Physicists from the STAR Collaboration have observed the antimatter hypernucleus antihyperhydrogen-4 — composed of an antihyperon, an antiproton and two antineutrons — in collisions of atomic nuclei at the Relativistic Heavy Ion Collider (RHIC) at DOE’s Brookhaven National Laboratory.
An artistic representation of antihyperhydrogen-4 created in a collision of two gold nuclei. Image credit: Institute of Modern Physics.
“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,” said Junlin Wu, a graduate student at Lanzhou University and China’s Institute of Modern Physics.
“But the reality is that our Universe is made of matter rather than antimatter, even though both are believed to have been created in equal amounts at the time of the Big Bang some 14 billion years ago.”
“Why our Universe is dominated by matter is still a question, and we don’t know the full answer.”
“To study the matter-antimatter asymmetry, the first step is to discover new antimatter particles. That’s the basic logic behind this study,” added Dr. Hao Qiu, a researcher at the Institute of Modern Physics.
STAR physicists had previously observed nuclei made of antimatter created in RHIC collisions.
In 2010, they detected the antihypertriton; this was the first instance of an antimatter nucleus containing a hyperon, which is a particle containing at least one strange quark rather than just the lighter up and down quarks that make up ordinary protons and neutrons.
Then, just a year later, STAR physicists toppled that heavyweight antimatter record by detecting the antimatter equivalent of the helium nucleus: antihelium-4.
A more recent analysis suggested that antihyperhydrogen-4 might also be within reach.
But detecting this unstable antihypernucleus would be a rare event. It would require all four components — one antiproton, two antineutrons, and one antilambda — to be emitted from the quark-gluon soup generated in RHIC collisions in just the right place, headed in the same direction, and at the right time to clump together into a temporarily bound state.
“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,” said Brookhaven Lab physicist Lijuan Ruan, one of two co-spokespersons for the STAR Collaboration.
To find antihyperhydrogen-4, the STAR physicists looked at the tracks of the particles this unstable antihypernucleus decays into.
One of those decay products is the previously detected antihelium-4 nucleus; the other is a simple positively charged particle called a pion (pi+).
“Since antihelium-4 was already discovered in STAR, we used the same method used previously to pick up those events and then reconstructed them with pi+ tracks to find these particles,” Wu said.
“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,” said Dr. Lijuan Ruan, a researcher at Brookhaven National Laboratory.
RHIC smashups produce a lot of pions. And to find the rare antihypernuclei, the physicists were sifting through billions of collision events.
Each antihelium-4 emerging from a collision could be paired with hundreds or even 1,000 pi+ particles.
“The key was to find the ones where the two particle tracks have a crossing point, or decay vertex, with particular characteristics,” Dr. Ruan said.
“That is, the decay vertex has to be far enough from the collision point that the two particles could have originated from the decay of an antihypernucleus formed just after the collision from particles initially generated in the fireball.”
The STAR researchers worked hard to rule out the background of all the other potential decay pair partners.
In the end, their analysis turned up 22 candidate events with an estimated background count of 6.4.
“That means around six of the ones that look like decays from antihyperhydrogen-4 may just be random noise,” said Emilie Duckworth, a doctoral student at Kent State University.
Subtracting that background from 22 gives the physicists confidence they’ve detected about 16 actual antihyperhydrogen-4 nuclei.
The result was significant enough for the scientists to do some direct matter-antimatter comparisons.
They compared the lifetime of antihyperhydrogen-4 with that of hyperhydrogen-4, which is made of the ordinary-matter varieties of the same building blocks.
They also compared lifetimes for another matter-antimatter pair: the antihypertriton and the hypertriton.
Neither showed a significant difference, which did not surprise the authors.
“The experiments were a test of a particularly strong form of symmetry,” they said.
“Physicists generally agree that a violation of this symmetry would be extremely rare and will not hold the answer to the matter-antimatter imbalance in the Universe.”
“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,” Duckworth said.
“So, in this case, it was sort of comforting that the symmetry still works.”
“We agreed the results further confirmed that our models are correct and are a great step forward in the experimental research on antimatter.”
The team’s work appears in the journal Nature.
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STAR Collaboration. Observation of the antimatter hypernucleus antihyperhydrogen-4. Nature, published online August 21, 2024; doi: 10.1038/s41586-024-07823-0
This article was adapted from an original release by Brookhaven National Laboratory.
