On August 22, Xinhua News Agency reported from Beijing that researchers from the Chinese Academy of Sciences have observed a new type of antimatter hypernucleus—antihydrogen-4—in a recent international collaborative study led by Chinese scientists. This is the heaviest antimatter hypernucleus discovered to date in experiments, contributing to a deeper understanding of the mysteries surrounding antimatter and the symmetry between matter and antimatter.

The study was led by a research team headed by researcher Qiu Hao from the Institute of Modern Physics at the Chinese Academy of Sciences, with the results published in the international academic journal *Nature* on August 21.

Schematic of antiparticle collision producing antihydrogen-4. (Image provided by the Institute of Modern Physics, Chinese Academy of Sciences)

Qiu Hao explained that current knowledge in physics suggests that the properties of matter and antimatter are symmetric, meaning that equal amounts of matter and antimatter should have existed at the birth of the universe. Antimatter is the "reverse state" of matter, and when matter and antimatter meet, they annihilate each other. Fortunately, some mysterious physical mechanism resulted in a slight asymmetry in the quantities of matter and antimatter in the early universe, allowing about one billionth of the matter to "survive" after the majority of matter-antimatter pairs annihilated, forming the material world we see today.

Is the understanding of the symmetry in the properties of matter and antimatter correct? What causes the difference in the quantities of matter and antimatter in the universe? To answer these questions, an important approach is to create new antimatter in the laboratory and study its properties.

However, antimatter is extremely rare, and the production of antimatter nuclei and antimatter hypernuclei (which contain hyperons) formed from several antinucleons is even more challenging. Previously, scientists had only discovered six types of antimatter nuclei and hypernuclei.

Antihydrogen-4 was observed by the research team during collision experiments at the Relativistic Heavy Ion Collider in the United States, consisting of one antiproton, two antineutrons, and one anti-Lambda hyperon. Due to the presence of the unstable anti-Lambda hyperon, antihydrogen-4 decays after traveling just a few centimeters. The team analyzed data from approximately 6.6 billion heavy ion collision events and ultimately detected around 16 signals of antihydrogen-4.

The team also measured the lifetime of antihydrogen-4 and compared it with that of its corresponding matter counterpart, hydrogen-4. Within the measurement precision, there was no significant difference in the lifetimes of the two, further confirming the symmetry in the properties of matter and antimatter.