Two papers published this week showcase the perplexing origins and potential uses of antimatter, a type of matter that flips the rules governing ordinary matter onto their heads.
One paper, published today in JCAP, found that antinuclei from cosmic rays may be an indicator of a specific kind of a dark matter. In a separate paper, published earlier this week in AIP Advances, researchers describe a method of detecting nuclear reactors’ locations and activity using antineutrinos produced by the facilities’ nuclear reactions.
Antimatter is important because it may help to explain fundamental cosmic mysteries, like why the universe is made of matter instead of an equal mix of matter and antimatter. These studies fit into a larger effort to crack some of physics’ biggest puzzles, including the nature of dark matter, physics at the smallest scales, and possibly even the origin of the universe itself.
Despite its name, antimatter is literally matter. It has mass. Antimatter refers to a group of particles that have opposite electrical charges to their ordinary counterparts. You’ve heard of electrons (which have a negative charge) and protons (with a positive charge); their antimatter counterparts are positrons (with a positive charge) and an antiproton (a negative charge).
Though there are differences in the charge of the particles, antimatter isn’t entirely alien to the fundamental forces. Last year, a team of physicists found that antimatter reacts to gravity the same way as ordinary matter, a finding that affirmed both Einstein and the Standard Model of Particle Physics.
Something more similar to the idea of “antimatter” you may have in your head is dark matter—which also has mass—but is invisible to every kind of detector humankind has so far devised. Scientists know dark matter exists because its gravitational effects are visible, even though the particle (or particles!) responsible cannot be directly observed.
Antimatter remains a matter of confusion (sorry, awful pun) for a few reasons. As explained by Gizmodo in 2022:
The universe rocked into being 14 billion years ago, with a Big Bang that in theory should have created equal amounts of matter and antimatter. But look around you, or at the latest Webb telescope images: We live in a universe dominated by matter. An outstanding question in physics is what happened to all the antimatter.
Antimatter and dark matter dovetail neatly in the recent JCAP paper, which posits that the amount of antimatter detected by experiments is more than there should be—and they believe dark matter is the culprit.
A few different particles (and other, more exotic objects) have been posited as responsible for dark matter. Among them: axions, a particle named for a laundry detergent; MAssive Compact Halo Objects, or MACHOs; dark photons, which despite their name are more like axions than some insidious version of light; and primordial black holes, which would be minuscule black holes birthed at the beginning of the universe, floating through space.
The recent research focuses on another type—Weakly Interacting Massive Particles, or WIMPs—as the guilty party. The theory is essentially that when WIMPs collide, they sometimes annihilate—destroy one another—emitting energy and particles of matter and antimatter.
In the aforementioned 2022 research, a team of physicists using the ALICE experiment at CERN found that antimatter could travel through our galaxy with ease instead of being snuffed out by the matter in the interstellar medium, a redeeming conclusion for antinuclei detectors like the AMS-02 experiment aboard the International Space Station.
“Theoretical predictions suggested that, even though cosmic rays can produce antiparticles through interactions with gas in the interstellar medium, the amount of antinuclei, especially antihelium, should be extremely low,” said Pedro De la Torre Luque, a physicist at the Institute of Theoretical Physicists in Madrid and lead author of the JCAP paper, in a SISSA Medialab release.
“We expected to detect one antihelium event every few tens of years, but the around ten antihelium events observed by AMS-02 are many orders of magnitude higher than the predictions based on standard cosmic-ray interactions,” De la Torre Luque added. “That’s why these antinuclei are a plausible clue to WIMP annihilation.”
However, De la Torre Luque added that WIMPs could only explain the amount of antihelium-3—one antimatter isotope detected by AMS-02—and not detected amounts of the rarer, heavier antihelium-4. In other words, even if WIMPs are responsible for dark matter, they don’t tell the whole story.
WIMPs could be responsible for the antimatter detections that space-based detectors are collecting. But regardless of the dark matter question—one that will take a long time to answer—the design of an antimatter-sniffing detector to monitor nuclear reactors on Earth shows practical applications in the here and now. Together, these findings on antimatter could offer new ways to harness the strange properties of the universe for practical use, while also helping us better understand both the cosmos and our own planet.
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