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Wednesday, December 6, 2023

Antihelium Offers Hope in the Search for Dark Matter

In 2010, physicists at the Large Hadron Collider began producing an exotic form of antimatter known as antihelium. Antimatter is that elusive substance that annihilates upon meeting regular matter, and antihelium is the antimatter twin of the classic helium atom, the stuff you find in party balloons. While no human has ever conclusively found a naturally occurring antihelium particle on Earth, it could be key to answering one of the biggest outstanding mysteries in physics: the nature of dark matter.

While this beast may be rare on Earth, physicists think it could be abundant in our galaxy, according to physicist Ivan Vorobyev, a researcher at CERN. That's because they think antihelium could form in the decay of dark matter, an invisible substance that seems to make up 85 percent of the universe’s matter. On Monday, Vorobyev’s team announced that they’d generated about 18,000 antihelium nuclei—and more notably, that they used their result to calculate the odds that Earth-based detectors could capture antihelium drifting in from space, where it might signify the presence of dark matter. 

Between 2016 and 2018, Vorobyev’s team had smashed more than a billion particles in the LHC’s 16-mile ring, based in Geneva. They performed two types of particle collisions: protons with protons, and lead ions with lead ions, which break apart to reform a myriad of new particles, such as pions, kaons, and more protons. Recording the wreckage required petabytes—that’s thousands of portable hard drives—of data. Then, they began to sift through it. “We filtered out only the part that is interesting to us,” says Vorobyev, a member of the ALICE collaboration, which conducted the project. (The acronym stands for A Large Ion Collider Experiment.) 

Specifically, Vorobyev’s team zeroed in on a version of the antiparticle known as antihelium-3, composed of two antiprotons and one antineutron. Vorobyev’s team isn’t the first to create antihelium-3: Scientists observed the antiparticle for the first time in 1970 by producing it in a collider. Still, nobody has ever conclusively captured it in nature. While antimatter forms naturally on our planet, it usually consists of lightweight particles such as positrons, the antimatter counterpart of electrons, which are thousands of times less massive than antihelium. But antihelium-3 is relatively heavy, and the heavier the antimatter particle, the more rarely it will be produced. “If you collide heavy ions, each additional nucleon will cost you about a factor of 300 or 400,” says Vorobyev. “That means every next nucleus will be produced with a factor 350 less than the previous one.”

Although physicists have inferred the presence of dark matter through its gravitational influence on the rotation of galaxies, they still don’t know what it is made of. Hypotheses include objects as heavy as black holes and as lightweight as 100 millionths of an electron’s mass. Two decades ago, physicists first proposed that certain dark matter particles—known as Weakly Interacting Massive Particles, or WIMPs—could annihilate with anti-dark matter to produce matter and antimatter in equal amounts. If dark matter throws off antihelium as it annihilates, finding this antiparticle would be a clue that it truly exists. 

In theory, physicists searching for dark matter could actually hunt for either the matter or the antimatter it generates. “In many models, dark matter is its own antiparticle, or there's equal amounts of dark matter and anti-dark matter,” says physicist Tim Linden of Stockholm University in Sweden, who was not involved with the LHC experiment. “Either way, you tend to generate about as many anti-particles as particles from dark matter annihilation.” 

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However, stars and other astrophysical objects unrelated to dark matter also produce a lot of extraterrestrial matter particles, says Linden, which makes it difficult to identify their origin. “So we look for antimatter signatures, because astrophysical processes are bad at making them, and the background is smaller,” he says. In this sense, any detected antimatter particles from space are more likely to come from dark matter. 

The excitement about antimatter as a dark matter signature has grown because of a tantalizing signal astrophysicists announced in 2016. Researchers in charge of the Alpha Magnetic Spectrometer (AMS), an instrument on the International Space Station, told the community that they had probably detected eight antihelium nuclei. They have not formally published the result, and researchers still refer to the signal as “tentative,” but “it’s inspired this effort to figure out—if that signal was true—how could it have come here?” says Linden.

The LHC’s experiment and analysis are significant because they have bolstered the field’s confidence in detecting antihelium from space as a strategy for finding dark matter. After producing the nuclei in their detector, Vorobyev’s team analyzed how likely the antihelium would be to break apart or annihilate with regular matter as it moved through the machine. They used these findings to simulate a model of the Milky Way to estimate how likely it was for antihelium nuclei, originating up to tens of thousands of light years away, to reach Earth. Space is quite empty, but as the antihelium travels through the galaxy toward our planet, these nuclei still have some likelihood of colliding with clouds of gas and breaking apart. 

The results are promising: “We have seen that half of them will survive the trip to the detectors near Earth,” says Vorobyev. And that is a good sign that physicists’ antimatter detectors will eventually catch a traveling antihelium particle. AMS, which detected the probable signals reported in 2016, is still looking. A new instrument, called the General Antiparticle Spectrometer, is scheduled to launch in a balloon into the Antarctic atmosphere in late 2023, where it will look for antihelium along with other particles at an altitude of 25 miles. 

This new work illustrates how convoluted and uncertain the scientific process can be. To tackle a question as big as dark matter, theorists have had to brainstorm how researchers might be able to detect it on Earth. Experimentalists have then had to run tests like Vorobyev’s to verify the theorists’ ideas. Astrophysicists have had to build the instruments to look for antimatter signals. Now, the threads are coming together, at least for antihelium-based dark matter searches. “It's a really good melding of communities to try to come up with answers to these really difficult problems,” says Linden.

But these communities still have a lot of work ahead. For theorists like Linden, they are still figuring out the details of how dark matter might generate antihelium in the first place. Astrophysicists have to watch for antihelium signals from space, and if they see any, they’d have to check that the antiparticles are consistent with theorists’ predictions about dark matter. The ALICE experiment lays the groundwork for a new approach to solve the mystery of dark matter—but physicists still have a lot of the rabbit hole left to explore.Update 12-14-2022 12:27 pm ET: This story was updated to correct the timeframe during which physicists at LHC began producing antihelium.

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