CDEX-10 Experiment: Low-Mass WIMP Search

• CDEX-10 Experiment is a direct dark matter search utilizing p-type point-contact germanium detectors in a deep underground environment.
• It features an innovative design with LN₂ immersion for integrated cooling and shielding, and employs advanced pulse shape discrimination techniques.
• The experiment sets stringent limits on WIMP-nucleon cross sections, paving the way for future large-scale dark matter and neutrinoless double beta decay projects.

The CDEX-10 experiment is a second-generation direct dark matter search carried out at the China Jinping Underground Laboratory (CJPL). Designed to probe weakly interacting massive particles (WIMPs) in the light mass regime, CDEX-10 employs an array of p-type point-contact germanium detectors with ultra-low electronic thresholds and operates within a carefully engineered, ultra-low-background environment deep underground. It is a critical technological and scientific bridge connecting the initial CDEX-1 results to the ongoing construction of large-mass, ton-scale CDEX germanium experiments.

1. Detector Architecture and Experimental Approach

CDEX-10 utilizes a 10 kg array of p-type point-contact germanium (pPCGe) detectors configured into three vertical strings, each comprising three detector modules (nine elements in total, labeled C10A, C10B, C10C; Ge1–Ge3 in each). For the first science run, due to technical constraints, results are based on data from the C10B-Ge1 detector (fiducial mass 0.939 kg after accounting for surface dead layers).

The entire array is directly immersed in liquid nitrogen (LN₂) within a stainless steel tank (1.5 m diameter × 1.9 m height), a configuration that serves both as cryogenic cooling and as integrated gamma shielding. This tank sits inside a polyethylene room with 1 m thick walls and is further enclosed by 20 cm of high-purity oxygen-free copper also bathed in LN₂ for additional passive shielding. The experiment is sited at CJPL-I, with an overburden of over 2400 meters of rock, yielding a cosmic-ray flux several orders of magnitude below that at the surface.

Key architectural innovations:

• Integrated shielding and cooling via full LN₂ immersion (adopted from GENIUS/GERDA), enabling both low-energy threshold operation and passive background suppression.
• Modular detector string design for scalability.
• Deep utility of the underground laboratory setting for muon suppression.

2. Operational Parameters and Exposure

The first CDEX-10 data release covered a period with the operational detector providing a live exposure of 102.8 kg·day (product of 0.939 kg fiducial mass and 109.4 days of active data-taking). The experiment achieved an analysis threshold of 160 eVee (electron-equivalent energy, calibrated to electron recoils such as X-rays), which is the lowest threshold among all CDEX datasets to date.

This threshold enabled sensitivity to nuclear recoils from WIMP-nucleus scattering events down to 2 GeV/c², a mass regime previously not accessible in germanium-based searches with similar exposure.

3. Data Acquisition, Analysis Methodology, and Background Control

The data acquisition and spectrum analysis workflow is adapted and improved from the prior CDEX-1B program, with modifications for the multi-element array context. Advanced event selection and pulse shape discrimination techniques are employed to differentiate putative WIMP-induced nuclear recoil candidates (bulk events) from surface background events arising from incomplete charge collection and from electronic or noise-dominated pulses.

• Pulse shape analysis: Rise time and other waveform metrics are used to discriminate bulk from surface events and electronic noise.
• Cosmogenic background subtraction: X-ray lines from isotopic decays (Ge, Zn, etc.) are modeled and statistically subtracted.
• Systematic uncertainties are rigorously propagated throughout, with particular attention paid to fiducial volume determination, energy calibrations, and background modeling.

These procedures ensure that identified events passing all criteria represent a cleanizable dataset for rare-event analysis.

4. Results: Limits on WIMP-Nucleon Cross Sections

No statistically significant excess of WIMP-candidate nuclear recoil events was observed above background expectations. The collaboration set stringent upper limits on both spin-independent (SI) and spin-dependent (SD) WIMP-nucleon scattering cross sections:

• For spin-independent interactions at $m_\chi = 5$ GeV/c²:

$\sigma^{\rm SI}_{\chi N} < 8 \times 10^{-42}~\text{cm}^2 \quad \text{(90% CL)}$

• For spin-dependent interactions at the same mass:

$\sigma^{\rm SD}_{\chi N} < 3 \times 10^{-36}~\text{cm}^2 \quad \text{(90% CL)}$

• The experiment extends mass reach down to $m_\chi = 2$ GeV/c².
• Exclusion curves show nearly an order of magnitude improvement over previous CDEX limits for $m_\chi \sim 2$–5 GeV/c².

The resulting exclusion is visualized in the form of standard WIMP parameter space plots (cross section vs. $m_\chi$), highlighting the newly excluded region for low-mass WIMPs in the SI regime.

5. Implications for Low-Mass Dark Matter Searches

CDEX-10 distinctly advances the global effort to probe low-mass ($m_\chi < 10$ GeV/c²) WIMP parameter space. Key aspects:

• Threshold-limited sensitivity: Owing to the 160 eVee analysis threshold, the experiment enters the low-mass regime inaccessible to most xenon or liquid noble TPCs with higher thresholds.
• Background competitiveness: Ultra-low bulk event backgrounds ($\sim$2 counts/(keV·kg·day) in the 2–4 keV region) impact both exposure–sensitivity and systematic reliability.
• Technological validation: The operation of a 10 kg pPCGe array fully immersed in LN₂ demonstrates large-mass upscaling with robust background rejection — directly informing future experiments at the 100–1000 kg scale.

CDEX-10’s performance demonstrates that modular germanium arrays in ultra-low background, deep-underground, and fully cryogenic configurations are poised to set competitive limits or possibly observe rare nuclear recoils from low-mass WIMPs.

6. Technological Significance and Future Prospects

The CDEX-10 project is a testbed for the technologies and analytic strategies necessary for next-generation experiments. The viability of the integrated LN₂-based shielding/cooling, array construction, and systematic background control directly supports the planned CDEX-100 (100 kg) and ultimately CDEX-1T (ton-scale) efforts. Additionally, the architecture is suitable for neutrinoless double beta decay searches, enabling multipurpose science delivery.

Future stages at CJPL-II are projected to employ LN₂ tanks approaching 1700 m³, substantially amplifying total detector mass and further reducing external gamma backgrounds. The technical groundwork laid by CDEX-10 also synchronizes with the goals of LEGEND1000 (next-generation $0\nu\beta\beta$ searches).

7. Comparative and Theoretical Context

While model-dependent LHC searches set stronger bounds for SI WIMP interactions, CDEX-10 and other direct detection experiments provide complementary, model-independent constraints. Solid-state detector platforms like CDEX-10 are uniquely positioned in the global landscape for low-mass WIMP searches due to their lower energy thresholds and superior energy resolution.

The exclusion results of CDEX-10 are directly comparable to those from contemporaneous efforts (e.g., SuperCDMS, EDELWEISS, DAMIC), but the combined exposure, threshold, and background suppression in CDEX-10 represent an advancement in the parameter space for light WIMP sensitivity.

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Summary Table: Main CDEX-10 Results (from (Jiang et al., 2018)

Parameter	Value/Range	Comments
Detector array	10 kg pPCGe	3 strings × 3 detectors
Active mass in first run	0.939 kg (C10B-Ge1)	After dead layer correction
Exposure	102.8 kg·day	109.4 d live time
Analysis threshold	160 eVee	Electron-equivalent energy
Background (bulk, 2–4 keV)	~2 counts/(keV·kg·day)	Bulk events, after cuts
$\sigma^{\rm SI}_{\chi N}$ limit	$8 \times 10^{-42}$ cm² (5 GeV/c², 90% CL)	Spin-independent
$\sigma^{\rm SD}_{\chi N}$ limit	$3 \times 10^{-36}$ cm² (5 GeV/c², 90% CL)	Spin-dependent
WIMP mass lower reach	2 GeV/c²	Threshold-limited

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CDEX-10’s first results confirm the experiment’s capacity for precision rare-event searches at and below the GeV/c² WIMP mass scale, establishing a technological and scientific foundation for the next generation of germanium-based dark matter experiments and providing benchmark exclusions in the search for light WIMPs at deep underground laboratories.