The deepest underground laboratory for searching for dark matter has significantly increased the sensitivity of detectors

In December 2023, it was reported that a new scientific facility had been launched in China — the world's deepest laboratory for searching for dark matter and other discoveries.. The other day an article was published in the journal Nature , from which more details became known about the experiments being prepared there.. At a depth of 2400 m, sensors with significantly increased sensitivity were installed to detect candidate elementary particles of dark matter.

Image source: Xinhua

The first stage of detectors in an underground laboratory in the Jinping mountain range in China — in the tunnels of a hydroelectric power station built there — was launched in 2010. At the same time, two experimental installations were launched: CDEX (China Dark Matter Experiment) and PandaX (Particle and Astrophysical Xenon Detector).

The CDEX detector is a solid-state semiconductor sensor in the form of 10 kg of pure germanium in a crystalline state. It is believed that 10 kg of sensitive medium is capable of detecting one event per year. The CDEX sensor is designed to capture hypothetical WIMP particles (Weakly Interacting Massive Particles) in the lower mass range.

Particles of dark matter or dark matter, which is more correct from the point of view of determining the nature of this phenomenon, can interact with other particles only with the help of gravity. Rare collisions are possible with the transfer of energy to the atoms of the sensors, which will manifest itself in the form of a glow. Based on the glow intensity and trajectory, the parameters of the original particle can be calculated.

Since the Earth in its orbital and galactic motion must cross the flows of dark matter, sooner or later these particles will manifest themselves in detectors. As we create new sensors and detection conditions, we are gradually creating new boundary conditions of physical states that can correspond to elusive and invisible dark matter particles in the electromagnetic range.

Image source: Liu Weiping et al. / EPJ Web of Conferences

The new germanium semiconductor sensor at the newly launched CJPL-II (China Jinping Underground Laboratory-II) has a mass of about 100 kg, which has increased the frequency of possible detection of candidate dark matter particles by an order of magnitude. At the same time, conditions have been created for further increasing the mass of the semiconductor sensor to approximately one ton. When it is created, China will be ahead of the rest in this.

The second experiment relies on a sensor made of xenon, which is in two phase states — liquid and gaseous. The PandaX experiment started with 120 kg. In the new halls of the CJPL-II laboratory, the mass of the substance in the PandaX-4T sensor has been increased to 4 tons and in the future will increase to 40 or even 50 tons. Today, the Italians have the most sensitive sensor of this kind in the LNGS XENONnT project — it includes 8.6 tons. In the American project LUX-ZEPLIN, the xenon mass reaches 7 tons. For such experiments, repeatability is extremely important to eliminate random detector activations.

Image source: Science China Press

The underground laboratory CJPL-II in China has much better conditions for shielding sensors from the effects of cosmic particles. Also, marble in Chinese mines is less phonite than in Europe and the USA, which also simplifies the design and operation of sensors. The elusive dark matter will become harder and harder to hide from scientists, and over time they will discover what its secret is.