Scientists examining the particle trails from six billion atomic nucleus collisions at the Relativistic Heavy Ion Collider (RHIC) — a powerful machine that simulates the conditions of the early universe — have identified a new form of antimatter nucleus, marking the heaviest ever found. This nucleus, known as antihyperhydrogen-4, is made up of four antimatter components: one antiproton, two antineutrons, and one antihyperon.
Scientists examining particle trails from six billion collisions of atomic nuclei at the Relativistic Heavy Ion Collider (RHIC) — a powerful “atom smasher” that mimics early universe conditions — have identified a new type of antimatter nucleus, the heaviest ever discovered. This exotic structure, named antihyperhydrogen-4, consists of four antimatter particles: one antiproton, two antineutrons, and one antihyperon.
Researchers from RHIC’s STAR Collaboration made this groundbreaking discovery using their large particle detector to scrutinize the remnants of the collisions. They presented their findings in the journal Nature and explained how these unique antiparticles are being used to investigate differences between matter and antimatter.
“Our understanding of matter and antimatter tells us that aside from having opposite electric charges, antimatter shares the same properties as matter — identical mass, decay lifetime, and interactions,” stated STAR collaborator Junlin Wu, a graduate student affiliated with both Lanzhou University and the Institute of Modern Physics in China. However, the universe is predominantly composed of matter, despite the belief that both matter and antimatter were produced in equal amounts during the Big Bang approximately 14 billion years ago.
“The reason behind the prevalence of matter in our universe remains a mystery, and we lack a comprehensive explanation,” Wu added.
RHIC, operated by the U.S. Department of Energy (DOE) Office of Science at Brookhaven National Laboratory, is an ideal environment for studying antimatter. By colliding heavy ions — atomic nuclei stripped of electrons and propelled near light speed — the distinct boundaries of protons and neutrons merge. The energy created in this quark-gluon soup, which constitutes the fundamental components of observable matter, results in the generation of thousands of new particles. Similarly to the conditions found in the early universe, RHIC produces matter and antimatter almost equally. Analyzing the properties of the matter and antimatter generated in these collisions may help unravel the asymmetry that led to a predominance of matter in our current universe.
Detecting heavy antimatter
“The initial step in examining the matter-antimatter imbalance involves identifying new antimatter particles,” said STAR physicist Hao Qiu, Wu’s advisor at IMP. “This reasoning drives our research.”
Previously, STAR physicists had documented antimatter nuclei formed during RHIC collisions. In 2010, they discovered the antihypertriton, representing the first detection of an antimatter nucleus that included a hyperon—a particle containing at least one “strange” quark—as opposed to the lighter “up” and “down” quarks found in usual protons and neutrons. The following year, STAR physicists surpassed that record with the discovery of antihelium-4, the antimatter counterpart to the helium nucleus.
Recent analyses indicated that antihyperhydrogen-4 was also within reach. However, capturing this unstable antihypernucleus, which would surpass the heavyweight record holder by including an antilambda particle in place of one of the protons in antihelium, is a rarity. All four components—one antiproton, two antineutrons, and one antilambda—must emerge from the quark-gluon soup produced in RHIC collisions in a specific way, converging at the right time and direction to form a transient bound state.
“It’s purely coincidental that these four constituent particles emerge from the RHIC collisions close enough to one another to combine into this antihypernucleus,” explained Brookhaven Lab physicist Lijuan Ruan, a co-spokesperson for the STAR Collaboration.
Needle in a “pi” stack
To identify antihyperhydrogen-4, the STAR scientists examined the paths of particles resulting from the decay of this unstable antihypernucleus. Two of the decay products include the previously detected antihelium-4 nucleus and a positively charged particle known as a pion (pi+).
“Given that antihelium-4 was already discovered in STAR, we utilized the same method from before to track those events and then re-analyzed them with pi+ tracks to locate these particles,” Wu expressed.
Reconstruction here refers to retracing the paths of both the antihelium-4 and pi+ particles to determine if they originated from a common point. Identifying antihypernuclei is challenging due to the abundant production of pions in RHIC collisions, which necessitates a meticulous search through billions of collision events! Each antihelium-4 could be paired with numerous pi+ particles—potentially hundreds or even up to a thousand.
“The crucial aspect was finding instances where the two particle trajectories intersect at a specific point, or decay vertex,” noted Ruan. This decay vertex needs to be distant enough from the original collision point to suggest that the two particles originated from the decay of an antihypernucleus formed shortly after the collision from particles initially created during the energy release.
The STAR team invested considerable effort to eliminate background noise from all other possible decay particle combinations. Ultimately, their analysis identified 22 candidate events with an estimated background count of 6.4.
“That implies that around six of the events that resemble antihyperhydrogen-4 decays could simply be random occurrences,” said Emilie Duckworth, a doctoral student at Kent State University responsible for ensuring proper coding of the software used to process and identify the signals from these events.
Subtracting that backdrop from the 22 events gives the physicists confidence that they have observed approximately 16 actual antihyperhydrogen-4 nuclei.
Matter-antimatter comparison
This result was significant enough for the STAR team to conduct direct comparisons between matter and antimatter.
They compared the lifetimes of antihyperhydrogen-4 with its ordinary-matter counterpart, hyperhydrogen-4. They also assessed the lifetimes of another matter-antimatter pair: the antihypertriton versus the hypertriton.
Neither comparison revealed a notable difference, which did not astonish the researchers.
The experiments served as a test of a particularly strong symmetry principle. Physicists broadly agree that a breaking of this symmetry would be exceedingly rare and unlikely to provide answers regarding the matter-antimatter imbalance in the universe.
“If we observed a breach of this specific symmetry, it would essentially mean reevaluating much of our understanding of physics,” Duckworth remarked.
Thus, confirming that this symmetry still holds was somewhat reassuring. The team concluded that the results reinforce the accuracy of physicists’ models, representing a significant advancement in antimatter experimental research.
The next stage will involve evaluating the mass discrepancies between particles and antiparticles, a pursuit being led by Duckworth, who received funding from the DOE Office of Science Graduate Student Research program in 2022.
This research was supported by the DOE Office of Science, the U.S. National Science Foundation, and various international agencies and organizations detailed in the scientific paper. The researchers utilized computational resources from the Scientific Data and Computing Center at Brookhaven Lab, alongside the National Energy Research Scientific Computing Center (NERSC) located at DOE’s Lawrence Berkeley National Laboratory and the Open Science Grid consortium. NERSC is also a user facility under the DOE Office of Science.
