The largest running dark-matter experiment has its first result: No finding yet with the world best sensitivity

Xiangdong Ji, Spokesperson and Project Leader of the PandaX experiment located in the China Jin-Ping underground Laboratory (CJPL), announced the first dark matter search results from the PandaX-II 500 kg liquid xenon detector in the 2016 International Identification of Dark Matter conference at Sheffield, UK in the evening of July 21, 2016 (Beijing Time). He reported that no trace of dark matter was observed with an exposure of 33,000 kg·day of liquid xenon, providing the newest constraints on the existence of dark matter. The sensitivity of the detection has reached the best level in the world.

PandaX collaboration was founded in 2009 by Prof. Ji who splits his time between Shanghai Jiao Tong University and University of Maryland, USA. The collaboration consists of a numbers of institutions in China, including Shanghai Jiao Tong University, Peking University, Shandong University, Shanghai Institute of Applied Physics, Sun Yat-Sen University, University of Science and Technology of China, China Institute of Atomic Energy and Yalong Hydropower Company. Scientists from the University of Maryland and University of Michigan also joined the collaboration. The experimental hall of PandaX is located within the China JinPing underground Laboratory, which has been developed by the Tsinghua University and the Yalong Hydropower Company in 2010.

Dark matter comprises of 85% of the matters in the universe according to the observations from astronomy and cosmology. Lack of electromagnetic interactions to ordinary matter makes the dark matter impossible to be “seen” with common methods, acting like “ghosts”. Many scientists believe that between the dark matter particle and ordinary matter there exists a weak interaction. Such interaction makes it possible for detection of dark matter with detectors in the satellites, such as “WuKong” launched in the end of 2015 in China, or with particle colliders, such as the large hadron collider at CERN (LHC), or with the deep underground dark matter detectors such as PandaX. However, the detection is far from a trivial task due to the extremely weakness of the interaction.

PandaX experiment uses xenon atoms as the detection target. It searches for extremely tiny signals caused by possible collisions between the xenon atoms and billions of dark matter particles around the earth. Recoiling energy from collisions produces photons and electrons, signals of which can be recorded by sensitive photomultiplier tubes (PMTs) in one single event. PandaX experiment, for the first time, makes use of 110 new type 3-inch PMTs to collect these signals. The latest result is equivalent to the fact that there is no single collision between these dark matter particles and 33000 kg of xenon atoms in one day or 1 kg of xenon atoms in 33000 days. This fact sets unprecedented stringent constraints for possible dark matter particles and provides vital inputs for theoretical research on dark matter. As a matter of fact, previous dark matter detection results have excluded many dark matter theories. The most popular theory, the supersymmetric dark matter theory, is also constrained more tightly by the latest PandaX-II data.

The most challenging part of the experiment is that background particles such as gamma rays can also produce photon and electron signals. Despite the fact that the PandaX detector is installed underneath the 2400 m of mountain rocks and is surrounded by hundreds of tons of high purity materials (they stop environmental background by not dark matter), the detector recorded approximately 24 millions of events from March 2016 to June 2016. All these events are processed and scrutinized by the data analysis. In the end, there is only one suspicious event, which happened at the Beijing time June 11, 2016, 03:03:06. However, careful analysis indicates that this event is likely due to radioactivity in the detector but not from dark matter collisions. This high rejection power for background interferences demonstrates the great potential of detecting dark matter particles with the PandaX detector. Prof. Jianglai Liu of Shanghai Jiao Tong University, who in charge of the data analysis, says that the most challenging part of data analysis is to develop an unprecedented discrimination algorithm between dark matter signals and background noises based on “pattern recognition” for all events collected by this brand new detector. Two teams of people, in Shanghai and Jinping, spending more than 14 hours per day including weekends for the analysis and discussion, successfully completed the data analysis work in a very short time.

The 500 kg-xenon PandaX detector was designed by the collaboration and built in the laboratory at Shanghai Jiao Tong University. This detector is capable of detecting a single photon or electron with high sensitivity. Being the largest running dark matter detector in the world to date, it has the highest sensitivity for dark matter detection. After this first run, it will continue to detect dark matter with even higher sensitivity, and will keep leading the results in the world in the near future.

The PandaX experiment is located inside the JinPing Mountain where people rarely venture. CJPL is in the middle of a 17-km long tunnel. The logistics there is still immature. The collaborators from the PandaX Collaboration had stationed there since 2012. Many members skipped vacations including the Spring Festivals these years. “Being through all those frustrations and tortures, we finally conquered all challenges during the construction and running of the PandaX-II detector and reached the world-leading sensitivity. I cannot find words to describe the happiness deep in my heart. For a scientist, nothing is more exciting than a little step crossing boundary of our knowledge to the Nature.” said Andi Tan, the on-site coordinator of the PandaX-II experiment, a graduate student from the University of Maryland, College Park who presented detailed analysis of this result on IDM2016.

An Academician of the Chinese Academy of Sciences, the former director of the Institute of High Energy Physics, HeSheng Chen said that this result of the PandaX-II experiment demonstrated that the underground detection of dark matter in China has reached the world-leading level. Another academician and prominent high energy physicist, Prof. Zhenguo Zhao of the University of Science and Technology of China, commented that PandaX-II, the largest running dark matter experiment in the world has produced a world leading upper limit to the dark matter-ordinary matter interactions. This shows that Chinese scientists are now at the frontline of the most cutting edge research.

PandaX has been supported in part by the Ministry of Education, Shanghai Jiao Tong University, China Jinping underground Laboratory, and all collaboration institutes. The Ministry of Science and Technology, the Natural Science Foundation of China, and the Hong Kong Hong Wen Foundation have all provided critical financial support to the project.

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