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Constraints on inelastic dark matter from the CDEX-1B experiment

Y. F. Liang, L. T. Yang, Q. Yue, K. J. Kang, Y. J. Li, H. P. An, Greeshma C., J. P. Chang, H. Chen, Y. H. Chen, J. P. Cheng, J. Y. Cui, W. H. Dai, Z. Deng, Y. X. Dong, C. H. Fang, H. Gong, Q. J. Guo, T. Guo, X. Y. Guo, L. He, J. R. He, H. X. Huang, T. C. Huang, S. Karmakar, Y. S. Lan, H. B. Li, H. Y. Li, J. M. Li, J. Li, M. C. Li, Q. Y. Li, R. M. J. Li, X. Q. Li, Y. L. Li, B. Liao, F. K. Lin, S. T. Lin, J. X. Liu, R. Z. Liu, S. K. Liu, Y. D. Liu, Y. Liu, Y. Y. Liu, H. Ma, Y. C. Mao, A. Mureed, H. Pan, N. C. Qi, J. Ren, X. C. Ruan, M. B. Shen, H. Y. Shi, M. K. Singh, T. X. Sun, W. L. Sun, C. J. Tang, Y. Tian, H. F. Wan, G. F. Wang, J. Z. Wang, L. Wang, Q. Wang, Q. Wang, Y. F. Wang, Y. X. Wang, H. T. Wong, Y. C. Wu, H. Y. Xing, K. Z. Xiong, R. Xu, Y. Xu, T. Xue, Y. L. Yan, N. Yi, C. X. Yu, H. J. Yu, X. Yu, M. Zeng, Z. Zeng, F. S. Zhang, P. Zhang, P. Zhang, Z. Y. Zhang, M. G. Zhao, J. F. Zhou, Z. Y. Zhou, J. J. Zhu

TL;DR

This study constrains spin-independent inelastic WIMP-nucleon scattering using 737.1 kg·day of CDEX-1B data by constructing a detailed Geant4-based background model and applying maximum-likelihood fits together with Markov chain Monte Carlo to extract 90% CL upper limits on the cross section across a range of WIMP masses and mass splittings. The authors demonstrate that the resulting limits exclude certain DAMA/LIBRA allowed regions for specific ($m_\chi$, $\delta$) combinations and emphasize that the analysis method is adaptable to other iDM scenarios. They also acknowledge that systematic downward deviations in the 10–70 keVee region render the limits conservative and outline future improvements, including background optimization and the CDEX-50 program, which could enhance sensitivity by roughly four orders of magnitude with a 50-detector HPGe array and ~150 kg·yr exposure. Overall, the work provides a rigorous framework for iDM searches in HPGe detectors and demonstrates meaningful constraints on iDM models while charting a path toward substantially stronger future limits.

Abstract

We present limits on spin-independent inelastic weakly interacting massive particles (WIMP)-nucleus scattering using the 737.1 kg$\cdot$day dataset from the CDEX-1B experiment. Expected nuclear recoil spectra for various inelastic WIMP masses $m_χ$ and mass splittings $δ$ are calculated under the standard halo model. An accurate background model of CDEX-1B is constructed by simulating all major background sources. The model parameters are then determined through maximum likelihood estimation and Markov chain Monte Carlo fitting. The resulting 90\% confidence level upper limits on the WIMP-nucleon cross section $σ_{\mathrm{n}}$ exclude certain DAMA/LIBRA allowed regions: the $χ^2 < 4$ regions for $δ< 30$ keV at $m_χ= 250$ GeV and the $χ^2 < 9$ region for $δ< 50$ keV at $m_χ= 500$ GeV. The method is applicable to other inelastic dark matter scenarios, and the upcoming CDEX-50 experiment is expected to improve sensitivity by four orders of magnitude.

Constraints on inelastic dark matter from the CDEX-1B experiment

TL;DR

This study constrains spin-independent inelastic WIMP-nucleon scattering using 737.1 kg·day of CDEX-1B data by constructing a detailed Geant4-based background model and applying maximum-likelihood fits together with Markov chain Monte Carlo to extract 90% CL upper limits on the cross section across a range of WIMP masses and mass splittings. The authors demonstrate that the resulting limits exclude certain DAMA/LIBRA allowed regions for specific (, ) combinations and emphasize that the analysis method is adaptable to other iDM scenarios. They also acknowledge that systematic downward deviations in the 10–70 keVee region render the limits conservative and outline future improvements, including background optimization and the CDEX-50 program, which could enhance sensitivity by roughly four orders of magnitude with a 50-detector HPGe array and ~150 kg·yr exposure. Overall, the work provides a rigorous framework for iDM searches in HPGe detectors and demonstrates meaningful constraints on iDM models while charting a path toward substantially stronger future limits.

Abstract

We present limits on spin-independent inelastic weakly interacting massive particles (WIMP)-nucleus scattering using the 737.1 kgday dataset from the CDEX-1B experiment. Expected nuclear recoil spectra for various inelastic WIMP masses and mass splittings are calculated under the standard halo model. An accurate background model of CDEX-1B is constructed by simulating all major background sources. The model parameters are then determined through maximum likelihood estimation and Markov chain Monte Carlo fitting. The resulting 90\% confidence level upper limits on the WIMP-nucleon cross section exclude certain DAMA/LIBRA allowed regions: the regions for keV at GeV and the region for keV at GeV. The method is applicable to other inelastic dark matter scenarios, and the upcoming CDEX-50 experiment is expected to improve sensitivity by four orders of magnitude.

Paper Structure

This paper contains 10 sections, 7 equations, 8 figures, 1 table.

Table of Contents

  1. Introduction
  2. Expected iDM spectra
  3. CDEX-1B experiment
  4. CDEX-1B background model
  5. Background simulation
  6. Background fitting
  7. Constraints on iDM
  8. Calculation method
  9. Results
  10. Conclusions and discussions

Figures (8)

  • Figure 1: Expected (a) nuclear recoil and (b) electron-equivalent spectra of inelastic WIMP-nucleus scattering in the HPGe detector with $m_\chi = 100$ GeV, $\delta = 100$ keV, and $\sigma_{\mathrm{n}} = 10^{-40}\ \mathrm{cm}^2$. The energy resolution of the CDEX-1B detector cdex1b2018 is applied to the electron-equivalent spectrum.
  • Figure 2: The expected spectra of inelastic WIMP-nucleus scattering in the HPGe detector for $\delta$ = 40, 80, 100, and 120 keV with $m_\chi = 100$ GeV and $\sigma_{\mathrm{n}} = 10^{-40}\ \mathrm{cm}^2$, compared with the expected spectrum of the elastic scattering.
  • Figure 3: Expected spectra of the inelastic scattering in the HPGe detector for $m_\chi$ = 75, 100, 250, and 500 GeV with $\delta = 100$ keV and $\sigma_{\mathrm{n}} = 10^{-40}\ \mathrm{cm}^2$.
  • Figure 4: Background spectrum with error bars based on the 737.1 kg$\cdot$day dataset of the CDEX-1B experiment cdex1b2018. The spectrum is processed using anti-Compton veto and bulk-surface event discrimination cdex1b2018. The bin width is 100 eVee and the energy range is 1.5--200 keVee.
  • Figure 5: Background model of the CDEX-1B experiment. The upper panel displays the experimental background spectrum (black points with error bars) alongside the best-fit simulated spectrum (red line) and its constituent components (colored lines). The "Additional Pb-210" refers to the additional $^{210}$Pb contained in the lead materials inside the detector vacuum chamber, independent of the "U Series" and the "Rn Progeny." The lower panel quantifies the agreement through normalized residuals $(Count_{\mathrm{simulated,\ i}} - Count_{\mathrm{experimental,\ i}})/\sigma_{\mathrm{experimental,\ i}}$, where $Count_{\mathrm{simulated,\ i}}$ and $Count_{\mathrm{experimental,\ i}}$ are simulated and experimental counts in bin i, respectively, and $\sigma{_\mathrm{experimental,\ i}}$ is the experimental error of bin i. Each peak region defined by the identified peaks in the experimental spectrum is merged into a single composite bin. Consequently, the peaks in the simulated spectra within these regions exhibit a triangular rather than a Gaussian profile in the upper panel.
  • ...and 3 more figures