Jinping Neutrino Experiment Overview

• The Jinping Neutrino Experiment is a next-generation ultra-low-background facility at the deepest underground laboratory, dedicated to precise measurements of solar, geo-, and supernova neutrinos.
• Its advanced detector design features a large acrylic vessel, high-PDE MCP-PMTs, and innovative DAQ systems, enabling sub-percent precision and effective background discrimination.
• The experiment provides critical tests of the Standard Solar Model, insights into Earth's radiogenic heat, and opportunities to explore physics beyond the Standard Model.

The Jinping Neutrino Experiment is a next-generation, ultra-low-background neutrino physics facility located at the China Jinping Underground Laboratory (CJPL), the deepest underground laboratory in the world. Its scientific objectives, experimental configurations, and technological innovations are designed to enable precision measurements of solar and geoneutrinos, searches for rare events such as supernova relic neutrinos and neutrinoless double beta decay, and to test new physics beyond the Standard Model by exploiting the unique features of CJPL: maximum overburden, exceptional cosmic-ray shielding, and minimal environmental radioactivity.

1. Scientific Goals and Distinctive Scientific Mission

The Jinping Neutrino Experiment is dedicated to the precision measurement of low-energy neutrinos from solar, terrestrial, and astrophysical sources. Its primary scientific goals include:

• Solar neutrino spectroscopy: Achieving percent-level or better precision on individual flux components: pp, $^7$Be, pep, $^8$B, and CNO-cycle neutrinos. This enables rigorous testing of the Standard Solar Model and the neutrino flavor conversion mechanism, including mapping the transition from vacuum to matter-enhanced (MSW) oscillations. The precise measurement of the CNO neutrino flux directly probes the stellar metallicity puzzle and nuclear burning pathways (Beacom et al., 2016).
• Geo-neutrino detection: Separating and measuring the geoneutrino flux from $^{238}$U and $^{232}$Th decay chains, including the U/Th ratio with about 10% precision. This constrains Earth's radiogenic heat production and compositional models of the mantle and crust (Wan et al., 2016).
• Supernova relic neutrinos and rare event searches: The ultra-low background conditions and large target mass enable sensitivity to the diffuse supernova neutrino background and open opportunities for searches for dark matter annihilation neutrinos, sterile neutrinos, and new interaction channels.
• Neutrino magnetic moment and new physics: High statistics and superb energy resolution provide sensitivity to exotic interactions such as neutrino magnetic moments down to $1.1{\times}10^{-11}~\mu_B$ at 90% CL with artificial sources, and $1.2\times 10^{-11}~\mu_B$ with ten years of solar data (Yue et al., 2021).

This scientific focus is distinguished from experiments like JUNO, which are optimized for high-statistics, medium-baseline reactor antineutrino oscillation studies and mass hierarchy resolution. Jinping leverages its unrivaled low-background situation to pursue measurements that require both large statistics and unprecedented suppression of external and internal radioactive backgrounds (He, 2014).

2. Underground Laboratory, Backgrounds, and Environmental Shielding

CJPL’s underground setting is fundamental to the experiment’s capabilities:

• Depth: At 2,400 m of vertical rock overburden (${\sim}6,720~\mathrm{m.w.e.}$), CJPL realizes a cosmic-ray muon flux of $$(3.56 \pm 0.16_{stat.} \pm 0.10_{syst.} ) \times 10^{-10}~\mathrm{cm}^{-2}~\mathrm{s}^{-1}$$ (Zhang et al., 6 Sep 2024). This is $\sim200$ times lower than Borexino, and more than an order of magnitude below Gran Sasso or Kamioka (Beacom et al., 2016, Guo et al., 2020).
• Low reactor background: The closest commercial nuclear reactors are over 1,200 km distant, yielding a negligible reactor antineutrino background ($\sim$6.8 TNU in the 1.8–3.3 MeV window) (Wan et al., 2016). This is critical for unambiguous separation of the geoneutrino signal.
• Suppressed local radioactivity: Managed through judicious selection and custom assay (e.g., smelting process for low-background stainless steel yielding $^{238}$U and $^{232}$Th content down to $\sim1$–$10~\mathrm{mBq/kg}$) (Hussain et al., 2017).
• Fast neutron and muon-induced neutron studies: Measured neutron fluxes in Hall A of CJPL are $$(1.51\pm0.03_{stat.}\pm0.10_{syst.})\times10^{-7}~\mathrm{cm}^{-2}\mathrm{s}^{-1}$$ for 1–10 MeV (Du et al., 2017); background can be further reduced by one order of magnitude using passive polyethylene shielding. Cosmogenic neutron yields are measured as $$(3.37 \pm 1.41_{stat.} \pm 0.31_{syst.}) \times 10^{-4}~\mu^{-1}~\mathrm{g}^{-1}\mathrm{cm}^{2}$$ at an average muon energy of 360 GeV (Zhang et al., 6 Sep 2024), consistent with the results of a global power-law scaling (exponent $b=0.77\pm0.03$).
• Radon mitigation: Introduction of a nitrogen bubbling and sealing system reduces $^{222}$Rn-induced backgrounds by a factor of 4.8, as corroborated by $^{214}$Bi–$^{214}$Po event tag rates (Wu et al., 2022).

3. Detector Design, Prototyping, and Structural Performance

The detector employs a hybrid design strategy, evolving from a validated 1-ton prototype to a multi-kiloton target (Wang et al., 2017, Wu et al., 2022, Wang et al., 8 Jun 2024):

• Vessel: A spherical acrylic vessel of 9.96 m diameter, 50 mm thickness, supported by a network of synthetic fiber ropes arranged in horizontal and vertical layers, designed to maintain a maximum von Mises stress of $<$3.5 MPa for long-term loads (creep threshold) and to survive rope failure events without exceeding 7.0 MPa (short-term) (Wang et al., 8 Jun 2024). Finite element analyses confirm displacements well below the allowable 600 mm, and global buckling safety factors $>$5.
• Support structures: All-stainless-steel trusses (S31608) validated with finite element and Riks analysis; maximum truss stress measured at 128.7 MPa under worst-case loading, with a safety factor of 6.9 in buckling (Wang et al., 2017).
• Photodetectors: Jinping is a pioneer in the use of high-PDE (photon detection efficiency) 8-inch MCP-PMTs, achieving $\sim1.7{\times}$ the PDE of conventional reference PMTs and $>$10% improvement in the energy resolution figure-of-merit, with transit time spread of $1.73\pm0.08$ ns and peak-to-valley ratios of 5.9 (Zhang et al., 2023). The deployment of such tubes supports the separation of Cherenkov and scintillation light—crucial for low-background spectroscopy.
• Electronics and DAQ: The upgraded Tsinghua THDAQ system demonstrates an ENOB $>$9.8 bits (a 14% increase over CAEN V1751), single-chassis clock synchrony of 85.6 ps, QSFP+ optical links at 82.5 Gbps per board, and a PCIe Gen3 throughput of 100.2 Gbps. This anticipates a scaling path to 4,000 channels, sufficient for multi-kiloton detectors with high parallelization (Yang et al., 16 Apr 2024).

4. Experimental Methods, Calibration, and Data Analysis

Jinping’s experimental techniques emphasize robust calibration, background discrimination, and simulation fidelity:

• **Calibration procedures rely on dark noise-based “rolling gain” single-PE charge fitting; in situ calibrations use radioactive decays (208Tl, 40K, 214Po) and careful matching of simulation (GEANT4-based) and measured spectra (Wu et al., 2022).
• **Vertex reconstruction is implemented via a charge-weighted centroid corrected by a geometry-derived scale factor, yielding a typical vertex resolution of $\sim$11.5 cm for central events (Wu et al., 2022).
• **Background models integrate laboratory measurements, Monte Carlo predictions, and time/space-coincidence selections; e.g., $^{214}$Bi–$^{214}$Po delayed coincidence tagging for U/Th chain backgrounds, and pulse-shape discrimination for neutron and $\gamma$ separation (Du et al., 2017, Wu et al., 2022).
• **Neutrino oscillometry for sterile neutrino searches utilizes precise $L/E$ pattern reconstruction with artificial sources ($^{144}$Ce–$^{144}$Pr or accelerator-based $^{8}$Li), leveraging both energy and position resolutions of $5\%/\sqrt{E(\mathrm{MeV})}$ and $10~\mathrm{cm}/\sqrt{E(\mathrm{MeV})}$, respectively (Smirnov et al., 2020).
• **Comprehensive systematic studies have been completed for all essential detector subsystems—materials radiopurity, PMT after-pulse/dark rates, temperature effects on the vessel, and mechanical resilience under rope failure (Hussain et al., 2017, Zhang et al., 2023, Wang et al., 8 Jun 2024).

5. Physics Reach and Broader Impact

• Solar neutrinos: Simulations with a 2-kiloton fiducial mass indicate sub-percent-level statistical uncertainties for pp, $^7$Be, pep, and CNO neutrino fluxes; the CNO metallicity discrimination is projected at the $7$–$10~\sigma$ level (Beacom et al., 2016).
• Geo-neutrinos: A 3-kiloton detector and 1,500 live days can secure total flux precision of $4\%$ and U/Th ratio determination to $10\%$ (Beacom et al., 2016). Uniquely low reactor background allows unambiguous separation from competing signals (Wan et al., 2016).
• Supernova relic neutrinos: The low background and excellent energy resolution make Jinping sensitive to the diffuse supernova neutrino background. While exact rates are not quoted, the experimental design is consistent with global ambitions in this domain (Beacom et al., 2016).
• Beyond the Standard Model: Measurement of the neutrino magnetic moment is feasible. $\mu_\nu^{eff}$ can be probed down to $1.2\times 10^{-11}~\mu_B$ (90% CL) in solar neutrino data, and to $1.1\times 10^{-11}~\mu_B$ (90% CL) with a $^{51}$Cr source (Yue et al., 2021).
• Backgrounds constraints: The experiment provides the world’s best measurements of the cosmic-ray muon flux, with terrain-dependent corrections, yielding valuable constraints (factor of 4 difference for mountain vs mine-shaft sites at equal vertical overburden) (Guo et al., 2020, Zhang et al., 6 Sep 2024).
• Technology development: The structural design of the acrylic vessel, integration of high-PDE MCP-PMTs, and adoption of a 4000-channel DAQ architecture positions Jinping as a model for future ultra-low-background calorimetry at large scale (Zhang et al., 2023, Yang et al., 16 Apr 2024, Wang et al., 8 Jun 2024).

6. Relation to Other Experiments and Synergies

Jinping’s scientific program is complementary to that of JUNO, which is optimized for reactor neutrino oscillation physics at medium baseline with an emphasis on resolving fine $L/E$ spectral features to determine the mass hierarchy (He, 2014). Jinping, by comparison, is optimized for rare event and precision low-energy neutrino observations due to its ultra-low backgrounds. Both are international collaborations and form pillars of China’s forefront participation in global neutrino physics.

At CJPL, ancillary experiments such as PandaX-III for neutrinoless double-beta decay search, and PandaX-xT for next-generation dark matter and neutrino rare-event physics, further enrich the scientific landscape (Chen et al., 2016, Collaboration et al., 6 Feb 2024). The low background environment also redefines the “neutrino floor” for dark matter searches, with CJPL's suppressed atmospheric neutrino flux reducing the irreducible background by nearly 30% compared to LNGS, allowing PandaX-xT to reach spin-independent cross-section sensitivities of $\sim3\times 10^{-49}~\text{cm}^{2}$ at $m_\chi \sim40~\text{GeV}$ with a 500 ton-year exposure (Fan et al., 28 Mar 2025).

7. Future Prospects and Long-Term Outlook

The Jinping Neutrino Experiment is positioned as a flagship observatory for low-background rare event physics:

• Scaling up: The validated mechanical design and background mitigation strategies are directly portable to multi-kiloton (and, by extension, multi-decade) detectors.
• Versatile physics reach: The experiment encompasses solar, geo, supernova, and exotic neutrino physics, and is also instrumental in characterization for dark matter “neutrino fog” boundary studies (Fan et al., 28 Mar 2025).
• Synergies and upgrades: Detector R&D continues with exploration of novel water-based liquid scintillators (WbLS) for enhanced Cherenkov-scintillation separation, incremental increases in PMT coverage, and advanced DAQ upgrades for next-generation event topology reconstruction.

This suite of technical, scientific, and operational innovations ensures that the Jinping Neutrino Experiment will play a central role in resolving outstanding questions in neutrino astrophysics, Earth science, and the search for new physics, providing a uniquely quiet window into the low-energy universe.