Researchers from the Institute of Nuclear Physics of the Polish Academy of Sciences have developed a new model incorporating quark-gluon interactions while accounting for nucleon pairing.
For the first time, quarks and gluons were used to describe properties of atomic nuclei, which until now had been explained by the existence of protons and neutrons. The temporary pair of correlated nucleons is highlighted in purple. Image Credit: IFJ PAN
This breakthrough, confirmed by experimental data, allows for a unified understanding of atomic nuclei across energy scales, potentially revolutionizing our understanding of nuclear structures. The study was published in Physical Review Letters.
Atomic nuclei are composed of neutrons and protons, particles existing through interacting quarks bonded by glucons. Reproducing its properties observed in nuclear experiments using only gluons and quarks should, therefore, not be too challenging. Yet it is only now that physicists, including those from Cracow's Institute of Nuclear Physics of the Polish Academy of Sciences, have achieved this.
Protons and neutrons, the fundamental building blocks of atomic nuclei, were discovered around a century ago. The new particles were initially thought to be indivisible. However, it was proposed in the 1960s that protons and neutrons would exhibit their intrinsic structure, the existence of quarks linked together by gluons at high enough energies. The existence of quarks was experimentally verified shortly after.
It may appear surprising, therefore, that no one has managed to reproduce the results of nuclear investigations at low energies with quark-gluon models when only neutrons and protons are visible within atomic nuclei.
Until now, there have been two parallel descriptions of atomic nuclei, one based on protons and neutrons which we can see at low energies, and another, for high energies, based on quarks and gluons. In our work, we have managed to bring these two so far separated worlds together.
Dr. Aleksander Kusina, Theoretician, Polish Academy of Sciences
Humans perceive their surroundings with their natural ability to detect and register scattered photons that have already interacted with the atoms and molecules that comprise the objects in our environment. Similarly, physicists learn about atomic nuclei by colliding them with smaller particles and carefully examining the collision's outcomes.
However, for pragmatic reasons, they employ elementary particles with a charge, often electrons, rather than electrically neutral photons. Experiments demonstrate that atomic nuclei behave as though they were composed of nucleons (protons and neutrons) when electrons have relatively low energy.
Partons (quarks and gluons) are “visible” inside the atomic nuclei at high energies. The outcomes of atomic nuclei and electron collisions have been accurately replicated.
Using models that assume the existence of nucleons alone to represent low-energy collisions and partons alone for high-energy collisions, the consequences of atomic nuclei colliding with electrons have been rather accurately replicated. Nevertheless, thus far, it has been impossible to integrate these two descriptions into a cohesive image.
In their investigation, IFJ PAN physicists used high-energy collision data, including data from the LHC accelerator at the CERN laboratory in Geneva. The primary goal was to study the partonic structure of atomic nuclei at high energies, which is presently defined by parton distribution functions (PDFs).
The distribution of quarks and gluons within protons, neutrons, and the atomic nucleus is mapped using these functions. Experimentally observable parameters, such as the likelihood of a certain particle being produced in an electron or proton collision with the nucleus, can be found using PDF functions for the atomic nucleus.
As protons and neutrons were thought to combine to form strongly interacting pairs of nucleons (proton-neutron, proton-proton, and neutron-neutron), the main innovation suggested in this paper from a theoretical perspective was the skillful extension of parton distribution functions, which was inspired by those nuclear models used to describe low-energy collisions.
Thanks to the innovative method, the researchers could ascertain parton distribution functions in atomic nuclei, parton distributions in correlated nucleon pairs, and even the number of such associated pairs for the 18 atomic nuclei under study.
The findings supported the finding from low-energy studies that proton-neutron pairings make up the majority of correlated pairs; this finding is especially intriguing for heavier nuclei, such as lead or gold. The method suggested in this study also has the advantage of better describing actual data than the conventional techniques for figuring out parton distributions in atomic nuclei.
In our model, we made improvements to simulate the phenomenon of pairing of certain nucleons. This is because we recognized that this effect could also be relevant at the parton level. Interestingly, this allowed for a conceptual simplification of the theoretical description, which should in future enable us to study parton distributions for individual atomic nuclei more precisely.
Dr. Aleksander Kusina, Theoretician, Polish Academy of Sciences
The agreement between theoretical predictions and experimental data, the behavior of atomic nuclei that have previously been explained only by nucleonic description, and data from low-energy collisions can now be reproduced for the first time using the parton model and data from the high-energy region.
The investigations' findings provide fresh insights into the structure of the atomic nucleus, bringing together its high- and low-energy components. The Polish National Science Center funded the IFJ PAN physicists' work on using the parton model to rebuild the nucleonic structure.
Journal Reference:
Denniston, A. W., et al. (2024) Modification of Quark-Gluon Distributions in Nuclei by Correlated Nucleon Pairs. Physical Review Letters. doi.org/10.1103/physrevlett.133.152502.