Physicists from the STAR Collaboration have observed an antimatter hypernucleus, antihyperhydrogen-4, consisting of an antihypernucleus, an antiproton, and two antineutrons, in nuclear collisions at the Relativistic Heavy Ion Collider (RHIC) at the U.S. Department of Energy's Brookhaven National Laboratory.

“What we know in physics about matter and antimatter is that, apart from the opposite charge, antimatter has the same properties as matter – the same mass, the same lifetime before decaying, and the same interactions,” said Junlin Wu, a graduate student at Lanzhou University and the China Institute of Modern Physics.

“But in reality, our universe is made up of antimatter rather than matter, even though equal amounts of matter and antimatter are thought to have been created during the Big Bang about 14 billion years ago.”

“Why our universe is populated with matter remains a question, and we don't yet have a complete answer.”

“The first step in studying the asymmetry between matter and antimatter is to discover new antimatter particles. This is the basic idea of ​​this research,” added Dr Hao Qiu, a researcher at the Institute of Modern Physics.

STAR physicists had previously observed atomic nuclei made of antimatter produced in RHIC collisions.

In 2010, they detected an antihypertriton, the first example of an antimatter nucleus containing a hyperon, a particle that contains at least one strange quark rather than just the light up and down quarks that make up ordinary protons and neutrons.

Just a year later, STAR physicists broke that massive antimatter record by detecting antihelium-4, the antimatter equivalent of a helium nucleus.

Recent analysis suggests that antihyperhydrogen 4 may also be feasible.

But detecting this unstable antihypernucleus is a rare event: all four components (one antiproton, two antineutrons and one antilambda) need to be ejected from the quark-gluon soup produced in the RHIC collision in just the right place, in the same direction and at just the right time, briefly becoming bound together.

“It's just a coincidence that these four component particles appear close enough together in the RHIC collision that they can combine to form an antihypernucleus,” said Brookhaven National Laboratory physicist Lijuan Luan, one of the STAR collaboration's co-spokespeople.

To find antihyperhydrogen-4, STAR physicists studied the trajectories of particles produced when this unstable antihypernucleus decays.

One of these decay products is the previously detected antihelium-4 nucleus, and the other is a simple positively charged particle called a pion (pi+).

“Antihelium-4 had already been discovered with STAR, so we used the same methods as before to pick up those events and reconstruct them with the π+ track to find these particles,” Wu said.

“It is simply by chance that these four component particles emerge from the RHIC collision close enough together to combine to form an antihypernucleus,” said Dr. Lijuan Luan, a research scientist at Brookhaven National Laboratory.

RHIC's collisions produce huge amounts of pions, and physicists have been sifting through billions of collision events to find the rare antihypernuclei.

The antihelium-4 produced by the collision can pair up with hundreds or even a thousand pi+ particles.

“The key was to find an intersection point where the trajectories of the two particles had a particular characteristic – a collapse vertex,” Dr. Luan said.

“That is, the collapse apex must be far enough away from the collision point that the two particles could have originated from the decay of an antihypernucleus that formed shortly after the collision of the particle originally produced in the fireball.”

STAR researchers worked hard to eliminate the background of all other potential collapse pair partners.

Ultimately, their analysis found 22 candidate events with an estimated background count of 6.4.

“That means that about six of what appear to be antihyperhydrogen-4 decays could just be random noise,” said Emily Duckworth, a doctoral student at Kent State University.

Subtracting that background count from the 22, physicists can be confident that they have detected about 16 actual antihyperhydrogen-4 nuclei.

The results were significant enough to allow scientists to make a direct comparison between matter and antimatter.

They compared the lifespan of antihyperhydrogen 4 to that of hyperhydrogen 4, which is made from normal matter variants of the same building blocks.

They also compared the lifetimes of another matter-antimatter pair, antihypertritons and hypertritons.

Neither difference was significant, but the authors were not surprised.

“This experiment tested a particularly strong form of symmetry,” the researchers said.

“Physicists generally agree that this symmetry breaking is extremely rare and is not an answer to the imbalance of matter and antimatter in the universe.”

“If we saw this particular breaking of symmetry, we would basically have to throw a lot of what we know about physics out the window,” Duckworth said.

“So in a way it was reassuring that symmetry still worked in this case.”

“We agree that this result provides further confirmation that our model is correct and marks a major step forward in the experimental study of antimatter.”

Team work Published in a journal Nature.

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STAR Collaboration. Observation of the antimatter hypernucleus antihyperhydrogen 4. NaturePublished online August 21, 2024, doi: 10.1038/s41586-024-07823-0

This article is based on an original release from Brookhaven National Laboratory.