos: At low threshold (~0.2 MeV), JUNO is sensitive to ⁷Be and pep solar neutrinos at high statistics.
Supernova neutrinos: A galactic supernova would produce roughly 5,000 events in JUNO via inverse beta decay — competitive with Super-K’s expected supernova response.
Atmospheric neutrinos: At GeV energies, JUNO has modest sensitivity. Independent atmospheric oscillation parameters.
DSNB: Limited sensitivity but contributes to the global picture.
What JUNO won’t do
CP violation: JUNO measures reactor antineutrinos only. CP violation requires neutrino vs antineutrino asymmetries, which JUNO cannot provide. DUNE and Hyper-K handle this.
Tau neutrinos: Outside JUNO’s energy regime.
Cosmic neutrinos: Wrong energy regime entirely.
0νββ: Different physics.
Complementarity
JUNO’s mass-ordering measurement is independent of long-baseline experiments in important ways:
- No matter effects: JUNO works in essentially vacuum-oscillation regime. The signal is pure quantum-mechanical interference.
- No δ_CP dependence: JUNO’s spectrum shape doesn’t depend on the CP phase.
- Pure electron-antineutrino disappearance: No appearance channel.
Long-baseline experiments (DUNE, Hyper-K) measure mass-ordering through matter effects on $\nu_\mu \to \nu_e$ oscillation. Atmospheric experiments (IceCube, KM3NeT) measure it through MSW effects on through-Earth atmospheric neutrinos.
The three approaches’ different systematics make their combination essentially impossible to dispute. By 2035, the mass-ordering should be 5σ-determined.
The Chinese neutrino program
JUNO is the centerpiece of China’s neutrino physics program. The project is funded by the Chinese Academy of Sciences with major international collaboration (over 700 collaborators from 17 countries).
It represents a deliberate strategic investment: by hosting the world’s leading mass-ordering experiment, China establishes itself as a major player in fundamental physics, with the infrastructure and expertise needed for the next generation of experiments.
Beyond JUNO, China is investing in TRIDENT — a proposed cubic-kilometer-scale underwater neutrino telescope — and in the proposed Circular Electron-Positron Collider that would include neutrino measurements.
The next part of this series turns to the 0νββ ton-scale generation that will probe Majorana nature: LEGEND-1000, nEXO, and KamLAND2-Zen.
Frequently asked
What's JUNO measuring right now?
JUNO began full data-taking in 2025. The primary measurement is the energy spectrum of reactor antineutrinos at a 52.5 km baseline, with 3% energy resolution at 1 MeV. The shape of the spectrum encodes information about the neutrino mass ordering, the mixing angles θ_12 and θ_13, and the mass-squared differences.
When does JUNO settle the mass ordering?
First 3σ result expected by ~2030. Combined with DUNE and Hyper-K data, definitive 5σ result by mid-2030s. JUNO's approach is independent of long-baseline matter effects, so it provides a critical cross-check.
What else will JUNO measure?
(1) θ_12 and Δm²_21 (solar mixing parameters) at sub-percent precision — the world's most precise. (2) |Δm²_31| at improved precision. (3) Geoneutrino flux with improved geographic resolution. (4) Solar neutrinos (⁷Be, ⁸B) at high statistics. (5) Supernova readiness — JUNO would see thousands of events from a galactic supernova.
What's the 3% energy resolution about?
The mass-ordering signature in JUNO's reactor-antineutrino spectrum is a small, fast-oscillating wiggle riding on top of the dominant Δm²_21 oscillation envelope. Resolving the wiggle requires extraordinary energy resolution — about 3% at 1 MeV, roughly twice as good as KamLAND. Every design choice in JUNO was driven by hitting this 3% number.
Cite this article 5 formats
APA
Neutrino Times Editorial Team. (2026, April 27). The 2030s Roadmap — Part 4: JUNO at full pace. Neutrino Times. https://neutrino-times.com/articles/roadmap-2030s-part-4-juno/
Chicago
Neutrino Times Editorial Team. "The 2030s Roadmap — Part 4: JUNO at full pace." Neutrino Times, April 27, 2026. https://neutrino-times.com/articles/roadmap-2030s-part-4-juno/.
MLA
Neutrino Times Editorial Team. "The 2030s Roadmap — Part 4: JUNO at full pace." Neutrino Times, 27 Apr. 2026, https://neutrino-times.com/articles/roadmap-2030s-part-4-juno/.
BibTeX
@misc{neutrino-times-roadmap-2030s-part-4-juno,
author = {Neutrino Times Editorial Team},
title = {The 2030s Roadmap — Part 4: JUNO at full pace},
howpublished = {Neutrino Times},
year = {2026},
month = {apr},
url = {https://neutrino-times.com/articles/roadmap-2030s-part-4-juno/},
note = {Accessed: 2026-04-27}
} RIS
TY - GEN TI - The 2030s Roadmap — Part 4: JUNO at full pace AU - Neutrino Times Editorial Team PY - 2026 DA - 2026-04-27 PB - Neutrino Times UR - https://neutrino-times.com/articles/roadmap-2030s-part-4-juno/ ER -