Another antimatter record has been broken. In the smash-up of very energetic lead ions, researchers have uncovered evidence of the heaviest antimatter version of an atomic nucleus ever seen.
In 2024, researchers from the STAR Collaboration at Brookhaven National Laboratory鈥檚 Relativistic Heavy Ion Collider (RHIC) in New York reported briefly creating a then unprecedentedly heavy antimatter nucleus called antihyperhydrogen-4. Many particles have antimatter equivalents that are identical but with opposite charges, and these antiparticles can combine into larger antimatter nuclei the same way normal particles form atoms.
Now, at Goethe University Frankfurt in Germany and his colleagues from the Large Hadron Collider (LHC) at the CERN particle physics laboratory near Geneva, Switzerland, have upped the ante by creating an even heavier antimatter nucleus: antihyperhelium-4.
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In the past, the US-based STAR detector had always been quicker to find record-breaking antimatter particles. 鈥淓very time they [the LHC team] went for something鈥� these guys from [STAR], they always took it. Scooped!鈥� says at the Frankfurt Institute for Advanced Studies in Germany. 鈥淣ow, it鈥檚 the first time, a very, very first time, that there is something which the STAR team have not yet seen, but it鈥檚 seen at CERN.鈥�
鈥淎ll antimatter discoveries are something very interesting, and this is one that we were missing,鈥� says D枚nigus. His team used machine learning to analyse data from a 2018 experiment with the ALICE detector at the LHC to identify the antihyperhelium-4 with a significance of 3.5 standard deviations. While it does not rise to the 鈥済old standard鈥� of 5 standard deviations, this still indicates a high likelihood that the discovery is genuine and can鈥檛 be explained as a mere quirk of the data.
Antihyperhelium-4 comprises a mix of antimatter versions of聽protons, neutrons and particles called hyperons that are in themselves exotic because they contain one or more quarks of the strange type. This 鈥渟trangeness鈥� is hard to find and hard to make. Consequently, researchers still do not fully understand how hyperons behave in nature, where they are thought to occur in exotic settings such as the interiors of neutron stars, says D枚nigus. Additionally, questions remain about how antimatter versions of these, and other, particles interact with each other.
鈥淥nly two antimatter hypernuclei have been discovered and both in the last 15 years,鈥� says at Kent State University in Ohio. 鈥淎LICE [now] provides the evidence of the third.鈥�
St枚cker says that antihyperhelium-4 is also meaningful because the conditions within the collider that made it temporarily replicate the state of the universe just a millionth of a second after the big bang. This state is a 鈥渉ot soup鈥� of massless particles, and identifying the matter and antimatter particles that emerge from it could help pinpoint the details of how we came to live in a universe where the amount of matter, including the matter that forms our bodies, overshadows rare antimatter particles.
Going forward, the team wants to find even heavier antimatter particles as well as antimatter versions of exotic particles that have recently been discovered in other colliders, says D枚nigus.
Physical Review Letters
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