The key components of atomic nuclei - protons and neutrons - were discovered nearly a century ago and initially thought to be indivisible. In the 1960s, it became evident that they are made up of smaller particles known as quarks, held together by gluons. Although quarks' existence was experimentally confirmed, reproducing results of nuclear experiments at low energies using quark-gluon models remained elusive. This deadlock has now been broken, as detailed in a new paper published in *Physical Review Letters* by scientists from the nCTEQ collaboration, including theoreticians from IFJ PAN in Cracow.

"Until now, there have been two parallel descriptions of atomic nuclei, one based on protons and neutrons at low energies, and another at high energies using quarks and gluons," said Dr. Aleksander Kusina, a theoretician from IFJ PAN. "In our work, we have managed to bring these two so far separated worlds together."

Similar to how human eyes detect photons to perceive objects, physicists study atomic nuclei by colliding them with smaller particles and analyzing the outcomes. At low collision energies, nuclei appear to consist of nucleons - protons and neutrons - while at higher energies, quarks and gluons (partons) are observed. Historically, models that relied on nucleons alone explained low-energy collisions, while parton-based models were used for high-energy scenarios. These two approaches had not been unified into a single coherent model - until now.

The researchers utilized data from high-energy collisions, including those from the Large Hadron Collider at CERN, to study the partonic structure of atomic nuclei. They employed parton distribution functions (PDFs), which describe how quarks and gluons are distributed inside protons, neutrons, and entire atomic nuclei. The approach allowed the researchers to determine the probability of various particles forming during collisions involving atomic nuclei.

The breakthrough came from extending the PDFs by incorporating elements from traditional nuclear models, where nucleons were thought to form pairs (e.g., proton-neutron, proton-proton, neutron-neutron). This innovative approach enabled the team to study 18 different atomic nuclei, revealing how quarks and gluons are distributed in correlated nucleon pairs and confirming that most pairs involve proton-neutron combinations. This finding is especially relevant for heavy nuclei such as gold and lead. The new method also yielded a better match with experimental data compared to previous models.

"In our model, we improved simulations of nucleon pairing, recognizing its relevance at the parton level," explained Dr. Kusina. "This led to a conceptual simplification that could help us study parton distributions for individual atomic nuclei more precisely in the future."

The successful agreement between theoretical predictions and experimental results signifies that the parton model can now accurately reproduce the behavior of atomic nuclei at both high and low energies, unifying these two previously distinct areas of nuclear physics. The researchers' findings open new avenues for understanding the structure of the atomic nucleus, integrating its quark-gluon and nucleon-based aspects.

The work was funded by the Polish National Science Centre.

Research Report:Evidence for Modified Quark-Gluon Distributions in Nuclei by Correlated Nucleon Pairs

Related Links
The Henryk Niewodniczanski Institute of Nuclear Physics Polish Academy of Sciences
Understanding Time and Space