Physicists have for the first time used quarks and gluons to describe the properties of atomic nuclei.

Nearly 100 years have passed since the discovery of the fundamental components of atomic nuclei: protons and neutrons. Initially, they were considered indivisible, but in the 1960s, scientists proposed that at sufficiently high energies, protons and neutrons would reveal their internal structure—the presence of quarks, held together by gluons.

Shortly thereafter, the existence of quarks was experimentally confirmed. However, no one succeeded in reproducing the results of nuclear experiments at low energies using quark-gluon models, where only protons and neutrons are visible within atomic nuclei.

Experiments indicate that at relatively low energies, atomic nuclei behave as if they are made up solely of nucleons—protons and neutrons—while at high energies, quarks and gluons become "visible" within atomic nuclei. The results of collisions between atomic nuclei and electrons are quite well reproduced using models that assume the existence of only nucleons for low-energy collisions or only partons—quarks and gluons—for high-energy collisions. This long-standing impasse has been overcome by scientists from the international collaboration nCTEQ focused on quark-gluon distributions.

Physicists from the Institute of Nuclear Physics of the Polish Academy of Sciences utilized data from high-energy collisions, including those collected at the Large Hadron Collider at CERN. The primary goal was to study the parton structure of atomic nuclei at high energies, which is currently described by parton distribution functions (PDFs).

These functions are used to represent the distribution of quarks and gluons within protons and neutrons and throughout the atomic nucleus. Using the PDF functions for the atomic nucleus, it is possible to experimentally determine the likelihood of producing a specific particle when an electron or proton collides with a nucleus.

The approach proposed in this new scientific paper expands the description of parton distribution functions. The researchers were inspired by nuclear models applied to describe low-energy collisions. These models assume that protons and neutrons combine into strongly interacting correlated pairs of nucleons: proton-neutron, proton-proton, and neutron-neutron.

The new approach enabled the determination of parton distribution functions in atomic nuclei for 18 studied atomic nuclei, the parton distributions in correlated pairs of nucleons, and even the number of such correlated pairs. The results confirmed an observation known from low-energy experiments: the majority of correlated pairs are proton-neutron pairs.

The advantage of the proposed approach is that it provides a more accurate description of experimental data compared to traditional methods used to determine parton distributions in atomic nuclei. The researchers made enhancements to model the phenomenon of pairing certain nucleons. They acknowledged that this effect could be significant at the parton level. This allowed for a conceptual simplification of the theoretical description, which should enable more precise studies of parton distributions for individual atomic nuclei in the future.

The agreement between theoretical predictions and experimental data indicates that, by applying the parton model and high-energy data, it has been possible for the first time to reproduce the behavior of atomic nuclei. Until now, this behavior was explained solely by nucleon descriptions and low-energy collision data. The results of the described research open new prospects for a better understanding of atomic nucleus structure, integrating its high- and low-energy aspects.

The scientific work has been published in the journal Physical Review Letters.