JUNO: Jiangmen Underground Neutrino Observatory

• JUNO is a 20 kt liquid scintillator detector designed for precise neutrino oscillation studies and unambiguous determination of neutrino mass ordering.
• The experiment features dual arrays of large and small PMTs with double calorimetry, achieving ~3%/√E energy resolution and sub-percent systematic precision.
• Beyond reactor oscillations, JUNO supports diverse programs including supernova, geo-neutrino, and rare event searches, backed by robust muon and background suppression.

The Jiangmen Underground Neutrino Observatory (JUNO) is a 20 kiloton liquid scintillator (LS) detector located 700 meters underground in Jiangmen, China. It represents the world’s largest LS neutrino experiment, designed to provide a definitive determination of the neutrino mass ordering (NMO) via precision vacuum oscillation measurements at medium baseline, while simultaneously pursuing a broad program in neutrino oscillations, astroparticle physics, and rare event searches (Stock, 12 May 2024, Zhang, 2021, Steiger, 2022, Salamanna, 2018, He, 2014).

1. Detector Architecture and Technical Specifications

JUNO’s central detector (CD) comprises a spherical acrylic vessel of 35.4 m diameter, containing 20 kt of ultra-pure LS composed of linear alkyl benzene (LAB) solvent, 2.5–3 g/L PPO fluor, and 1–3 mg/L bis-MSB wavelength shifter. The LS exhibits a light yield of approximately 10,000 photons/MeV and attenuation length exceeding 20 m at 430 nm (Stock, 12 May 2024, Steiger, 2022, Steiger, 2019).

Surrounding the vessel are two concentric arrays of photomultiplier tubes (PMTs):

• 17,612 large 20-inch PMTs (LPMTs; Hamamatsu R12860 HQE and NNVT MCP-PMTs), providing ~78% photocathode coverage and typical detection efficiency QE×collection ~30% at 420 nm.
• 25,600 small 3-inch PMTs (SPMTs), interleaved for double calorimetry and improved spatial granularity (He, 2017, Steiger, 2019).

The CD is immersed in a 43.5 m diameter cylindrical water-Cherenkov detector (WCD) serving as a gamma/neutron shield and muon veto, equipped with 2,400 20-inch PMTs. Overhead, a Top Tracker array of recycled OPERA plastic scintillator modules covers ~60% of the CD/WCD footprint, providing cosmic muon momentum resolution of 0.2% for through-going muons (Guo et al., 2018, Stock, 12 May 2024).

Auxiliary systems include a multi-axis calibration platform (ACU, CLS, GTCS, ROV), online optical and radiopurity monitoring (OSIRIS), and a near-field reference detector (TAO) employing high-density SiPM calorimetry (Stock, 12 May 2024, Rodphai et al., 25 Feb 2024, Steiger, 2022).

2. Neutrino Oscillation Physics and Mass Ordering Sensitivity

JUNO is positioned at 52.5 km baseline from the Yangjiang and Taishan nuclear reactor complexes (combined 26.6 GW_th power), a distance chosen to maximize sensitivity to both “solar” ($\Delta m^2_{21}$, $\sin^2 2\theta_{12}$) and “atmospheric” ($\Delta m^2_{31}$, $\sin^2 2\theta_{13}$) oscillation frequencies in vacuum (Cerrone, 25 Mar 2024, Salamanna, 2018, Li, 2016).

The $3\%/\sqrt{E}$ energy resolution (target $<3\%$ at 1 MeV) achieves the spectral fidelity necessary to resolve the fine interference between slow and fast oscillation wiggles: $$P(\bar\nu_e \to \bar\nu_e) = 1 - \cos^4\theta_{13}\sin^2 2\theta_{12}\sin^2\left(\frac{\Delta m^2_{21}L}{4E}\right) - \sin^2 2\theta_{13}\left[\cos^2\theta_{12}\sin^2\left(\frac{\Delta m^2_{31}L}{4E}\right)+\sin^2\theta_{12}\sin^2\left(\frac{\Delta m^2_{32}L}{4E}\right)\right]$$ (Stock, 12 May 2024, Steiger, 2022, Cerrone, 25 Mar 2024).

The vacuum-dominant setting at the first solar oscillation maximum ($\Delta_{21}\simeq\frac{\pi}{2}$) allows JUNO to disentangle normal and inverted mass orderings via the observed phase shift in high-frequency oscillation peaks (Cerrone, 25 Mar 2024, Li, 2016). After six years, JUNO is projected to achieve a NMO discrimination of $\Delta\chi^2\sim9$ (3σ significance) (Stock, 12 May 2024, Steiger, 2022, Zhang, 2021).

The sub-percent precision goals for oscillation parameters, attainable in $\sim$6 years, include:

• $\Delta m^2_{21}$: $\sim$0.3–0.5%
• $\sin^2\theta_{12}$: $\sim$0.5–0.7%
• $|\Delta m^2_{31}|$: $\sim$0.2–0.5% (Stock, 12 May 2024, Zhang, 2021, Li, 2016, He, 2014).

3. Extended Physics Program: Astroparticle, Geo, and Exotic Searches

The detector mass and background control enable diverse scientific programs beyond reactor oscillations:

• Supernova neutrinos: For a galactic SN at 10 kpc, JUNO will record O(5,000) inverse beta decay (IBD) events and several thousand elastic and nuclear interactions (Collaboration et al., 2021, Steiger, 2022, Salamanna, 2018).
• Diffuse supernova neutrino background (DSNB): A $\sim$3σ detection in $\sim$10 years is feasible through IBD tagging in 10–30 MeV (Collaboration et al., 2021, Steiger, 2022).
• Geo-neutrinos: Antineutrinos from U/Th decay chains will be detected at $\sim$400 ev/yr; precision on mantle flux and radiogenic heat at O(10%) over a decade (Stock, 12 May 2024, Giaz, 2018, Li, 2016).
• Solar neutrinos: High event rates for $^7$Be, $^8$B, pep, CNO chains via $\nu_e$–$e^-$ elastic scattering, with challenging stringent radiopurity requirements ($^{238}$U, $^{232}$Th $<10^{-17}$g/g for optimal S/B) (Zhang, 2021, Grassi, 2016, Steiger, 2022).
• Atmospheric neutrinos: Charged-current $\nu_e$, $\nu_\mu$ events provide complementary NMO handles, flavor and CP phase sensitivity (Zhang, 2021, Giaz, 2018).
• Proton decay: Sensitivity in the $p\to\bar\nu K^+$ channel projected to set lifetime limits %%%%28$\sim$29%%%% years in a decade (Stock, 12 May 2024, Salamanna, 2018).
• Sterile neutrinos, exotic models: Short–baseline oscillations (TAO), non-standard interactions, Lorentz/CPT violations, axion-like particles, neutrino magnetic moment (Zhang, 2021, Steiger, 2022, Giaz, 2018).

4. Muon Backgrounds and Veto System Performance

A principal background in JUNO arises from cosmogenic isotopes ($^{9}$Li/$^{8}$He, fast neutrons), produced by cosmic muons traversing the detector. Showering muons—comprising $\sim$10% of through-going muons—generate $>$85% of cosmogenic isotope yield, localized within $\sim$3 m of parent tracks (Zhang, 2022).

Muon tracking utilizes PMT waveform analysis, reconstructing shower vertices to $\lesssim$1 m spatial resolution for showers with $E_\text{dep}>1$ GeV. Spherical vetoes of radius 3 m about reconstructed vertices remove $\sim$95% of cosmogenic backgrounds, preserving live time and enabling high-purity IBD signal extraction. Neutron-capture vetoes yield even higher efficiency ($>$99.7%), albeit with model-dependent association (Zhang, 2022, Zhang et al., 2018, Guo et al., 2018).

The Water Cherenkov Veto (WCV) and Top Tracker (TT) combine to yield $>$99.9% muon tagging, with ultra-pure water maintained at $<$0.2 Bq/m$^3$ Rn via membrane degassing and circulation (Guo et al., 2018, Steiger, 2019).

5. Double Calorimetry, Calibration, and Simulation Frameworks

Energy measurement in JUNO employs a double calorimetry approach:

• Large PMTs (high photon statistics, nonlinear charge response, dominant in stochastic term).
• Small PMTs (single photoelectron regime, negligible nonlinearity, used for photon counting and cross-calibration) (He, 2017, Steiger, 2019).

The correlation $Q_{20}(E, r) = \alpha(r)\cdot N_3(E, r) + \beta(r)$ enables systematic correction of spatial and electronics nonuniformities, yielding overall energy-scale uncertainty $<$1% and total resolution of $3\%/\sqrt{E}$ (He, 2017, Steiger, 2019).

Calibration uses redundant systems (ACU, CLS, GTCS, ROV) deploying gamma, neutron, positron sources and pulsed lasers for mapping position-dependent and nonlinearity effects (Salamanna, 2018, Giaz, 2018, Steiger, 2022). OSIRIS provides batch-level radiopurity assays to $10^{-16}$ g/g via Bi–Po delayed coincidence signatures (Rodphai et al., 25 Feb 2024).

Simulation software, built on the SNiPER C++/Python framework, leverages full Geant4-based chain, accelerated via GPU (Opticks), and incorporates multi-threaded event mixing, voxelized fast photon transport, and realistic electronics digitization (Lin et al., 2022). Key models include Birks’ law for scintillation quenching, Rayleigh/Cerenkov/attenuation parameterizations, and detailed time-correlated event mixing for precise physics sensitivity studies (Lin et al., 2022).

6. Construction, Commissioning Milestones, and Future Prospects

The underground facility (700 m, $\sim$1,800 m.w.e.) was completed in December 2021, with stainless-steel shell and PMT assembly finalized by June 2022. Acrylic sphere paneling and LS purification plants, together with OSIRIS and TAO near detector construction, proceed toward sequence filling by late 2024; first physics data is scheduled to commence by the end of 2024 (Stock, 12 May 2024, Steiger, 2022, Steiger, 2019).

Current and future upgrades:

• Enhanced calibration (LED/laser mapping for non-linearity $<$1%)
• Background minimization (pulse-shape discrimination, PMT waveform tagging)
• Multi-messenger SN early warning via low-threshold triggers
• Gadolinium loading in LS for improved neutron-capture signal
• Expanded TAO reference spectrum for reactor flux anchoring (Stock, 12 May 2024, Steiger, 2022, Giaz, 2018).

JUNO thus inaugurates a new era in precision vacuum oscillation physics, astroparticle neutrino detection, and next-generation technical platforms for rare-event and exotic physics searches.

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Table: Key Parameters of JUNO Central Detector

Quantity	Value	Feature
LS mass	20 kt	Acrylic sphere, D=35.4 m
Large PMTs (20″)	17,612	Optical coverage, 78%
Small PMTs (3″)	25,600	Double calorimetry
Energy resolution	3%/√E(MeV)	$<$1% non-linearity
Reactor baseline	52.5–53 km	Yangjiang & Taishan NPPs
Muon veto tagging	$>$99.9%	WCD/TT/ultra-pure H$_2$O

JUNO’s integrated design provides combined sensitivity to neutrino mass ordering and allows sub-percent precision measurements of oscillation parameters, all supported by robust muon/background suppression and calibration systems (Stock, 12 May 2024, Zhang, 2022, He, 2017, Lin et al., 2022, Guo et al., 2018).