Our Galaxy is Partially Transparent to Light Antimatter Nuclei, New Study


Antimatter particles such as positrons and antiprotons abound in the cosmos. 

Much less common are light antinuclei, composed of antiprotons and antineutrons, which can be produced in our Galaxy via high-energy cosmic-ray collisions with the interstellar medium or could also originate from the annihilation of the still undiscovered dark-matter particles. 

On Earth, the only way to produce and study antinuclei with high precision is to create them at high-energy particle accelerators like CERN’s Large Hadron Collider (LHC). Though the properties of elementary antiparticles have been studied in detail, knowledge of the interaction of light antinuclei with matter is rather limited. 

In new research, physicists from the ALICE Collaboration at the LHC focused on the determination of the disappearance probability of antihelium-3 when it encounters matter particles and annihilates or disintegrates.

“To find out whether dark matter is the source behind any potential detections of light antinuclei from outer space, we need to determine the flux of light antinuclei that is expected to reach the near-Earth location of such experiments,” explained members of the ALICE Collaboration.

“This flux depends on features such as the exact type of antimatter source in our Galaxy and the rate at which it produces antinuclei, but also on the rate at which the antinuclei should later disappear through annihilation or absorption when they encounter normal matter on their journey to Earth.”

“The latter is where the new study from ALICE comes in.”

By investigating how antihelium-3 nuclei produced in collisions of heavy ions and of protons at the LHC interact with the ALICE detector, the physicists were able to measure, for the first time, the rate at which antihelium-3 nuclei disappear when they encounter normal matter.

In this analysis, the ALICE detector’s material serves as the normal matter with which the antinuclei interact.

Next, the researchers incorporated the obtained disappearance rate into a publicly available computer program called GALPROP, which simulates the propagation of cosmic particles, including antinuclei, in the Milky Way.

They considered two models of the flux of antihelium-3 nuclei expected near Earth after the nuclei’s journey from sources in the Galaxy.

One model assumes that the sources are cosmic-ray collisions with the interstellar medium, and the other describes them as hypothetical dark-matter particles called weakly interacting massive particles (WIMPs).

For each model, the authors then estimated the transparency of the Milky Way to antihelium-3 nuclei, that is, the Galaxy’s ability to let the nuclei through without being absorbed.

They did so by dividing the flux obtained with and without antinuclei disappearance.

For the dark-matter model, the team obtained a transparency of about 50%, whereas for the cosmic-ray model the transparency ranged from 25% to 90% depending on the energy of the antinucleus.

These transparency values show that antihelium-3 nuclei originating from dark matter or cosmic-ray collisions can travel long distances — of several kiloparsecs — in the Milky Way without being absorbed.

“Our results show, for the first time on the basis of a direct absorption measurement, that antihelium-3 nuclei coming from as far as the centre of our Galaxy can reach near-Earth locations,” said Dr. Andrea Dainese, a physicist at Italy’s National Institute for Nuclear Physics and ALICE physics coordinator.

“Our findings demonstrate that searches for light antimatter nuclei from outer space remain a powerful way to hunt for dark matter,” added Dr. Luciano Musa, spokesperson of the ALICE Collaboration.

“This is an excellent example of an interdisciplinary analysis that illustrates how measurements at particle accelerators can be directly linked with the study of cosmic rays in space,” said Professor Laura Fabbietti, a physicist at the Technical University of Munich and a member of the ALICE Collaboration.

“The results from the ALICE experiment are of great importance for the search for antimatter in space with the AMS-02 module (Alpha Magnetic Spectrometer) on the International Space Station (ISS).”

“Starting in 2025 the GAPS balloon experiment over the Arctic will also examine incoming cosmic rays for antihelium-3.”


The ALICE Collaboration. Measurement of anti-3He nuclei absorption in matter and impact on their propagation in the Galaxy. Nat. Phys, published online December 12, 2022; doi: 10.1038/s41567-022-01804-8