On June 12, CDEX (China Dark matter EXperiment) collaboration, led by Tsinghua University, published an article entitled “Limits on Light Weakly Interacting Massive Particles from the First 102.8kg×day Data of the CDEX-10 Experiment” on Physical Review Letters (Phys. Rev. Lett. 120, 241301). In this work, the most stringent limits on spin-dependent WIMP-nucleon cross sections were set for a WIMP mass of 4-5 GeV/c² with the lowest exclusion at 8×10^-42cm² using Point Contact Germanium detectors directly immersed into liquid nitrogen.                    

Figure 1. Configuration of CDEX-10 experimental setup and C10B detector layout inside the string.

        The first results of a light weakly interacting massive particles (WIMPs) search were from the CDEX-10 experiment with a 10 kg germanium detector array immersed in liquid nitrogen at the China Jinping Underground Laboratory (CJPL) with a physics data size of 102.8 kg day. At an analysis threshold of 160 eVee, the lowest threshold and background among the various CDEX data set to date were achieved, which extends the reach of WIMP mass lower to 2 GeV/c². At the same time, this brings forth almost an order of magnitude improvement over their previous exclusion bounds, and the most stringent limits were given with the same detection technology. Especially for spin-independent cross sections, new regions for WIMP mass of 4–5 GeV/c² are probed and excluded.

Figure 2. Exclusion plots of (a) SI χ-N coupling and (b) SD χ-neutron coupling at 90% C.L., superimposed with results from other benchmark direct search experiments.

The CDEX-10 detector array will be installed in a new large LN₂ cryotank with a volume of about 1700 m³ at Hall-C of CJPL-II by the end of 2018, where shielding from ambient radioactivity is provided by the 6 m-thick LN₂. By then, the CDEX-10 experiment will run in this cryotank toward a future ton-scale experiment (CDEX-1T) searching for dark matter and neutrino-less double beta decay.

Figure 3. Conceptual configuration of a future CDEX-1T experiment at CJPL Phase-II.

Prof. Qian Yue, Asso. Prof. Hao Ma and postdoc Litao Yang are the co-corresponding authors of this paper. Hao Jiang and Liping Jia, Ph.D. students in Deparment of Engineering Physics, are the co-first authors. This work was supported by the National Key Research and Development Program of China and the National Natural Science Foundation of China, and the dark matter experimental platform of Tsinghua University.                            

The article link: https://link.aps.org/doi/10.1103/PhysRevLett.120.241301