Scientists have made a groundbreaking discovery at a major collider, revealing the existence of an exotic antimatter nucleus. This astonishing find could unlock secrets about the universe...

Collisions of heavy ions in the Large Hadron Collider (LHC) create a quark-gluon plasma—a dense and extremely hot state of matter. In 2015, the LHC produced a quark-gluon plasma with a temperature of 10 trillion degrees Celsius. Scientists believe that this type of substance filled the Universe just a millionth of a second after the Big Bang.

Particle collisions in the LHC also create conditions for the formation of atomic nuclei, exotic hypernuclei, and their antimatter counterparts—anti-nuclei and anti-hypernuclei. Hypernuclei are composed not only of neutrons and protons but also include another elementary particle—a hyperon.

Studying such forms of matter is crucial for physics. By investigating extreme conditions, scientists gain a clearer understanding of the processes that form hadrons from quarks and gluons in the plasma, as well as the matter-antimatter asymmetry observed in the Universe today.

As a result of heavy ion collisions, only the lightest hypernuclear cores, hypertriton and hyperhydrogen, had been observed until recently. The antihypertriton was discovered in 2010, while antihyperhydrogen-4 was identified only in 2024. Antihyperhydrogen-4 consists of an antiproton, two antineutrons, and an anti-lambda hyperon.

Now, scientists have presented evidence for the existence of antihyperhelium-4. The data were collected during the ALICE experiment (A Large Ion Collider Experiment, a detector for heavy ion collisions), one of the eight major detectors at the LHC. The exotic nucleus comprises two antiprotons, an antineutron, and an anti-lambda hyperon.

The result has a statistical significance of 3.5 standard deviations, indicating that scientists are confident in the existence of antihyperhelium-4. This exotic nucleus becomes the heaviest antimatter hypernuclear core experimentally found at the LHC. The experiment's results are described in a paper published on the preprint server arXiv.

Measurements from ALICE were gathered during lead nucleus collisions in 2018 at an energy of 5.02 tera-electronvolts per colliding pair of particles. Researchers sought signals of hyperhydrogen-4, hyperhelium-4, and their antimatter partners from a vast amount of data using a specially designed machine learning algorithm.

Candidates for antihyperhydrogen-4 were identified by their decay into an antihydrogen-4 nucleus and a charged pion, while candidates for antihyperhelium-4 were identified by their decay into an antihydrogen-3 nucleus, an antiproton, and a charged pion.

In addition to discovering exotic nuclei, the ALICE team measured the quantities and masses of both hypernuclei. The masses align with other experiments conducted by physicists worldwide. The obtained results were compared with calculations from a statistical model of hadronization, which accurately describes the process of forming hadrons and nuclei in heavy ion collisions, and the model agrees well with experimental data. The size of the hypernuclei is about two femtometers (2⋅10⁻¹⁵ meters).

The ratio of antiparticles to particles for both hypernuclei is 1:1. This confirms the equal formation of matter and antimatter at the energies of the LHC experiment. High-energy physics has yet to explain the matter-antimatter imbalance in the Universe, but each experiment brings scientists closer to understanding the matter asymmetry.