In Liangshan, Sichuan, there is a place 2500 meters underground. In this "China Jinping Deep Underground Dark Matter Laboratory", there is an experimental hall 40 meters long, 6.5 meters high and 8.5 meters wide. This is the deepest and best underground laboratory in the world at present, and it is also the place where the "Panda Project" is implemented.
In Columbus, Ohio, USA, it was reported at the 2017 International Conference on High Energy Particle Astrophysics a few days ago that China’s "Panda Project" project team issued the strongest restriction on the measurement of dark matter particle properties, and China scientists once again took the lead in the sensitivity of direct detection of dark matter and took the lead in achieving significant detection results. This is the second time that the "Panda Project" released the most sensitive dark matter detection results in the world after the first detection results of the 580 kg detector were released two years ago.
What is dark matter? There is dark matter in the universe, dark matter in galaxies, and dark matter around us, but we can’t see it with our eyes. It is silent and difficult to leave traces. It does not emit electromagnetic waves of any band, nor does it interact with any electromagnetic waves. Because there is no direct electromagnetic interaction with ordinary matter, dark matter does not emit light, which is similar to "ghost" and cannot be seen in the usual way. Nevertheless, dark matter should not be underestimated by anyone, because the research and analysis so far show that dark matter accounts for more than 80% of the total mass of cosmic matter.
Uncovering the essence of dark matter particles is one of the most important scientific goals in physics and astronomy in the 21st century, and the international scientific research on detecting dark matter particles is in full swing. In order to detect dark matter, scientists have thought of many methods. The method of China’s "Panda Project" is to hit a xenon atom as a target. If dark matter particles collide with the xenon atom, this collision will be converted into recoil energy of the xenon atom, and a photoelectric signal will be sent out, which will be captured by the detector.
In the view of Dr. Ji Xiangdong, the leader of the Panda Project and professor of Hongwen at Shanghai Jiaotong University, scientists’ method of detecting dark matter particles is similar to "waiting for a rabbit". Dark matter particles are like rabbits running in droves, and xenon atoms are like a tree in the forest. It is inevitable that one or two rabbits will be killed under a tree and become the trophy of scientists. The crux of the problem is that there are not only rabbits in the big forest, but also lions, tigers and other animals. Whoever bumps into them is louder than rabbits. Therefore, to find a "forest" with no or few other creatures, China Jinping Underground Laboratory is the ideal "forest".
In Jinping Mountain, Sichuan, there is a 2400-meter-high mountain cover, just like a natural barrier blocking cosmic rays. According to the measurement, the cosmic ray flux in the laboratory can be reduced to one-tenth to one-hundredth of the ground level. However, the underground laboratory is located in marble instead of granite, and its natural radioactivity is quite low. Experimental tests show that its natural radioactivity is lower than that of rocks outside the cave, which provides very good conditions for dark matter direct detection experiments.
Under this condition, in the past year, the long-term operation of the 580 kg liquid xenon detector in China has improved its sensitivity to the interaction between dark matter and ordinary matter by more than four times. However, the 580 kg liquid xenon detector used in the "Panda Project" has not detected a collision event of dark matter particles, so China scientists have reached the upper limit of collision probability. This discovery has once again set a new world record for limiting the properties of dark matter particles.
In scientific research, finding a road is as important as blocking a road, and "not seeing it" is equally meaningful.
As one of the most important mysteries in basic physics, the study of dark matter has attracted many physicists all over the world to work hard for it. Several international cooperation groups are working hard to "catch" dark matter. In order to seize the opportunity, it can be said that every minute is lost. All experimental cooperation groups expect to be the first to find the signal of dark matter particles, and the international competition for dark matter detection is fierce.
In the international dark matter detection competition, the LUX cooperation team of the United States, which has finished its operation, officially released the final data of the 250 kg detector in the past four years in January 2017, exceeding the world’s most sensitive detection results published by the Panda Project in July 2016. The XENON1T detector located in Italy has become the largest dark matter detector in operation in the world at present, and the amount of liquid xenon in the detection area is 2 tons. This experimental cooperation group also published the first result in May this year, and its sensitivity once again surpassed that of LUX in the United States.
Although the volume of the second phase detector of "Panda Project" is less than one-third of that of XENON1T in Europe, the cooperative team has obtained the latest detection results beyond XENON1T through the continuous and stable operation for nearly half a year, and achieved the second lead.
Chen Hesheng, an academician of the China Academy of Sciences and former director of the Institute of High Energy Physics of the Chinese Academy of Sciences, said that the latest results of the second phase of the Panda Project show that the deep dark matter detection experiment in China has been at the international leading level; Zhao Zhengguo, an academician of China Academy of Sciences and an experimenter of high-energy physics, thinks that it shows that scientists in China can make international leading achievements in the most advanced fields of exploring the universe and nature, and also in China.
Talking about the next step, Liu Jiang, a scientist of Panda Project and a professor of Shanghai Jiaotong University who is in charge of experimental data analysis, said that the first step is to increase the volume of the detector and the second step is to reduce the noise as much as possible. He revealed that preparations for the next generation of detectors are in full swing, and the 580 kg liquid xenon dark matter detector being used is upgraded to 4 tons. The components of the new detector are already under construction, followed by assembly, alignment and testing. Scientists in China hope to stop the operation of the 580 kg liquid xenon detector in 2018, fully carry out the experiment of the upgraded detector, and strive to obtain the first batch of data of the new detector in 2019. Our reporter Cai Wenqing
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