Tuesday, December 20, 2022

Antimatter particles could cross the galaxy without being destroyed

Antimatter from far away should be tricky to find. It annihilates when it meets regular matter – and the more space it crosses, the more chances there are for these particles to meet their end. But an experiment at a particle collider suggests that some antimatter particles can travel across our galaxy without getting destroyed.

In space, the antimatter version of helium atoms’ nuclei – the antihelium nuclei – are thought to form when cosmic rays collide with free-floating atoms. Theories suggest they also arise when particles of dark matter, a mysterious substance that fills most of the universe, annihilate with each other. If antinuclei made in such annihilations were detected, they could reveal new properties of dark matter.

Stefan Königstorfer at the Technical University of Munich in Germany and his colleagues at the Large Hadron Collider (LHC) wanted to see whether antinuclei created in space could make it to detectors in Earth’s neighbourhood intact.

First, they measured how many antihelium nuclei get destroyed when they hit regular matter inside a particle collider. Using the ALICE detector at the CERN particle physics laboratory in Switzerland, they analysed collisions of very high-energy protons and charged atoms, which produced both helium nuclei and antihelium nuclei. Both should be produced in equal number, so the researchers counted how many nuclei survived to infer how many antinuclei annihilated against the steel, carbon and other materials that make up the ALICE (A Large Ion Collider Experiment) detector.

Königstorfer says they used this “disappearance probability” in a computer simulation of antimatter’s journey towards Earth from distant space, such as the centre of our galaxy. Simulations of antinuclei being created by dark matter showed that about half of such particles would be detectable near Earth unscathed, even after traversing thousands of trillions of kilometres.

The researchers also simulated antinuclei being created by cosmic rays, which are theorised to form at fewer places in the universe and typically with higher energies than those created by dark matter. They found that only the most energetic of them would reach Earth in high numbers.

This shows that any low-energy antihelium nuclei we detect on Earth will likely have come from dark matter, says Jonas Tjemsland at the Norwegian University of Science and Technology.

“This experiment says that if any astrophysical object for any reason produces antihelium, we can detect it near Earth with standard detectors. And the signal-to-noise ratio will be very high for dark matter,” says Tim Linden at Stockholm University in Sweden.

Understanding how antinuclei interact with interstellar matter is one part of the puzzle, but the LHC could also investigate how they form, says Stefano Profumo at the University of California, Santa Cruz. He says that understanding this better could help researchers fine-tune theories of dark matter.

Königstorfer and his colleagues are now planning such experiments. The Alpha Magnetic Spectrometer experiment at the International Space Station could detect antinuclei already, and another detector, General AntiParticle Spectrometer, carried by a balloon above Antarctica, will launch soon.

The ALICE Collaboration. Measurement of anti-3He nuclei absorption in matter and impact on their propagation in the Galaxy. Nat. Phys. (2022). https://doi.org/10.1038/s41567-022-01804-8

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