---
title: 'JUNO Experiment: Neutrino Oscillation & Mass Ordering'
url: https://www.emergentmind.com/topics/juno-experiment
type: topic
---

# JUNO Experiment: Neutrino Oscillation & Mass Ordering

The Jiangmen Underground Neutrino Observatory (JUNO) is a next-generation, multipurpose liquid scintillator neutrino experiment located 700 meters underground in Jianmen, Guangdong Province, South China. Designed around a 20 kiloton ultra-pure linear alkylbenzene (LAB) scintillator target housed in a 35.4-meter-diameter acrylic sphere, JUNO aims to resolve the neutrino mass ordering through spectral analysis of reactor antineutrinos, and to reach sub-percent precision in neutrino oscillation parameters. The experiment’s scope includes solar, atmospheric, geo-, supernova, and diffuse supernova background neutrino studies, as well as searches for rare processes such as nucleon decay. With a photocathode coverage exceeding 75% via a hybrid system of 17,612 20-inch and 25,600 3-inch photomultiplier tubes (PMTs), JUNO achieves an energy resolution target of 3% at 1 MeV—critical for its oscillation physics program. Full physics data-taking is planned to commence in late 2024, following the completion of detector construction, purification, and calibration campaigns [2405.07321, 1412.4195].

## 1. Physics Program and Motivation

JUNO’s central aim is the determination of the neutrino mass ordering (NMO), exploiting interference effects in the energy-dependent oscillation pattern of reactor antineutrinos at a medium baseline of approximately 52.5 km between the detector and the Yangjiang and Taishan nuclear power plant complexes (total power 26.6 GW_th). At this baseline, the survival probability of reactor $\bar\nu_e$ is sensitive to both the Δm$_{21}^2$ and Δm$_{31}^2$ driven oscillation frequencies:

$$
P_{\bar\nu_e\to\bar\nu_e} = 1 - \cos^4\theta_{13}\sin^2 2\theta_{12} \sin^2 \Delta_{21} - \sin^2 2\theta_{13} [\cos^2\theta_{12} \sin^2\Delta_{31} + \sin^2\theta_{12} \sin^2\Delta_{32}]
$$

with $\Delta_{ij} = 1.267\,\Delta m_{ij}^2[\text{eV}^2]\,L[\text{km}]/E[\text{MeV}]$ [2403.16817, 1506.01152]. The relative phase shift in the fast oscillation components between normal and inverted mass ordering creates a subtle but statistically resolvable distortion in the IBD energy spectrum, provided high statistics and $\leq 3\%/\sqrt{E\,(\text{MeV})}$ energy resolution are achieved.

Beyond this flagship goal, the broad physics program includes:

- Precision measurements of $\Delta m^2_{21}$, $\Delta m^2_{31}$, $\sin^2\theta_{12}$ to sub-percent accuracy.
- Detection and study of solar neutrinos (^7Be, ^8B, pep, CNO), requiring low backgrounds and sub-MeV thresholds.
- Measurement of atmospheric neutrino spectra (100 MeV–10 GeV) and geoneutrinos.
- Supernova and diffuse supernova neutrino detection, with O(5,000) events expected for a Galactic core-collapse at 10 kpc.
- Searches for nucleon decay (e.g. $p\to\bar\nu K^+$) with multi-layered coincidence tagging [2405.07321, 1606.04743, 1605.09118].

The experiment is designed to remain insensitive to the CP phase $\delta_{CP}$, yielding a robust NMO determination independent of CP-violating effects [1506.01152].

## 2. Detector Architecture and Subsystems

### Central Detector

JUNO’s central detector consists of:

- 20,000 tonnes of LAB-based scintillator, ~2.5 g/L PPO, 3 mg/L bis-MSB; light yield $\sim 10,000$ photons/MeV; attenuation length >20 m.
- Spherical acrylic vessel, 35.4 m in diameter, 12 cm wall thickness.
- Photodetection by 17,612 20-inch PMTs (MCP-based) and 25,600 3-inch PMTs (photon-counting regime); total optical coverage $\sim$78% [2405.07321, 1706.08761].
- Stainless steel truss; water pool (43.5 m diameter) serving as both passive shielding and active muon veto (2,400 20-inch PMTs for Cherenkov light).
- Top Tracker: recycled OPERA plastic-scintillator modules, covering 60% of the CD top, provide precise muon-vector reconstruction.

### Double Calorimetry System

JUNO employs a dual calorimetry system:

| System         | Number of PMTs       | Operational Regime      | Calorimetric Role                     |
|----------------|---------------------|------------------------|---------------------------------------|
| Large PMTs     | 17,612 × 20-inch    | 10–2000 p.e./tube      | Photon-statistics-limited, high yield |
| Small PMTs     | 25,600 × 3-inch     | 0–1 p.e. (photon count)| Linearity standard, low-energy range  |

Photon-counting with small PMTs enables direct calibration and linearity cross-checks of the large channels, reducing non-linear systematic uncertainties below 0.5% [1706.08761].

### OSIRIS Subdetector

OSIRIS (Online Scintillator Internal Radioactivity Investigation System) is a precursor subdetector dedicated to verifying LS batch radiopurity ($<$1e–16 g/g U/Th), using Bi–Po coincidence tagging within a 18-ton LS volume and water Cherenkov veto. Only LS batches certified by OSIRIS are transferred to the main detector, protecting the solar and geo-neutrino low-background programs [2403.07898].

## 3. Simulation, Electronics, and Data Acquisition

### Simulation Architecture

The JUNO simulation stack is built around:

- The SNiPER framework orchestrating workflow; Geant4 toolkit for detector modeling and particle propagation; ROOT for I/O [2212.10741].
- Full simulation of all primary and secondary processes, including optical photon tracking, necessary to match the requirements imposed by the 3%$/\sqrt{E}$ energy resolution target.
- Multi-threaded event processing via MT-SNiPER, with shared immutable geometry/physics data (Intel TBB backend); per-core resident memory 2–2.5 GB (muon/atm-ν events), with near-linear throughput scaling up to 8–16 threads for uniform workloads [2503.20219].
- Deferred optical photon workflow: optical photons are only fully simulated for event classes passing physically motivated pre-selections, reducing CPU/memory in background simulations by up to an order of magnitude [2202.06589].
- Fast optical parameterization ("voxel method") and GPU offloading (Opticks) for efficient handling of high-energy, high-photon-yield events [2212.10741].

### Readout Electronics

- Underwater front-end: each 20-inch PMT is powered, amplified, and digitized via custom ASIC/FPGA platforms. Three PMTs per box share digitization; 3-inch PMTs read out via CATIROC ASICs (128 channels per box).
- Backend DAQ: IPBUS (1 Gbps) for control/data, Trigger and Timing Control (TTC, 125 Mbps) for synchronized clock and trigger communication.
- Dynamic range: 1–4,000 p.e. per channel; single-p.e. noise floor $<$0.1 p.e.; system timing resolution $<$1 ns (dominated at system level by TTS and clock skew).
- Global trigger constructed from box-local primitives, with sub-microsecond latency, then event data are packaged (±500 ns windows) and written out at aggregate rates $>$300 Gbps. Sustained DAQ uptime $>$99% over multi-year campaigns, underwater electronics design loss rate $<$1%/6 years [2110.12277].

## 4. Event Reconstruction and Background Suppression

### Muon and Spallation Backgrounds

- Cosmic muons (3 Hz rate underground) are tracked with sub-3 cm spatial and $<$0.4° angular resolution using a global least-squares fit of first-hit PMT times (FHT) and data-driven corrections for systematics (e.g. light propagation, TTS, scintillation tails) [1803.10407].
- Shower vertex (spallation) reconstruction via χ² minimization of multi-peak PMT waveform times, achieving $<$1 m spatial resolution for $>$50 MeV energy deposits; 90% efficiency above 20 MeV [2203.00402].
- Use of cylindrical and spherical veto techniques (3 m radius around track or shower) enables reduction of cosmogenic β–n background dead-time to 10–12%, compared to full-volume veto (which would be prohibitively costly in live time).

### Trigger and Low-Energy Sensitivity

- Standard multiplicity N-hit triggers (300 ns) provide 0.5 MeV detection thresholds for IBD physics, quantified analytically by Poissonian rates of dark-coincidence triggers.
- Sophisticated energy deposit triggers: 80 ns time-of-flight (TOF)-corrected voxel-based FPGA scheme achieves 0.1–0.2 MeV thresholds while suppressing noise to $<$1 kHz (assuming central $^{14}$C contamination $<$1e–18 g/g), crucial for elastic solar and supernova neutrino channels [1912.01864].

## 5. Calibration, Purification, and Systematic Controls

- Energy scale and non-linearity are controlled to $<$1% through a combination of:
  - Automated deployment (ACU, CLS, GTCS, ROV) of $\gamma$, neutron, and $\alpha$ sources throughout the volume and along boundaries.
  - Small-PMT photon-counting calibration as an absolute linearity standard.
  - Fiber-injected laser and LEDs for PMT/electronics timing, gain, and linearity calibrations.
- LS purification employs alumina filtration, vacuum distillation, water extraction, and gas stripping, monitored in real time with OSIRIS, reaching attenuation lengths $>$20 m at 430 nm and U/Th levels $<$10$^{-15}$ g/g (for reactor program), with further reduction for solar program [2403.07898, 2104.02565].
- Ongoing material screening and surface-clean assembly procedures for the acrylic vessel, PMT glass, and support structure.
- Radon and other noble gas backgrounds are suppressed via degassing membranes and micro-bubble stripping in buffer water pools.

## 6. Sensitivity Projections and Scientific Reach

- Mass ordering sensitivity: six years’ data at full power and design energy resolution yield $\Delta\chi^2 \sim 9$ (3$\sigma$) for NMO via spectral shape analysis, improving to $>$4$\sigma$ when combined with external constraints (1% prior on atmospheric $\Delta m^2_{ee}$) [2403.16817, 1412.4195].
- Oscillation parameters: sub-percent measurement projected—$\Delta m^2_{21}$ to 0.3–0.6%, $\Delta m^2_{31}$ to 0.2–0.6%, $\sin^2\theta_{12}$ to 0.5–0.6% with six years’ data [2405.07321].
- Supernova burst response: $>$5,000 IBD events for a 10 kpc source, with all-flavor sensitivity (IBD, $\nu$–p, $\nu$–e, $\nu$–$^{12}$C channels), ns timing for profile studies [1606.04743].
- Atmospheric neutrino spectral reconstruction at 10–25% precision (in 5 years), θ$_{23}$ octant sensitivity, complementary NMO reach [2103.09908].
- Solar neutrino program: $^7$Be flux achievable at 3–10% uncertainty, $^8$B to 5–8% depending on background; day-night asymmetry and MSW upturn accessible [1809.03821].
- Geo-neutrinos: $>$400 events/year, enabling $<$5% measurement of Earth's radiogenic heat budget [2104.02565].
- Proton decay ($p\to\bar\nu K^+$): lifetime reach $>9\times 10^{33}$ yr (90% CL, ten-year exposure) [2405.07321].

## 7. Construction, Commissioning, and Outlook

- Civil construction of the underground laboratory and caverns was completed by late 2021.
- Acrylic sphere assembly, PMT installation, calibration system deployment, and LS purification plant commissioning are ongoing, with all major components tested and characterized for performance and radiopurity.
- Data taking is planned for late 2024, with the TAO near detector for reactor spectrum monitoring also being commissioned at the Taishan site.
- The scale, energy resolution, and systematic control of JUNO will enable it to serve as a precision instrument for oscillation physics and as a key node in the global multi-messenger astronomy network [2405.07321, 2104.02565].

Source: https://www.emergentmind.com/topics/juno-experiment