An international group of scientists has provided evidence of the existence of a new anti-hypernucleus, the heaviest yet found. Studying antihypernuclei and their properties brings physicists closer to explaining the asymmetry of matter and antimatter in the universe. The collision of heavy ions at the Large Hadron Collider (LHC) creates quark-gluon plasma, a dense and very hot main state. In 2015, a quark-gluon plasma with a temperature of 10 trillion degrees Celsius was created at VAK. Scientists believe that this type of matter filled the universe within a millionth of a second after the Big Bang.

The collision of particles in VAK also creates conditions for the formation of atomic nuclei, exotic hypernuclei and their antimatter analogues – antinuclei and antihypernuclei. Hypernuclei consist not only of neutrons and protons, there is another fundamental particle – a hyperon.

The study of such forms of matter is important for physics. By studying extreme conditions, scientists gain a more precise understanding of the processes of formation of hadrons from quarks and gluons in plasma, as well as the asymmetry of matter and antimatter observed in the universe today.

Until recently, only the lightest hypernuclear nuclei, hypertriton and hyperhydrogen, were observed as a result of the collision of heavy ions. The hypertriton antiparticle was discovered in 2010, and antihyperhydrogen-4 was discovered only in 2024. Antihyperhydrogen-4 consists of an antiproton, two antineutrons and an anti-lambda-hyperon.

So far, scientists have provided evidence for the existence of antihyperhelium-4. Data were collected in one experiment BUYING (A Large Ion Collider Experimentheavy ion collision detector), one of the eight main VAK detectors. An exotic nucleus consists of two antiprotons, an antineutron and an anti-lambda-hyperon.

The statistical significance of the result is 3.5 standard deviations, which means that scientists are confident in the existence of antihyperhelium-4. This exotic nucleus becomes the heaviest antimatter hypernuclear nucleus experimentally detected at VAK. The results of the experiment are described in the article published on the preprint server. arXiv.

Measurement BUYING During the collision of lead nuclei in 2018, an energy of 5.02 teraelectron volts was collected per pair of colliding particles. Using a specially designed machine learning algorithm, the researchers searched for signals of hyperhydrogen-4, hyperhelium-4, and their antimatter partners in large amounts of data.

Candidate particles for the role of antihyperhydrogen-4 were identified from decay to the antihelium-4 nucleus and charged pion, while candidates for antihyperhelium-4 were identified from decay to antihelium-3, antiproton and charged pion nucleus.

In addition to detecting exotic nuclei, the team BUYING measured the number and masses of both hypernuclei. The masses are consistent with other experiments by physicists around the world. The obtained results were compared with calculations of the hadronization statistical model, which describes well the process of formation of hadrons and nuclei in the collision of heavy ions, and the model is in good agreement with experimental data. The size of hypernuclei is approximately two femtometers (2⋅10⁻¹⁵ meters).

For both hypernuclei, the ratio of antiparticles to particles is 1:1. This confirms the formation of equal matter and antimatter at the energies of the experiment at VAK. High-energy physics cannot yet explain the imbalance of matter and antimatter in the universe, but each experiment brings scientists closer to explaining the asymmetry of matter.