The XENONnT Dark Matter Experiment is a huge laboratory located 1400 meters below the surface, using up to several tons of liquid xenon and a series of sophisticated experimental instruments to search for mysterious dark matter.
In the center of this photo you can see the bottom of the cylindrical "time projection chamber". The interior of this central chamber is dotted with 120 golden "eyes" that receive light from interactions caused by subatomic particles (neutrons).
Michelle Galloway, a senior researcher at the University of Zurich in Switzerland, was a participant in the experimental program. She usually drives to work, passing through a tunnel on the way and then into the interior of a mountain. At the entrance to the experimental facility, the guard would ask her for a secret code, "and then the door in the rock opened, like the scene in the James Bond movie, which is super cool."
Behind the gate is the Gran Sasso National Laboratory in Italy. This is the world's largest underground laboratory, located at a depth of 1400 meters below the surface. In the rocks of the Apennines, cave-like halls were built, and an advanced machine inside might change our understanding of the entire universe.
Through the XENONnT experiment, Galloway and her colleagues hope to achieve one goal: to determine exactly what dark matter is made of. Dark matter is a substance that does not interact with electromagnetic forces, i.e. does not absorb, reflect or emit light. Scientists speculate that dark matter accounts for about 85 percent of the universe's total mass; it bends light to hold galaxies together and prevent them from separating from each other — and physicists know that there is a lot of dark matter in the universe from gravitational effects like these.
The remaining 15 percent of the universe's mass — from countless stars and planets to the cells that make up the human body — could be incorporated into the Standard Model, a theory by which scientists describe all known elementary matter particles.
Dark matter presents scientists with a conundrum because it does not conform to the Standard Model. One idea, known as supersymmetry, is that there are many difficult particles in the universe that interact with particles we already know. "If we can find some evidence to support supersymmetry, that will give us a way to extend the Standard Model," Galloway explains.
A worker carefully paced the floor of the outer detection chamber, wearing a special experimental suit that would avoid contaminating the equipment. The main detector is located behind a white cloth above. When operating, the whole space is filled with water.
Researchers such as Galloway hope to use 8.6 tons of liquid xenon to unravel the mystery of dark matter particles. Xenon is an inert gas that is sometimes used as a general anesthetic. Galloway pointed out that this gas is extremely rare, so it is very expensive. The last time the team bought it, it cost around €12 per liter. At this price, 8.6 tons of liquid xenon would cost around €17 million. However, these liquid xenon are available in batches and can be recycled.
About 5 tons of liquid xenon, kept at minus 100 degrees Celsius, are pumped into the probe's time projection chamber (TPC), the smallest of the detector's three chambers, which has just recently undergone a massive upgrade. The time projection chamber can determine the three-dimensional coordinates of the trail by using the particle trail to generate the drift time of the ionized electrons and the projection position in the direction of the drift. In the XENONnT experiment, the time projection chamber was designed to receive weak signals from dark matter particles traveling through Earth. One theoretical candidate particle the team hopes to detect is the "massive weak interaction particle," or WIMP for short. Galloway said XENONnT should be able to effectively capture the "wind of massive weakly interacting particles" traveling through the universe.
If it works as planned, the WIMP will enter a cylindrical time projection chamber, hitting the nucleus of the xenon atom, causing a small amount of light to escape. In this "nuclear recoil" event, some electrons are also released from the xenon; they reach the top of the time projection chamber, emitting more light signals when interacting with a layer of xenon gas.
The problem is that although the light detectors 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 the light-emitting events, the team can map exactly where they occur. If multiple interactions at the appropriate energy levels occur in the center of the time projection chamber, in the middle of liquid xenon, the researchers can determine that these signals are caused by weakly interacting particles.
In such experiments, eliminating noisy signals is one of the biggest challenges scientists face. Xenon must be constantly purified in order to extract substances that naturally accumulate in liquid xenon from the detector's material. The two outer chambers are filled with a special salt solution that slows down particles that interfere with WIMP detection, and additional photosensitive devices in these external chambers can also detect WIMP-independent interactions, thus knocking out these unrelated signals. Imagine that in a forest where the wind is howling, if you want to hear a small bird's faint call, you need to block out 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, the XENONnT experiment should bring us closer to the answer to this question. At the very least, this experimental project is yet another reminder that humanity still knows 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 gives us a special perspective." ”