Getting under the skin of atomic nuclei using antimatter
The AEgIS Collaboration presents a new method to probe nuclei
Researchers at the AEgIS experiment have piloted a new method to delve further into the heart of atoms. This proof-of-principle study, reported today in Physical Review Research, shows how antiprotons – the antimatter counterparts of protons – could be used to probe the outer edges of a wider range of atomic nuclei. This technique marks a major step towards uncovering what happens to the nucleus when an antiproton annihilates at its periphery. And with CERN’s recent world-first demonstration of antiproton transportation, this technique could become accessible to laboratories across Europe.
Understanding how protons and neutrons arrange themselves within the nucleus may give a deeper insight into the strong nuclear force that holds these particles together. It could also shed light on the structure of neutron stars – the dense astronomical objects left after a massive star explodes as a supernova. The interior of a neutron star is not, as the name suggests, just neutrons but is still somewhat of a mystery and some researchers think they could be a useful testing ground for fundamental physics.
Previous experiments investigating nuclear structure have found that in many nuclei there are more neutrons around the periphery of the nucleus, a feature which is known as a neutron skin. Measuring the neutron skin is very challenging given its scale – a fraction of a femtometre, which is a quadrillionth of a metre. Researchers have therefore had to come up with innovative techniques to study this phenomenon.
However, recent measurements of neutron skins have provided results that differ so much that they cannot be accurately explained by theory. This has led researchers to search for new ways of measuring the neutron skin.
A particularly sensitive probe is the antiproton. When it approaches a nucleus, the antiproton may annihilate with one of the protons or neutrons at the edge of the nucleus. In most cases, the energy released as a result of this annihilation is enough to tear the nucleus apart, yet roughly 10–20% of the nuclei may avoid the fallout from the annihilation, in what is called a ‘cold’ annihilation event.
Researchers can identify and study these nuclear remnants that are left behind to determine if a proton or neutron was lost to the cold annihilation. The ratio of protons and neutrons lost provides an indication of the neutron–to–proton ratio at the annihilation site. Additionally, x-ray measurements indicate how far out from the centre of the nucleus the antiproton was before its annihilation. By combining all this data, researchers can determine a value for the neutron skin thickness.
Previous experiments have identified the nuclear remnant that are left after the annihilation by measuring their radioactive decay. However, these methods left a significant gap in the nuclei whose neutron skins could be investigated, as many nuclei do not undergo radioactive decay after the antiproton annihilation.
To address this issue, the AEgIS experiment presents a novel approach that marks the first step towards capturing and studying the nuclear remnants produced following the cold antiproton annihilations. This allows researchers to perform detailed studies of the annihilation process and lays the groundwork for the future identification of the nuclear remnants, including the previously invisible non-radioactive ones, via time-of-flight spectrometry. Such studies could provide new insight into the outer edge of the nucleus that would complement other methods, such as that used at CERN’s PUMA experiment. This approach is only now possible thanks to the technical advancements at CERN’s Antimatter Factory, where millions of antiprotons can be produced and held in sophisticated antimatter traps in a matter of minutes.
In this recent proof-of-principle study, the AEgIS Collaboration used argon and helium as test subjects for their study. The researchers successfully demonstrated their method by capturing the highly charged ions produced from the antiproton annihilation inside the antimatter trap and identifying them via time-of-flight spectrometry.
“Our study establishes a foundation for capturing and directly studying the nuclear remnants left behind after cold antiproton annihilations for a wider range of nuclei” said Fredrik Parnefjord Gustafsson, the lead author of the experiment. "We hope this new method, combined with portable antimatter traps, could allow laboratories, big and small, to use antiprotons as a tool for nuclear research."
Author: Rory Harris
More information:
‘Spectrometry of captured highly charged ions produced following antiproton annihilations’, F. P. Gustafsson, et al. (AEgIS collaboration), Phys. Rev. Research 8, 023202 – Published 22 May, 2026
DOI: https://doi.org/10.1103/gg1j-vtvy
* Image copyright AEgIS collaboration