At 4:00 p.m. on July 8th, 2021 Beijing time, the spokesperson of the PandaX Experiment of China’s Jinping Underground Laboratory Professor Liu Jianglai published the first dark matter search results in the PandaX-4T Experiment at the triennial Marcel Grossmann Meeting on Recent Developments in Theoretical and Experimental General Relativity, Gravitation, and Relativistic Field Theory. The results were based on the data of the PandaX-4T after 95 days of trial operations, which, with an exposure of 0.63 tonne/year, has again updated the boundary value of dark matter.

The astronomical and cosmographic studies over the past 100 years or so have demonstrated that the invisible dark matter in the universe is more than 5 times the ordinary matter as we know! It is generally believed in the scientific community that there is very likely to be a new weakly interaction between dark matter and ordinary matter that can be found deep underground with an ultra-sensitive detector.

The PandaX Experiment was initiated by Professor Ji Xiangdong in 2009, in collaboration with Shandong University, Peking University, University of Science and Technology of China, Sun Yat-sen University, Beihang University, Nankai University, Fudan University, the China Institute of Atomic Energy and Yalong River Hydropower Development Co., Ltd. (Yalong Hydro) with over 70 researchers involved, as well as a few institutions from the US, France, Spain and Thailand. The PandaX Experiment was conducted in the Jinping Underground Laboratory in Liangshan Prefecture, Sichuan Province. 2,400 meters from the ground, it is the world’s deepest and lowest cosmic ray background underground laboratory, which was jointly developed and managed by Tsinghua University and Yalong Hydro.

Liquid xenon was used as the dark matter target and detector in the experiment. The dark matter particles dispersed in the Milky Way may bump into the xenon nucleus whose recoil will result in light or the ionization of other xenon atoms in the detector, which can be recorded by the sensitive photomultipliers located on the upper and lower part of the detector. The PandaX team has completed two-generation dark matter experiments (PandaX-I: 120kg; PandaX-II: 580kg) over the past decade and more. The PandaX-II experiment has produced internationally leading results relating to dark matter participles, which was selected as part of the top ten scientific progress published in the Science & Technology Review in 2016 and as one of the highlights of the American Physical Society in 2017. To make the detection for dark matter more sensitive, the team started to build the new generation of PandaX-4T at the Jinping dual phase B2 laboratory in 2009. The multi-ton liquid xenon experiments are also conducted in XENONnT in the Italian Gran Sasso Laboratory and LX in the US Sanford Laboratory.

The Jinping dual phase laboratory for PandaX-4T is one of the major scientific infrastructure projects commissioned by China’s National Development and Reform Commission for the Thirteen Five-Year Plan period, expected to be completed in 2023. To accelerate the experiment, the PandaX team built temporary facilities and 900-tonne ultra-pure water shielding facilities at the B2 laboratory. The experiment group moved in August 2019 and installed the detector by the end of May 2020 after overcoming a myriad of technical challenges and the COVID-19 impacts. The detector was tested on November 28th 2020, and was put into trial operations till April 15th 2021, with 95 live days.

There were 5.6 tons of highly purified xenon in the PandaX-4T detector, with 3.7 tons of it in the sensitive areas. The key to dark matter detection lies in the reduction of disturbance to background. Associate Professor of SJTU for detector development and data analysis Zhou Ning said that the background of PandaX-4T, which is PandaX-II, decreased four times under the same exposure based on several improvements to low-background technologies. The main ingredients of the background of PandaX-4T are the chemically inert radon and tritium in trace amounts dispersed in the detector. The tritium pollution was highly likely to come from an injection after the completion of the PandaX dual phase experiment. In the 95-day operations, the PandaX-4T detector recorded approximately 1.37 billion incidents, of which only 1,058 incidents were put into the final dark matter analysis data set. 6 of them fell into the areas of signals, and proved to meet the background expectations without any signs of excessive dark matter incidents. Due to exceptional sensitivity, the data of PandaX-4T covered the parameter space where undetected dark matter had interacted with ordinary matter, and hence produced the best dark matter search results.

Following this trial operations, PandaX experiment group will accelerate the purification of tritium background in the detector, which is expected to reduce the background by a factor of two to search dark matter with higher sensitivity.

The PandaX Experiment has received strong support from China’s Ministry of Education, SJTU, the Jinping Underground Laboratory and other partners. The National Natural Science Foundation of China, China’s Ministry of Science and Technology, the Shanghai Municipal Science and Technology Commission/the Shanghai Office for the Promotion of Science and Technology Innovation Center, the Hong Kong Hongwen Foundation and Tencent Foundation have all provided important financial support to the PandaX project.