The PandaX collaboration released the latest results on the search of WIMPs and axions based on the full exposure of data at the International High Energy Physics Conference (ichep2020) on July 30, 2020 (PDF). Detailed analyses were reported with an online seminar on August 20(PDF1, PDF2 ). The most stringent constraint is set for WIMPs around $10 GeV/c^{2}$ using the nuclear recoil events. The intriguing electron recoil event excess observed by XENON1T [1] was tested with the electron recoil data, and was found to be within the constraint of the PandaX-II data.
The PandaX-II experiment started operation in 2016 and officially ended in June 2019. The total low background data exposure is about 132·ton-days. In 2016 and 2017, the most stringent limits for WIMP-nucleon interactions were set using the earlier 33 ton-day and 54 ton-day data sets. With the full data set and a series of improvements in analysis including reconstruction, detector response modeling, and background estimates, the median sensitivity of the WIMP search is improved by about a factor of two in comparison to the 54 ton-day results. Stringent limit on the spin independent scattering cross section of WIMPs (similar to that from XENON1T in 2018) is given at the WIMP mass of $10 GeV/c^{2}$. A detailed paper describing this analysis has been accepted by Chinese Physics C [2].
For the electron recoil events in the detector, the spectra of major background (krypton, radon, tritium and other radioactive impurities mixed in xenon) were accurately measured in PandaX-II. Based on this, the collaboration performed a direct test on the excess of low energy electron recoil events observed by XENON1T experiment. The data show that the XENON1T excess is within the constraints set by PandaX-II. More sensitive experiments are highly anticipated to set a definitive answer. A letter reporting this analysis has been submitted to Physical Review Letters [3].
The next generation of PandaX experiments, PandaX-4T, is being prepared in China Jinping underground Laboratory and expected to start operation in 2021.
The work on PandaX-II has been supported by Shanghai Jiao Tong University, Shanghai Science and Technology Commission, Ministry of Science and Technology of China, Natural Science Foundation of China, Shandong University, and Science and Technology Department of Sichuan Province. [1] Observation of Excess Electronic Recoil Events in XENON1T, E. Aprile et al. (XENON collaboration), arXiv:2006.09721[2] Results of Dark Matter Search using the Full PandaX-II Exposure, Qiuhong Wang et al. (PandaX-II collaboration), arXiv:2007.15469
[3] A search for solar axions and anomalous neutrino magnetic moment with the complete PandaX-II data, Xiaopeng Zhou et al. (PandaX-II collaboration), arXiv:2008.06485