JUNO Experiment: Neutrino Oscillation & Mass Ordering

• JUNO Experiment is a next-generation underground neutrino observatory employing a 20-kiloton liquid scintillator target to resolve neutrino mass ordering via reactor antineutrino spectral analysis.
• The detector uses over 43,000 PMTs in a dual calorimetry system to achieve 3% energy resolution at 1 MeV, enabling sub-percent precision in oscillation measurements.
• The experiment also studies solar, atmospheric, geo-, and supernova neutrinos, and searches for rare processes like nucleon decay, advancing our understanding of particle physics.

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 (Stock, 12 May 2024, He, 2014).

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 $$\Delta_{ij} = 1.267\,\Delta m_{ij}^2[\text{eV}^2]\,L[\text{km}]/E[\text{MeV}]$$ (Cerrone, 25 Mar 2024, Zhan, 2015). 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 paper 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 (Stock, 12 May 2024, Li, 2016, Grassi, 2016).

The experiment is designed to remain insensitive to the CP phase $\delta_{CP}$, yielding a robust NMO determination independent of CP-violating effects (Zhan, 2015).

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% (Stock, 12 May 2024, He, 2017).
• 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% (He, 2017).

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 (Rodphai et al., 25 Feb 2024).

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 (Lin et al., 2022).
• 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 (Yu et al., 26 Mar 2025).
• 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 (Lin, 2022).
• Fast optical parameterization ("voxel method") and GPU offloading (Opticks) for efficient handling of high-energy, high-photon-yield events (Lin et al., 2022).

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 (Petitjean et al., 2021).

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) (Zhang et al., 2018).
• 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 (Zhang, 2022).
• 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 (Fang et al., 2019).

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 $>$20 m at 430 nm and U/Th levels %%%%30$\delta_{CP}$31%%%% g/g (for reactor program), with further reduction for solar program (Rodphai et al., 25 Feb 2024, Collaboration et al., 2021).
• 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 %%%%34$$\Delta_{ij} = 1.267\,\Delta m_{ij}^2[\text{eV}^2]\,L[\text{km}]/E[\text{MeV}]$$35%%%% when combined with external constraints (1% prior on atmospheric $\Delta m^2_{ee}$) (Cerrone, 25 Mar 2024, He, 2014).
• 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 (Stock, 12 May 2024).
• 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 (Li, 2016).
• Atmospheric neutrino spectral reconstruction at 10–25% precision (in 5 years), θ$_{23}$ octant sensitivity, complementary NMO reach (Collaboration et al., 2021).
• 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 (Salamanna, 2018).
• Geo-neutrinos: $>$400 events/year, enabling $<$5% measurement of Earth's radiogenic heat budget (Collaboration et al., 2021).
• Proton decay ($p\to\bar\nu K^+$): lifetime reach $>9\times 10^{33}$ yr (90% CL, ten-year exposure) (Stock, 12 May 2024).

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 (Stock, 12 May 2024, Collaboration et al., 2021).