2021-10-18

Secret underground laboratory for searching dark matter: 1400 meters underground to change the understanding of the universe

By yqqlm yqqlm

Secret underground laboratory for searching dark matter: 1400 meters underground to change the understanding of the universe

the bottom of the cylindrical “time projection chamber” can be seen in the center of this photo. The central chamber is dotted with 120 golden “eyes”, which can receive the light emitted by the interaction caused by subatomic particles (neutrons)

behind the gate is the Gran Sasso National Laboratory in Italy. This is the world’s largest underground laboratory, located 1400 meters below the surface. In the rocks of the Apennine mountains, people have built a cave like hall. An advanced machine in it may change our understanding of the whole universe

through the xenonnt experiment, Galloway and her colleagues hope to achieve a goal: to determine what dark matter is composed of. Dark matter refers to the matter that does not interact with electromagnetic force, that is, it will not absorb, reflect or emit light. Scientists speculate that dark matter accounts for about 85% of the total mass of the universe; They can bend light, hold galaxies together and prevent them from separating from each other – physicists know from such gravitational effects that there is a lot of dark matter in the universe

the remaining 15% of the mass of the universe – from countless stars and planets to the cells that make up the human body – can be included in the standard model. Through this theory, scientists describe all known basic matter particles

dark matter poses a problem for scientists because it does not conform to the standard model. A view called supersymmetry theory holds that there are many difficult to detect particles in the universe that interact with our known particles. “If we can find some evidence supporting supersymmetry, it will provide us with a way to extend the standard model,” Galloway explained

Secret underground laboratory for searching dark matter: 1400 meters underground to change the understanding of the universe(1)

a worker carefully paced on the floor of the external detection room. He wore special experimental clothes to avoid polluting the equipment. The main detector is located behind the white cloth above. During operation, the whole space is filled with water

Galloway and other researchers hope to solve the mystery of dark matter particles with the help of 8.6 tons of liquid xenon. Xenon is an inert gas and is sometimes used as a general anesthetic. Galloway pointed out that this gas is extremely rare, so it is very expensive. When the team last purchased, the price was about 12 euros per litre. At this price, 8.6 tons of liquid xenon will cost about 17 million euros. However, these liquid xenon can be bought in batches and can be recycled

about 5 tons of liquid xenon kept at minus 100 ℃ will be pumped into the time projection chamber (TPC), which is the smallest of the three chambers of the detector, and has just been upgraded on a large scale. The time projection chamber can determine the three-dimensional coordinates of the track by using the drift time of ionized electrons generated by the particle track and the projection position of the drift direction. In the xenonnt experiment, the time projection chamber is designed to receive the weak signal of dark matter particles passing through the earth. One of the theoretical candidates that the research team hopes to detect is “large mass weakly interacting particles”, or wimp. Galloway said that xenonnt should be able to effectively capture the “large mass weakly interacting particle wind” across the universe

if it works as planned, wimp will enter the cylindrical time projection chamber and hit the nucleus of xenon atom, resulting in a small amount of light escape. In this “nuclear recoil” event, some electrons will also be released from xenon; They reach the top of the time projection chamber and emit more light signals when interacting with a layer of xenon gas

the problem is that although the photodetector used in the experiment can detect all physical interactions, including background radioactivity, this is not evidence of the existence of dark matter. However, by pinpointing the location of luminous events, the team can accurately map where they occur. If multiple interactions at an appropriate energy level occur in the center of the time projection chamber, that is, in the middle of liquid xenon, the researchers can determine that these signals are caused by weakly interacting particles

in such experiments, eliminating noise signals is one of the biggest challenges faced by scientists. Xenon must be continuously purified to extract substances naturally accumulated in liquid xenon from the material of the detector. The two external chambers are filled with special salt solution, which will slow down the speed of particles interfering with wimp detection. The additional photosensitive devices in these external chambers can also detect the interaction irrelevant to wimp, so as to eliminate these irrelevant signals. Imagine that if you want to hear the faint cry of a bird in the wind roaring forest, you need to shield the surrounding noise and listen very carefully to get better results

of course, dark matter may not be composed of wimp at all; It may be a mixture of different particles, or it may be something completely beyond human imagination. But in any case, xenonnt experiment should bring us closer to the answer to this question. At least, this experimental project reminds us once again that human beings still know so little about the universe. “Even in my lifetime, we still don’t know what dark matter is,” Galloway said. “But I think it still provides us with a special perspective.” (Ren Tian)