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Researchers from the Institute of Modern Physics, Chinese Academy of Sciences and other institutions participated in the RHIC-STAR international collaborative experimental research.For the first time, a new antimatter hypernucleus, antihyperhydrogen-4, was observed in relativistic heavy ion gold-gold collisions. This is the heaviest antimatter hypernucleus ever discovered experimentally.On August 21, the relevant research results were published in Nature.

Current knowledge of physics holds that the properties of matter and antimatter are symmetrical, and that equal amounts of matter and antimatter should have existed at the beginning of the universe. Fortunately, some mysterious physical mechanism led to an extremely small asymmetry in the amount of matter and antimatter in the early universe. After most of the matter and antimatter were annihilated, about one in ten billion of matter survived, forming today's material world and becoming the basis for the birth and existence of human civilization. What causes the difference in the amount of matter and antimatter in the universe? To answer this question, an important idea is to create new antimatter in the laboratory and study its properties.

Antimatter is extremely rare, and antimatter nuclei and antimatter hypernuclei (i.e., nuclei containing hyperons such as Lambda) formed by further combining several antibaryons are even more difficult to produce. Since the "negative energy solution" of the Dirac equation in 1928 predicted the existence of antimatter,Scientists have only discovered six antimatter (hyper) nuclei in the past century。

The anti-superhydrogen-4 discovered this time was produced in the relativistic heavy ion collision experimentThe Relativistic Heavy Ion Collider at Brookhaven National Laboratory in the United States can accelerate heavy ion beams to nearly the speed of light and collide them, simulating the state of the Big Bang in the early universe in the laboratory. This collision can produce a trillion-degree high-temperature fireball containing almost equal amounts of matter and antimatter. The fireball expands and cools rapidly, giving some antimatter a chance to escape the fate of annihilation with matter and be observed by the STAR experimental detectors orbiting the collision point.

Antihyperhydrogen-4 is composed of an antiproton, two antineutrons and an antihyperon. Because it contains unstable anti-Lambda hyperons, antihyperhydrogen-4 decays after flying only a few centimeters.The study analyzed experimental data from about 6.6 billion heavy ion collision events, and found that the decay of antihelium-4 and π⁺The muon reverse reconstruction of anti-superhydrogen-4 obtained the signals of about 16 anti-superhydrogen-4。

▲Anti-helium-4 and π⁺Anti-superhydrogen-4 signal in the muon invariant mass spectrum

The team also measured the lifetime of anti-superhydrogen-4 and compared it with its corresponding positive particle superhydrogen-4. The study showed that there was no significant difference in the lifetimes of the two within the measurement accuracy range.Once again verified the symmetry of the properties of matter and antimatter。

Anti-hyperhydrogen-4 is the heaviest antimatter hypernucleus observed by scientists so farIts discovery and study of its properties have enabled scientists to take an important step forward in the exploration of antimatter and the symmetry of matter and antimatter.

▲Heavy ion collision produces antimatter superhydrogen-4

STAR is a large international experimental collaboration at RHIC, consisting of more than 700 researchers from 74 institutions in 14 countries.

The work was led by the team of Qiu Hao, a researcher at the Institute of Modern Physics. The research team of the University of Science and Technology of China made contributions in decay particle reconstruction technology and efficiency calculation. The research work was supported by the National Natural Science Foundation of China, the Strategic Priority Research Program of the Chinese Academy of Sciences, and the Chinese Academy of Sciences' Stable Support for Young Basic Research Teams Program.