Antihyperhelium-4: World’s largest atom smasher detects heaviest antimatter to date

In addition to helping us understand the origins of the Universe, these experiments also produce new atomic nuclei and hypernuclei alongside their antimatter counterparts, antinuclei and antihypernuclei. 

What are hypernuclei? 

Hypernuclei are exotic nuclei, approximately two femtometers in size, formed after the mixing of protons, neutrons, and hyperons. While protons and neutrons are well-known and stable constituents of an atom, hyperons are unstable and are made up of either one or more quarks. 

Spotted in cosmic rays more than seven decades ago, hypernuclei are still fascinating because they are not usually found in nature, and they are difficult to create in the lab. Experiments like the LHC create them in large quantities, though, and this is where detectors of ALICE and Relativistic Heavy Ion Collider (RHIC) try to spot and study them. 

Earlier this year, a collaboration of researchers at RHIC reported the observation of antihyperhydrogen-4. This antimatter and hypernucleic atom consists of an antiproton, two antineutrons, and an antilambda particle. A lambda particle is a hyperon containing one strange quark, while the antilambda is its antimatter counterpart. 

Now, researchers at ALICE have confirmed the observation of antihyperhelium-4, which consists of two antiprotons, an antineutron, and an antilambda, and is the heaviest antimatter hypernucleus detected at the LHC till date. 

How was it observed? 

Instead of relying on conventional hypernuclei search techniques, the researchers used a machine learning approach to identify signals of hyperhydrogen-4, hyperhelium-4, and their antimatter counterparts. 

Key to the identification of these hypernuclei was the detection of particles like pions and antiprotons into which these hypernuclei decay. When these signals were observed with 3.5 standard deviations, the researchers were confident of having found significant evidence for their existence. 

The research team also measured masses and production yields for both antihyperhelium-4 and antihyperhydrogen-4 and found them to be compatible with current world average values. 

Statistical hadronization models help us understand the process of formation of hadrons and predict production yields from heavy-ion collisions. The researchers at ALICE measured production yields from antihyperhelium-4 candidates and found them to be comparable with hadronization model numbers. 

The approach also demonstrated that the model can predict the production of hypernuclei, the press release added. Additionally, the research team also determined anti-particle-to-particle yield ratios and found them to be within the experimental uncertainties. 

LHC’s experiments so far have shown that antimatter and matter are produced in equal quantities, and this information adds to the wealth of knowledge available about the matter-antimatter imbalance in the Universe.