Sichuan: 2.5 kilometers underground, “Panda” is detecting dark matter

Xinhua News Agency, Chengdu, January 27th: A “panda” is growing vigorously in the underground laboratory of China Jinping at 2,500 meters underground in the Jinping hydropower station on the Yalong River in Sichuan. It won't be a treasure, but it is peeping into the universe "ghost" - dark matter.

This "panda" named PandaX is an English shorthand for the "particle and astrophysics æ°™ detector." This experiment uses the inert element helium purified in the air as a detection medium to search for dark matter, led by Shanghai Jiaotong University.

The reporter recently learned from the experimental group that the current size of liquid helium in the core area of ​​the detector is being upgraded from the first stage of 120 kg to the second stage of 500 kg. After the completion of the upgrade in 2015, it is expected to become the most sensitive dark matter detector in the world.

Professor Shi Xiangdong, a professor at Shanghai Jiaotong University and an expert on the National Thousand Talents Program, likens dark matter to the “ghost” in the universe. He said that research and analysis so far have shown that the share of dark matter in the universe far exceeds what humans can currently see, and the common material we usually observe only accounts for 5% of the mass of the universe. Dark matter does not interact with ordinary substances, so detection is difficult. Dark matter and dark energy are referred to as "two dark clouds" in the modern sky in the 21st century modern physics and astronomy.

It is understood that PandaX uses liquid helium technology to cool helium to minus 100 degrees Celsius. When dark matter collides with helium atoms, helium atoms will emit light, and free electrons will also be generated. The scientist's job is to find ways to detect these faint light and several free electrons, and at the same time confirm that these signals are not caused by other reasons.

According to Liu Jiang, a researcher specializing in physics and astronomy at the Shanghai Jiaotong University, if dark matter is the "fog and haze" in the universe, the earth is a car that runs in fog, and when the particles in the fog hit the car, it will make a "sound". The task of the detection instrument is to record this "sound".

Since the beginning of this century, more than 20 dark matter detection experiments have been carried out in the world. The Italian DAMA/LIBRA experiment, the American CoGeNT and CDMS experiments, and the German CRESST experiment have allegedly discovered a signal suspected of a light-weight dark matter. The main difficulty in such experiments is the cosmic rays and the ubiquitous radioactivity on Earth. For PandaX, the 2500-meter-thick rock layer above it reduces the cosmic flux to very low levels. At the same time, the rock is mainly marble. There is no much radiation material, and it is far away from radiation sources that cause false signals.

In August 2014, the PandaX experimental group published the first batch of data obtained during the first phase of the experiment, questioning the previously internationally suspected suspected dark matter signal.

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