PHYSICISTS AT THE Large Hadron Collider began producing antihelium, an exotic form of antimatter, in 2010. Or we some call it Antihelium an Antimatter. Antimatter is the enigmatic substance that annihilates when it collides with regular matter. And antihelium is the antimatter twin of the classic helium atom, which is found in party balloons. While no human has ever conclusively discovered a naturally occurring antihelium particle on Earth. It could hold the key to solving one of physics’ most vexing mysteries: the nature of dark matter.
While this beast may be rare on Earth, physicists believe it may be common in our galaxy. This is because they believe antihelium could form during the decay of dark matter, an invisible substance that appears to account for 85 percent of the matter in the universe. On Monday, Vorobyev’s team announced that they had generated approximately 18,000 antihelium nuclei. And, more importantly, that they had used their findings to calculate the likelihood that Earth-based detectors could capture antihelium drifting in from space, where it could indicate the presence of dark matter.
Vorobyev’s team smashed over a billion particles in the LHC’s 16-mile ring in Geneva between 2016 and 2018. They collided protons with protons and lead ions with lead ions. It broke apart to form a slew of new particles such as pions, kaons, and more protons. Petabytes of data—the equivalent of thousands of portable hard drives—were required to record the wreckage. Then they started sorting through it. “We filtered out only what was interesting to us,” says a member of the ALICE collaboration.
Vorobyev’s team focused on an antiparticle known as antihelium-3, which is made up of two antiprotons and one antineutron. Vorobyev’s group is not the first to discover antihelium-3. In 1970, scientists observed the antiparticle for the first time by creating it in a collider. Nonetheless, no one has ever captured it conclusively in nature. While antimatter naturally forms on our planet. It is typically composed of lightweight particles such as positrons, the antimatter counterpart of electrons, which are thousands of times less massive than antihelium. Still, antihelium-3 is relatively heavy, and the heavier the antimatter particle. The rarer it is in the form of a person or something. “This means that each subsequent nucleus will be produced at factor 350 less than the previous one.”
Although physicists have deduced the existence of dark matter from its gravitational influence on the rotation of galaxies, they still do not know what it is made of. Hypotheses include objects as massive as black holes and as light as 100 millionths of the mass of an electron. Physicists first proposed two decades ago that certain dark matter particles, known as Weakly Interacting Massive Particles, or WIMPs, could annihilate with anti-dark matter to produce equal amounts of matter and antimatter. If dark matter emits antihelium as it annihilates, discovering this antiparticle would be proof that it exists.
Theoretically, physicists could look for either the matter or the antimatter that dark matter produces when they are looking for it. According to physicist Tim Linden of Stockholm University in Sweden, who was not involved with the LHC experiment, “in many models, dark matter is its own antiparticle, or there are equal amounts of dark matter and anti-dark matter.” In either case, you typically produce about the same amount of anti-particles as dark matter annihilation produces particles.
Origin of extraterrestrial matter particles
According to Linden, it is challenging to determine the origin of extraterrestrial matter particles because stars and other astrophysical objects unrelated to dark matter also generate a large number of these particles. Because astrophysical processes are poor at producing antimatter and the background is smaller, he explains, “we look for antimatter signatures.” In this way, dark matter is more likely to be the source of any antimatter particles that are discovered in space.
Tantalizing signal astrophysicists revealed in 2016 has increased interest in antimatter as a dark matter signature. The community was informed by scientists in charge of the International Space Station’s Alpha Magnetic Spectrometer (AMS) that they had likely found eight antihelium nuclei. Although the finding has not yet been formally published and the signal is still considered “tentative,” according to Linden. “it has inspired this effort to figure out—if that signal was true—how could it have come here?”
The LHC experiment and analysis are significant because they have increased confidence in detecting antihelium from space as a strategy for discovering dark matter. After creating the nuclei in their detector, Vorobyev’s team calculated the likelihood of the antihelium breaking apart or annihilating with the regular matter as it moved through the machine. They used these findings to create a model of the Milky Way. They did so to estimate the likelihood of antihelium nuclei reaching Earth from up to tens of thousands of light-years away. Although space is relatively empty, as antihelium travels through the galaxy toward our planet. These nuclei may collide with clouds of gas and break apart.
“We have seen that half of them will survive the journey to the detectors near Earth,” says Vorobyev. That is a promising sign that physicists’ antimatter detectors will eventually detect a traveling antihelium particle. AMS, which discovered the possible signals reported in 2016, is still searching. The General Antiparticle Spectrometer is a new instrument that will be launched in a balloon into the Antarctic atmosphere in late 2023 to look for antihelium and other particles at an altitude of 25 miles.
This new work exemplifies how perplexing and uncertain the scientific process can be. To address a question as large as dark matter, theorists must consider how researchers might be able to detect it on Earth. To validate the theorists’ ideas, experimentalists had to conduct tests like Vorobyev’s. Astrophysicists had to create instruments to detect antimatter signals. At least for antihelium-based dark matter searches, the threads are coming together. “It’s a really good melding of communities to try to find solutions to these really difficult problems,” Linden says.
However, these communities still have a lot of work ahead of them. For theorists like Linden, the details of how dark matter might generate antihelium in the first place remain a mystery. Astrophysicists must keep an eye out for antihelium signals from space. And if they do, they must ensure that the antiparticles are consistent with dark matter theorists’ predictions. The ALICE experiment lays the groundwork for a new approach to solving the mystery of dark matter. But physicists still have a long way to go down the rabbit hole.
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