Although normal matter dominates over antimatter in the universe, there are many physical processes that produce antimatter particles over and over again. Theoretically, one of these could be the interaction of dark matter particles – the antiparticles produced in the process would thus be an indication of their existence. But until now it was unclear whether such cosmic antiparticles could even penetrate to earth without being annihilated by contact with normal matter. Now, data from the ALICE experiment at CERN’s LHC particle accelerator show that our Milky Way would be transparent to antihelium nuclei: around 50 percent of them would reach us even over long distances.
Antimatter forms a kind of mirror image of normal matter: Although particles and their antiparticles are similar in terms of image and mirror image, they have opposite charges and spins. During the Big Bang, equal proportions of these forms of matter should actually have been created, but strangely enough, normal matter dominates over antimatter in the cosmos today – why has not yet been clarified. What is clear, however, is that antimatter is still formed in many physical processes, such as radioactive decay, collisions of high-energy particles from cosmic rays, and some lightning strikes in the Earth’s atmosphere. However, these antiparticles only survive as long as they do not come into contact with normal particles. If this happens, both particles cancel each other out in the annihilation with the release of energy. This behavior is relatively well researched from experiments with antimatter.
Dark matter as antimatter producer
So far, however, it has been unclear how long antiparticles can escape annihilation under cosmic conditions – for example, if they are formed several 10,000 light-years away from us and then fly through the Milky Way towards Earth. However, this is important for understanding processes in cosmic rays and also for clarifying the nature of dark matter. Although it shapes the appearance and behavior of entire galaxies, it is invisible and only interacts with normal matter via gravity. Among other things, it is still unknown which particles form the basis of dark matter. This is where the antiparticles come into play, because antimatter could also be created when certain dark matter particles are annihilated. “The observation of antinuclei such as antihelium-3 is one of the most promising signatures of the annihilation of dark matter Weakly Interacting Massive Particles (WIMP),” explain physicists from the ALICE collaboration at the CERN research center near Geneva. The WIMPs are considered a possible candidate for the long-sought dark matter particles.
In order to find out whether anti-helium nuclei from such hypothetical annihilations would reach us at all, the researchers evaluated data from the ALICE detector at CERN’s LHC particle accelerator. When protons and heavy lead nuclei collide with high energy in the accelerator, antihelium nuclei are formed in addition to many particles of normal matter. From the particle tracks recorded by the detector, it is possible to quantify the probability that an antihelium-3 nucleus will interact with the detector material and be annihilated. Physicists refer to this collision probability as the inelastic cross-section or as the transparency of a medium for these antiparticles. For their study, the physicists compared the number of antihelium nuclei of different energies produced in the collisions, known from earlier tests, with the number of particles still detected at different distances from the collision point. This allowed an estimation of the inelastic cross section of the antihelium nuclei at different energies.
Milky Way is 50 percent transparent to Antihelium
In the next step, the physicists fed the results obtained from the experiment into a simulation that depicts the environment of the Milky Way, including the matter, magnetic fields and other possible disruptive factors. This enabled them to determine how far the antihelium cores would fly under galactic conditions. In other words, how transparent the Milky Way is to antimatter particles of this type. The result: Our Milky Way is about 50 percent permeable to the antihelium nuclei that could arise from the interactions of the hypothetical dark matter particles. If these WIMPs and their interactions actually exist, around half of the antimatter particles they produce could reach Earth – even if they originated tens of thousands of light years away. “Our results show for the first time on the basis of a direct absorption experiment that even antihelium-3 nuclei from areas near the center of our galaxy can reach Earth,” says ALICE spokesman Luciano Musa from CERN.
For antihelium nuclei formed by cosmic rays, the transparency of the Milky Way varies from 25 to 90 percent, depending on the energy. Because these antihelium nuclei are usually much faster and therefore more energetic than those generated by dark matter, the two forms can be easily distinguished, as the team explains. “Our results thus demonstrate that the search for light antimatter cores from space can be a worthwhile avenue in the search for dark matter,” says Musa. The antimatter particles produced by dark matter can be captured, among other things, by the AMS-02 instrument already installed on the International Space Station ISS. From 2025, the GAPS balloon experiment over the Arctic will also examine the incoming cosmic rays for antihelium-3.
Source: The ALICE Collaboration (CERN, Geneva), Nature Physics, doi: 10.1038/s41567-022-01804-8