This Could Be the Closest We’ve Ever Been to Detecting Dark Matter
Scientists just announced that they have detected a completely unexpected number of antihelium particles—something that could mean they were generated by dark matter.
Specifically, they could have been created by decaying WIMPs, which are one of our best guesses as to what dark matter actually is.
On top of that, some of the detections point to the presence of a type of antihelium we were never supposed to detect, which could mean entirely new physics is needed.
Everything we’ve ever known or experienced as a species, everything we could ever hope to touch or see or hear, makes up about 5 percent of the universe. 5 percent. The other 95 percent is what experts call “dark,” basically meaning that we can’t interact with it directly. About 68 percent of the universe is made up of dark energy, and final 27 percent is made up of dark matter.
Dark matter, while making up the smaller percentage of the pie, is probably the more famous of the “dark” categories. And it’s not hard to understand why people are so taken with the concept. Floating out in space is a mess of particles that we can’t see or touch, but that can nonetheless bend literal spacetime with their gravity.
So, despite the fact that it has eluded us for decades, we search on for direct observations of this mysterious matter. And according to a new paper, published today in the Journal of Cosmology and Astroparticle Physics, we may have just taken a huge step towards getting there.
See, there’s an experiment running on the International Space Station (ISS) called the Alpha Magnetic Spectrometer (AMS-02). It’s job, according to a NASA website, is basically to collect and analyze cosmic rays, which can come from a wide variety of sources. Sometimes, when it analyzes those cosmic rays, it comes to the conclusion that the particles making them up are, in fact, particles of antimatter.
Antimatter, while it sounds incredibly sci-fi, is much better understood than dark matter (though, we still have a long way to go before we understand it completely). Often originating from the collisions and decay of other particles, antimatter particles are basically oppositely charged versions of the particles we know and love—positrons are positively charged electrons, for instance.
It’s not weird for AMS-02 to have identified antimatter particles. It does that pretty frequently. What is weird though is the fact that it detected antihelium—basically an opposite helium particle. The team expected to get about one detection of antihelium every few decades or so. Instead, AMS-02 has already made about 10 antihelium detections since its activation in 2011.
That might not sound like much, but it’s an orders-of-magnitude jump from what was expected. “If you see the production of antiparticles in the interstellar medium, where you expect very little, it means something unusual is happening,” Pedro De la Torre Luque, lead author on the study, said in a press release. “That’s why the observation of antihelium was so exciting.”
If we are lucky—very, very lucky—that “something unusual” is WIMPs.
WIMP is the exceedingly charming name for one of our best attempts at guessing what dark matter is actually made of. It stands for Weakly Interacting Massive Particles, and, in theory, these particles only interact with the universe through gravity and the weak nuclear force.
They check all the boxes we need to explain dark matter, and the hunt has been on for these things for years now. But, so far, it’s been no dice on detections, and the possibilities for exactly what types of particles WIMPs could be have dramatically dwindled. “Of the numerous best-motivated proposed models,” De la Torre Luque said in the press release, “most have been ruled out today and only a few of them survive today.”
So, what do a few random, too-frequent detections of antihelium have to do with trying to spot a theoretical particle that seems to be rapidly on its way to being ruled out? Well, remember how a lot of antimatter is formed from the collisions and decay of other particles? According to the scientists behind this new paper, there’s a solid chance that WIMP decay would be the best explanation for this way overblown presence of antihelium.
“Taken in whole,” the authors wrote in their study, “these results demonstrate that (standard) astrophysical mechanisms are highly unlikely to be capable of explaining the preliminary AMS-02 signal of a few antihelium events. If this signal is confirmed, production mechanisms stemming from dark mater annihilation […] remain the most reasonable explanation.”
And that’s not even the wildest part. Because there’s a chance that not all of the antihelium detected is the same flavor.
Particles come in a huge number of varieties, each often labeled with a number. Carbon-12, for example, has 12 total subatomic particles in its nucleus. But carbon also comes in varieties ranging from carbon-8 to carbon-22.
Now, that’s not to say all the flavors are created equal. carbon-12 is the most common carbon atom on Earth by a long shot, making up 99 percent of all the carbon we have. The next most common, carbon-13, only makes up 1 percent of our carbon, and you only get about one carbon-14 atom for every trillion carbon atoms of any stripe on our planet.
Antimatter works much the same way, and the antihelium detections made by AMS-02 came in two flavors: antihelium-3 and antihelium-4. Antihelium-3 was the flavor we were only supposed to see once every few decades. Antihelium-4... we weren’t supposed to see antihelium-4 at all. In fact, while WIMPs could explain the presence of the antihelium-3, even they can’t explain the antihelium-4.
Exciting. Tremendously so. But the experts behind this paper are cautious. Understandably so—if dark matter is your easiest, least complicated explanation, you better be ready to back your work up to the ends of the Earth and then some. Especially considering that WIMPs aren’t the only proposed explanation here. Various teams of scientists have put forth their own theories about what could have caused both the antihelium-3 and antihelium-4 detections.
And the detections themselves still need to be confirmed. Just seeing something once is exciting, but we need to make several of these waves of antihelium detection before anybody goes crying “eureka!”
So, caution it is—with a side of optimism, of course. This is a real “final frontier” kind of scientific experiment, and any kind of potentially promising result warrants further investigation. But if future confirmation studies back up the findings of this paper, well, we might have some seriously new science on our hands.
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