The research, carried out by the STAR Collaboration at RHIC, has been published in the journal *Nature*. The findings contribute to ongoing efforts to understand the subtle differences between matter and antimatter, particularly why the universe is composed predominantly of matter.
"Our physics knowledge about matter and antimatter is that, except for having opposite electric charges, antimatter has the same properties as matter - same mass, same lifetime before decaying, and same interactions," explained STAR collaborator Junlin Wu, a graduate student at Lanzhou University and the Institute of Modern Physics in China. Despite the assumption that equal amounts of matter and antimatter were created during the Big Bang, the universe today is almost entirely made of matter. "Why our universe is dominated by matter is still a question, and we don't know the full answer," Wu added.
RHIC, operated by the U.S. Department of Energy's Brookhaven National Laboratory, is an ideal environment for such investigations. Collisions between accelerated heavy ions at nearly the speed of light dissolve the boundaries of protons and neutrons, creating a high-energy environment that mimics the early universe. This process results in the production of both matter and antimatter in nearly equal proportions. Studying these particles could shed light on the asymmetry that led to the matter-dominated universe.
"To study the matter-antimatter asymmetry, the first step is to discover new antimatter particles," said STAR physicist Hao Qiu, Wu's advisor at IMP. The recent discovery follows a series of significant findings in antimatter research at RHIC, including the detection of antihypertriton in 2010 and antihelium-4 in 2011. The new antihyperhydrogen-4 antinucleus sets a new record as the heaviest antimatter nucleus identified.
Detecting such a rare and unstable antinucleus was a challenging task. According to Brookhaven Lab physicist Lijuan Ruan, a co-spokesperson for the STAR Collaboration, "It is only by chance that you have these four constituent particles emerge from the RHIC collisions close enough together that they can combine to form this antihypernucleus." The STAR team focused on identifying the decay products of antihyperhydrogen-4, which included an antihelium-4 nucleus and a pion (pi+), the latter being a simple positively charged particle.
By retracing the trajectories of these particles, the team was able to identify the decay events of the antihypernucleus, a task made difficult by the abundance of pions produced in RHIC collisions. "The key was to find the ones where the two particle tracks have a crossing point, or decay vertex, with particular characteristics," said Ruan. After rigorous analysis, the team identified 22 potential events, with an estimated 16 likely to be genuine antihyperhydrogen-4 nuclei.
The STAR team's discovery allowed them to compare the lifetimes of antihyperhydrogen-4 with its ordinary matter counterpart, hyperhydrogen-4, as well as the antihypertriton and hypertriton pair. No significant differences were observed, a result that did not surprise the scientists. "If we were to see a violation of [this particular] symmetry, basically we'd have to throw a lot of what we know about physics out the window," noted Emilie Duckworth, a doctoral student at Kent State University who contributed to the study.
The findings support the existing models of physics, and the researchers see this as a positive step in the experimental study of antimatter. The next phase of research will involve measuring the mass difference between matter and antimatter particles, a project Duckworth is currently pursuing with support from the DOE Office of Science Graduate Student Research program.
Research Report:Observation of the Antimatter Hypernucleus 4/(anti)?(anti)H"
Karen McNulty Walsh [email protected] [email protected]
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