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| Dark matter could produce antimatter with high enough energies to cross the galaxy sakkmesterke/Alamy |
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.
Reference:
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
Tags:
Physics