“There are indications that scientists are on the verge of the discovery of dark matter. The discovery will be sensational in the physics community.”On October 21, Professor Ji Xiangdong, head of Department of Physics, ShanghaiJiaoTongUniversity, gave an academic lecture to teachers and students to introduce the cause and effect of dark matter and analyze the possibility of detecting dark matter in laboratory.

Scientists have found the evidence of the existence of dark matter both in astronomy and cosmology. Ji Xiangdong said, fluctuations of cosmic background radiation could be explained by a simple inflation models and 20% of the entire universe is dark matter; the Milky Way contained over 95% dark matter. Scientists made judgment that dark matter was possibly a kind of particle, of electric neutrality and long life-span, but without strong interaction.

The numerical simulation of the universe evolution demonstrates that dark matter is a cold matter in all probability, so its quality should be relatively large. In the early universe, dark matter particles and other particles in the universe were in thermal equilibrium state. As the universe cooled, dark matter particles constantly annihilated to keep the thermal equilibrium with other particles in the universe. To a certain point of time, the particle density was too small, the speed of annihilation slowed down, and dark matter was left over as antiquity of the universe.

Dark matter may annihilate in space and produce common substances, such as electron-positron pair, quarks and anti-quarks, high-energy X-rays, gamma Rays, high-energy neutrinos, etc. According to these principles, scientists have tried various methods to detect dark matter. They captured high-energy positrons by satellite, tested high-energy electrons by balloon, explored high-energy neutrinos by burying detection volume in the Antarctic Ice Sheet to indirectly search for dark matter.

Ji Xiangdong introduced that dark matter could also be detected directly. By way of analogy, dark matter forms a huge ocean which surrounds the Milky Way and Earth (the detector) is like a boat sailing on the ocean. There is a very low probability for the floating dark matter particle to run into the nucleus of the detector and cause it to bounce. The direct detection of dark matter is to measure such a bounce.

In order to exclude the interference of the general living environment, scientists build the laboratory underground, which creates good experimental conditions for the search for dark matter. The United States, Italy, Japan, Koreaand other countries all have built the underground laboratory. At present, Department of Physics, ShanghaiJiaoTongUniversityand TsinghuaUniversityare building the laboratory which is of 2400-meter depth underground in Jinping Mountain, Sichuan. Its equivalent water depth is 6000 meters and is expected to be completed by 2010. Through the liquid xenon detector to search for dark matter, it is internationally competitive in the frontier area of physics.Professor Ji Xiangdong at the same time is head of Instituteof Nuclear, Particle, Astronomy, and Cosmology (INPAC). He leads the research team to strive to search for dark matter. He is optimistic to predict that in the next few years, significant discovery is most likely to be made in particle physics and cosmology. What is dark matter? Where does electron mass come from? These two interrelated issues may be expected to get settled. Ji Xiangdong hopes that the Chinese will make great contributions in these studies.