CDEX-50 Experiment: Next-Gen Dark Matter Search

• CDEX-50 experiment is a next-generation dark matter search that utilizes a 50-kg germanium array with sub-keV thresholds and ultra-low backgrounds.
• It employs advanced detector architectures with point-contact crystals arranged in strings and a liquid nitrogen cryostat to minimize cosmic and intrinsic radioactivity.
• The experiment targets low-mass WIMP detection, aiming for exclusion limits as low as 5.1×10⁻⁴⁵ cm² at 5 GeV/c², setting new benchmarks in sensitivity.

The CDEX-50 experiment is a next-generation direct dark matter search utilizing a 50-kg array of point-contact high-purity germanium detectors at the China Jinping Underground Laboratory (CJPL). It is designed to achieve ultra-low backgrounds and sub-keV energy thresholds, enabling high sensitivity to Weakly Interacting Massive Particles (WIMPs) in the low-mass regime. The project incorporates advanced background modeling, sophisticated detector technologies, and mitigation strategies for cosmogenic activation, with scientific goals targeting exclusion limits on spin-independent WIMP-nucleon cross-sections down to $5.1 \times 10^{-45}~\mathrm{cm}^2$ for a $5~\mathrm{GeV}/c^2$ WIMP mass (Geng et al., 2023).

1. Scientific Motivation and Experimental Framework

The CDEX-50 experiment is motivated by the search for WIMPs as dark matter candidates, with particular emphasis on the low-mass ($2.2$–$8~\mathrm{GeV}/c^2$) region, where nuclear recoils are limited to a few keV or below (Kang et al., 2013). The overarching objective is the direct detection of WIMP-nucleus elastic and inelastic scattering events through the measurement of ionization signals in germanium detectors with sub-keV thresholds ($\sim160~\mathrm{eV}_{ee}$). The CJPL offers a unique low-background environment due to its $2400~\mathrm{m}$ rock overburden, suppressing cosmic muon flux by $10^{-8}$ compared to the surface (Kang et al., 2013, Yue et al., 2016).

2. Detector Array Architecture and Readout

CDEX-50 deploys fifty $1~\mathrm{kg}$ cylindrical p-type point-contact germanium crystals arranged in five strings of ten (Geng et al., 2023). The PCGe technology maintains low capacitance ($\sim1~\mathrm{pF}$), essential for electronic noise suppression and for attaining sub-keV thresholds (target: $160~\mathrm{eV}_{ee}$). Each string is separated by $40~\mathrm{mm}$, and the crystals are spaced $54~\mathrm{mm}$ along each string for optimal coverage and background rejection. The entire array is immersed in a liquid nitrogen cryostat ($13~\mathrm{m} \times 13~\mathrm{m}$), which provides both cooling and passive shielding ($6.5~\mathrm{m}$ all around) (Geng et al., 2023).

Signal processing leverages pulsed feedback preamplifiers and high-speed FADCs ($100$–$1000~\mathrm{MHz}$), with trigger logic and pulse-shape discrimination (PSD) for noise and surface event rejection (Kang et al., 2013). The low-mass supports and materials are carefully selected and assayed to minimize intrinsic backgrounds.

3. Background Sources and Suppression

Table: Dominant Backgrounds in CDEX-50

Background Type	Principal Source/Process	Mitigation Strategy
Cosmogenic Isotopes	$^3$H,$^68$Ge,$^65$Zn...	Underground fabrication, shielded transport
Environmental γ/Neutron	U/Th chains, $^{40}$K	LN$_2$ shielding, material purity
Radon and airborne daughters	$^{222}$Rn, $^{214}$Pb	Air control, radon monitoring
Solar Neutrinos	CE$\nu$NS	Statistical separation, modeling

Background levels are projected to be $$\sim0.01~\mathrm{counts\,keV^{-1}\,kg^{-1}\,day^{-1}}$$ in the $2$–$2.5~\mathrm{keV}_{ee}$ region (Geng et al., 2023). Cosmogenic activation is quantified using Geant4/CRY simulations, and validated by measurements on CDEX-1B and CDEX-10 (Ma et al., 2018, Nie et al., 2023). Shielded transport containers, temporary underground storage during fabrication, extra neutron moderation, and extended cooling periods underground are deployed to suppress radioisotope accumulation—particularly $^68$Ge, which dominates the long-term background (Nie et al., 2023). Radioassay of materials yields U/$^{232}$Th/$^{40}$K levels for concrete at $6.8 \pm 1.5$, $5.4 \pm 0.6$, and $81.9 \pm 14.4~\mathrm{Bq/kg}$, respectively, with implications for background modeling (Ma et al., 2020).

4. Signal Modeling and Analysis Techniques

WIMP-induced nuclear recoils are calculated under the standard halo model with recoil rate (Geng et al., 2023, Liang et al., 9 Oct 2025):

$$\frac{dR}{dE_r} = N_T \frac{\rho_\chi}{m_\chi} \int_{v_\mathrm{min}}^\infty v f(\vec{v},\vec{v}_E) \frac{d\sigma^{SI}}{dE_r} d^3\vec{v}$$

where $N_T$ is target nuclei number, $\rho_\chi$ is local DM density, $m_\chi$ is WIMP mass, and $f(\vec{v},\vec{v}_E)$ is the velocity distribution.

For inelastic scenarios, the minimal velocity is shifted as (Liang et al., 9 Oct 2025):

$$v_\mathrm{min} = \frac{1}{\sqrt{2 E_{nr} m_N}} \left( \frac{E_{nr} m_N}{\mu} + \delta \right)$$

with $m_N$ nuclear mass, $\mu$ reduced mass, and $\delta$ mass splitting.

Analysis incorporates maximum likelihood estimation and Markov Chain Monte Carlo spectral fitting, with robust background templates generated from Geant4 simulations (Liang et al., 9 Oct 2025).

5. Projected Sensitivity and Scientific Reach

With a $150~\mathrm{kg\cdot yr}$ exposure, CDEX-50 is projected to set a 90% CL exclusion limit on spin-independent WIMP-nucleon cross-section at $5.1 \times 10^{-45}~\mathrm{cm}^2$ for $m_\chi = 5~\mathrm{GeV}/c^2$, outperforming CDEX-10 by three orders of magnitude (Geng et al., 2023). The experiment is optimized for unprecedented sensitivity in the $2.2$–$8~\mathrm{GeV}/c^2$ mass range. In inelastic DM searches, the increased mass and reduced background are expected to improve sensitivity by four orders of magnitude (Liang et al., 9 Oct 2025).

The controlled backgrounds, low thresholds, and statistical techniques position CDEX-50 as a leading experiment in probing parameter space below the neutrino floor. It excludes DAMA/LIBRA allowed regions for certain $\delta$ values and masses in inelastic scenarios—even with a much smaller exposure compared to some contemporaries (Liang et al., 9 Oct 2025).

6. Theoretical Implications and Future Directions

CDEX-50's projected sensitivity supports rigorous tests of WIMP effective field theory models (NREFT and ChEFT), facilitating exploration of nonstandard interaction channels such as WIMP-pion couplings (Wang et al., 2020). The capability to distinguish elastic, inelastic (including exothermic and isospin-violating DM), and EFT operator-driven interactions relies on spectral features and modulation analyses.

Strategies for background further suppression—including underground crystal growth, additional neutron shielding, and extended underground cooldown—are under active paper for scaling to ton-scale detectors such as CDEX-1T (Yue et al., 2016, Ma et al., 2018).

The reduction of cosmogenic radionuclide backgrounds also enables auxiliary physics programs, such as solar neutrino coherent scattering and neutrinoless double-beta decay searches (Ma et al., 2018), thus expanding the overall scientific impact of the experiment.

7. Impact and Integration in Global Dark Matter Searches

CDEX-50 integrates the technological and analytical advances from CDEX-0, CDEX-1, and CDEX-10 (Kang et al., 2013, Yue et al., 2016, Ma et al., 2017), reflecting a systematic approach toward ultra-low threshold, low-background direct detection at CJPL. The results are directly relevant to resolving tensions in the field, e.g., comparisons with previously allowed DAMA/LIBRA regions, and help constrain or exclude models for both elastic and inelastic WIMP interactions (Liang et al., 9 Oct 2025, Chen et al., 2014).

The experiment's performance benchmarks and methodological innovations—validated through simulation and measurement—provide foundational knowledge for future direct detection efforts, scaling strategies, and rare event search methodologies. The projected limits and background modeling establish CDEX-50 as a reference for next-generation germanium-based detectors in dark matter and rare event physics.