How Scientists Hunt for Dark Matter in the Deepest Cave on Earth
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How Scientists Hunt for Dark Matter in the Deepest Cave on Earth

In the mountains of South Dakota, USA, there is a cave with a depth of 1490 meters. This is one of the deepest underground science laboratories in the world. It is mainly used for neutrino physics, cosmic ray exploration, geothermal energy, nuclear fusion reactions, etc. Scientific research experiment. Because the cave is deep enough to effectively avoid cosmic rays, dust and noise from interfering with the experiment, the researchers proudly claim that this is the purest place on Earth and even in the solar system.
  In the laboratory stands a huge white cylindrical device, which is the world’s largest and most sensitive dark matter particle detector LUX-ZEPLIN (“LZ” for short). It is 30 times larger than its predecessor, the large underground xenon laboratory, and 100 times more sensitive to detecting dark matter signals than its predecessor. In July 2022, LZ will be officially opened. Now, a team of researchers is using it to find an as-yet-undiscovered particle and go directly to dark matter hiding in the universe.
  Why can scientists confirm the existence of dark matter?
  Dark matter is the mysterious matter hidden in the universe. They do not radiate any electromagnetic waves, nor do they carry or transmit any information. Simply put, they do not emit light or reflect light. Therefore, neither human eyes nor instruments can directly perceive dark matter, and they are also dubbed “cosmic ghosts in invisibility cloaks”.
  Although dark matter is very stealthy, it can interact with matter we can see. Long-term observations by astronomers have found that many galaxies, such as the Milky Way, spin very fast around the center. According to precise calculations, the galaxy rotates at such a fast speed that the outer stars will be thrown outward like people sitting on a merry-go-round. In this way, the entire universe will become a mess of loose sand, as if falling apart, and collisions between planets will inevitably occur. But in fact, all galaxies seem to be pulled together by some force, always running in an orderly manner.
  Is the physical knowledge we have wrong, or is there an invisible and intangible substance that maintains the normal order of the galaxy with its own gravity? Scientists concluded it was the latter.
  Through in-depth research and analysis, scientists call the force that causes galaxies to separate outwards “dark energy”, and the matter that maintains the normal rotation of galaxies is called “dark matter”. After a rough estimate, these two mysterious things account for about 95% of the universe. The mission of dark matter detectors is to find them.
  Looking for unique flashes deep underground
  According to the existing research results, dark matter is a special neutral particle that is neither positively nor negatively charged. It has almost no attraction and repulsion with ordinary elementary particles. It is a “weakly interacting massive particle”. Therefore, it can easily pass through any object or even a large celestial body, and travel freely in the vast space without a trace or a trace.
  Scientific experiments have found that some known heavy particles like dark matter in the universe interact with atoms of xenon gas to emit unique flashes of light. Therefore, the researchers set up two sealed tanks made of titanium metal in the dark matter detector, which were filled with 10 tons of liquid xenon. The reason for choosing xenon liquid is that its density is very high, about 3 kilograms per liter, which is denser than the interior of metal aluminum, which is conducive to resisting the passage of particles out of thin air and increasing the probability of collision with xenon atoms. At the same time, in order to allow the xenon atoms to “wait” for collisions, the temperature inside the titanium tank must be kept below -100°C to reduce the free vibration of the xenon atoms.
  In addition, the top of the titanium tank is also densely covered with sensitive instruments such as photomultiplier tubes, photodetectors and optical amplifiers, which are used to convert the energy generated by particle collisions into flashes of light and amplify them so that people can see them with the naked eye.
  When the heavy particles with strong penetrating ability from the universe break through the thick strata and enter the titanium tank to collide with xenon atoms, the electrons knocked out from the xenon atoms will drift to the top of the tank, where they will collide with the photomultiplier tubes. A series of complex actions take place in the instrument, which makes people see strange and beautiful flashes. Because of the heavy particles that collide with the xenon atoms, the flashes that appear are different. Therefore, according to the characteristics of this flash, scientists can use the method of exclusion to infer whether some mysterious heavy particle from the universe is dark matter.
  Once this goal is realized, it will subvert the human understanding of the vast universe. It turns out that not only the sun, moon and stars are above our heads, but most of the members of the universe have been secretly “hide and seek” with us.
  There are also many ways to detect dark matter in China
  In fact , there are many ways to detect dark matter, and Chinese scientists have explored several methods earlier. Due to the strong penetrating power of dark matter, laboratories can be built in pits thousands of meters deep. With the help of thick rock formations, interference from cosmic rays can be isolated, and only dark matter particles with strong penetrating power can penetrate in. Hitting it on the receiving screen of a sensitive instrument produces a weak signal, from which we can observe its “true face of Lushan Mountain”.
  On December 12, 2010, China’s first and deepest underground scientific laboratory, Jinping Underground Laboratory, was unveiled at the Jinping Hydropower Station on the Yalong River in Sichuan and officially put into use. This well-isolated underground dark matter capture site has a rock cover thickness of 2,400 meters, ranking first in the world. Its completion marks that my country has a world-class clean and low-radiation research platform, and can independently carry out international cutting-edge research such as dark matter detection. The dark matter detector of the experimental group of Tsinghua University entered the laboratory first and started the detection work. In 2011, Shanghai Jiaotong University and other research teams also entered here one after another to carry out dark matter detection research. In June 2018, the China Dark Matter Experimental Cooperation Group used the world’s first high-purity germanium detection system to increase the direct detection sensitivity of dark matter to the highest international level within a certain range.
  The space collection method is another important means for our country to detect dark matter. When the dark matter particles distributed in the universe collide and annihilate each other, they will produce particle streams such as electrons, protons, neutrons, and photons that we are familiar with, that is, various rays. Therefore, using instruments onboard satellites and space stations to collect the signals of these streams of particles holds the promise of finding dark matter particles. On December 17, 2015, my country successfully launched the “Wukong” dark matter particle detection satellite. As the dark matter detection spacecraft with the widest observation range and the best resolution in the world, it flies around the earth outside the atmosphere repeatedly, almost free from the interference of the atmosphere and geomagnetism, and is busy searching for dark matter in the near-vacuum near-Earth space. clues.
  What is even more gratifying is that in November 2021, the research team of Professor Peng Xinhua of the University of Science and Technology of China developed a new type of ultra-sensitive quantum precision measurement technology, which can be used to directly search for dark matter. The experimental results are at least higher than the previous best international level. 5 orders of magnitude. The core technology of this project is the magnetic field quantum amplifier developed by them, which can significantly amplify the magnetic field, thereby greatly improving the sensitivity of magnetic field measurement. Due to the weak interaction between dark matter and atomic nuclei, a very small magnetic field will be generated. Using this ultra-sensitive instrument comparable to the American titanium tank, if the magnetic field is measured in space, it can prove the existence of dark matter particles.

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