First coherent image of an atomic nucleus composed of quarks and gluons

The atomic nucleus is made up of protons and neutrons, particles that exist through the interaction of quarks bound by gluons. It therefore does not seem difficult to reproduce all the properties of atomic nuclei observed so far in nuclear experiments using only quarks and gluons. However, only now have physicists, notably those at the Institute of Nuclear Physics at the Polish Academy of Sciences in Krakow, achieved this.

It’s been almost a century since the discovery of the main components of atomic nuclei: protons and neutrons. Initially, the new particles were considered indivisible. In the 1960s, however, it was suggested that at high enough energies, protons and neutrons would reveal their internal structure, that is, the presence of quarks constantly held together by gluons.

Shortly thereafter, the existence of quarks was confirmed experimentally. It may therefore seem surprising that, despite many decades, no one has been able to reproduce with quark-gluon models the results of low-energy nuclear experiments, when only protons and neutrons are visible in atomic nuclei. .

This long-standing impasse has only just been resolved, in an article published in Physical Examination Letters. Its lead authors are scientists from the international nCTEQ collaboration on quark-gluon distributions, including those from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) in Krakow.

“Until now, there have been two parallel descriptions of atomic nuclei, one based on the protons and neutrons that can be observed at low energies, and the other, for high energies, based on the quarks and gluons In our work, we have succeeded in bringing these two previously separate worlds together,” explains Dr Aleksander Kusina, one of three IFJ PAN theorists participating in the research.

Humans see their surroundings because they use innate detectors (eyes) to record scattered photons that have already interacted with the atoms and molecules that make up the objects in our environment. Physicists gain knowledge about atomic nuclei in the same way: they collide them with smaller particles and carefully analyze the results of the collisions.

But for practical reasons, they do not use electrically neutral photons, but elementary particles carrying a charge, generally electrons. Experiments then show that when electrons have relatively low energies, atomic nuclei behave as if they were made up of nucleons (i.e. protons and neutrons), whereas at high energies, the partons (i.e. quarks and gluons) are “visible” inside atomic nuclei.

The results of collisions of atomic nuclei with electrons have been fairly well reproduced using models assuming the existence of single nucleons to describe low-energy collisions, and single partons for high-energy collisions. However, until now these two descriptions could not be combined into a coherent picture.

In their work, IFJ PAN physicists used data on high-energy collisions, notably those collected at the LHC accelerator at the CERN laboratory in Geneva. The main objective was to study the parton structure of atomic nuclei at high energies, currently described by parton distribution functions (PDF).

These functions are used to map how quarks and gluons are distributed inside protons and neutrons and throughout the atomic nucleus. With PDF functions for the atomic nucleus, it is possible to determine experimentally measurable parameters, such as the probability that a specific particle will be created when electrons or protons collide with the nucleus.

From a theoretical point of view, the essence of the innovation proposed in this paper was the skillful extension of parton distribution functions, inspired by nuclear models used to describe low-energy collisions, where protons and neutrons were expected to combine into strongly interacting pairs. of nucleons: proton-neutron, proton-proton and neutron-neutron.

This new approach allowed researchers to determine, for the 18 atomic nuclei studied, the distribution functions of partons in atomic nuclei, the distributions of partons in correlated pairs of nucleons and even the number of these correlated pairs.

The results confirmed the observation from low-energy experiments that most correlated pairs are proton-neutron pairs (this result is particularly interesting for heavy nuclei, for example gold or lead). Another advantage of the approach proposed in this paper is that it provides a better description of experimental data than traditional methods used to determine parton distributions in atomic nuclei.

“In our model, we made improvements to simulate the pairing phenomenon of certain nucleons. Indeed, we recognized that this effect could also be relevant at the parton level. It is interesting to note that this allowed a simplification conceptual of the theoretical description, which should allow us in the future to study the distribution of partons for individual atomic nuclei more precisely,” explains Dr Kusina.

The agreement between theoretical predictions and experimental data means that using the partonic model and data from the high energy region, it was possible for the first time to reproduce the behavior of atomic nuclei previously explained only by nucleonic description and low energy data. -energy collisions. The results of the studies described open new perspectives for a better understanding of the structure of the atomic nucleus, by unifying its high and low energy aspects.