Scientists Found One of the Heaviest Antiparticles Ever. It Could Help Explain Why the Universe Exists.
Understanding why we live in a matter-dominated universe demands that scientists recreate the quark-gluon plasma that existed one millionth of a second after the Big Bang.
A Large Ion Collider Experiment, or ALICE, at CERN's Large Hadron Collider does exactly that by smashing together heavy-ion particles and analyzing the results.
In late 2024, ALICE discovered hyperhelium-4 and its anti- counterpart. This is only the third hypernuclei ever discovered, and it could help scientists better understand things like the matter-antimatter asymmetry and the strange physics of neutron stars.
One of the biggest mysteries of the universe's creation goes by many names. Some call it the baryon asymmetry, others call it the matter asymmetry, and still others call it the matter-antimatter asymmetry. But all of these names speak to the same confounding question: Why does matter exist at all?
You, me, the countless stars across among the trillions of galaxies shouldn't exist—at least, not according to the Standard Model of physics. In those first moments of being, matter and antimatter should've been created in equal measure and annihilated each other, leaving only energy in their wake.
Fast-forward 13.8 billion years later, however, and it's obvious that we live in a matter-dominated universe—and particle physicists (understandably) want to know why. But understanding what might've caused matter to triumph over antimatter requires somehow recreating the conditions of the Big Bang. The best way to do that so far has been to slam particles together at near the speed of light and sift through the subatomic mess, and one of the best place in the world to do that is at CERN's Large Hadron Collider in Switzerland. Recently, scientists working on A Large Ion Collider Experiment (ALICE) once again showed why.
In late 2024, ALICE—which, as its name suggests, focuses on heavy-ion physics—discovered the heaviest antimatter particle to date at LHC—antihyperhelium-4. Exploring these kinds of exotic hypernuclei is important, as the quark-gluon plasma created by these ALICE's collisions mimic the conditions of the universe one millionth of a second after the Big Bang. So, if you want to understand why matter exists, then this is the place to look. You can read about the discovery in a preprint paper published on the server arXiv.
As you probably guessed, hyperhelium isn't the same as your typical helium atom. Instead, hyperhelium contain protons, neutrons, and particles known as hyperons that contain one or more quarks of the 'strange' type (but no charm, bottom, or top quarks). Although heavy-ion collisions create hypernuclei and antihypernuclei in sufficient quantities, finding them is immensely difficult, as they decay almost instantly. Antihyperhelium-4, for example, quickly becomes an antihelium-3 nucleus, an antiproton, and a charged pion, according to Physics World.
Scientists have also only discovered three of these antihypernuclei in the past 15 years. The other two—antihypertriton and antihyperhydrogen—were spotted by the STAR collaboration at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in 2010 and 2024, respectively.
'What caused the difference in quantities of matter and antimatter in the universe?' Qiu Hao, a collaborator with STAR who helped discover the antihyperhydrogen, said in August of 2024. 'To answer this question, an important approach is to create new antimatter in the laboratory and study its properties.'
Further study of hypernuclei and antihypernuclei can also help experts explore other mysteries of the universe. By understanding how they interact with neutrons and protons, astrophysicists can also gain a better grasp of the intensely strange physics believed to occur inside the heart of neutron stars.
To fully understand the mysteries of our existence—as well as the strange celestial objects that fill the universe—scientists need a better understanding of the raw materials used to fashion existence in the first place. And the finding these hypernuclei particles adds a new piece to that cosmic puzzle.
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