Context
Once of the things that physicists can do, even if our understanding of the laws of physics is complete, is to formally derive the properties of more complex structures, like atoms, from the fundamental laws of physics found in the Standard Model.
We understand the structure of atoms mostly in the context of a simplified proton-neutron-electron model, that is used in chemistry and even, for the most part, in nuclear physics, with a simplified (mostly experimentally fit) binding energy description of how protons and neutrons are held together in atomic nuclei that describes the residual strong force that holds protons and neutrons in the atom together in a nucleus (that we know is mediated mostly by composite pions as force carriers and to a secondary extent by force carrying composite kaons, rather than directly by gluons).
The Standard Model of Particle physics provides a more fundamental description of protons, neutrons, and their interactions in terms of quarks and gluons with its model of the strong force that binds quarks and gluons to each other that is mediated by force carrying gluons. This theory is called quantum chromodynamics (QCD) because the analogy to electric charge for the strong force is called "color charge". Quarks can have one of three color charges, and gluons come in eight combinations that involve pairs of color charges.
The New Paper
Half a century after the Standard Model was devised, a new paper has made a major breakthrough at advancing the unfinished project of explaining atomic nuclei in terms of quarks and gluons, rather than in terms of composite protons and neutrons bound by the residual strong force.
The new paper accurately reproduces the structure of 18 atomic different nuclei with quantum chromodynamics (the theory of the Standard Model strong force that binds quarks and gluons to each other).
The parton distribution functions (PDFs) describe the structure of a composite particle in terms of quarks and gluons. PDFs can be calculated, in theory, from first principles in the Standard Model without any experimental input beyond the values of the two dozen or so experimentally measured physical constants of the Standard Model.
But until less than a decade ago, in practice, parton distribution functions were almost always created by a vast statistical data dump from billions of collisions to which a mathematical function was fitted, that were very particular to particular particles at particular energy scales. These were updated from time to time with new data from more collisions.
When it has been done previously from first principles, this has mostly been confined to individual protons, neutrons, or other simple hadrons such as pions (hadrons are composite particles whose particles are bound by gluons), not to multi-hadron atoms.
The new paper make some big leaps beyond that, advancing the project of rigorously demonstrating what we had merely assumed (for some good reasons) for the last fifty years: that the structure of atomics can be described fully from first principles using the Standard Model.
I quote at length from a secondary source account of what the paper is doing, because the paper itself is too technical for a general audience and what the paper is doing is sufficiently technical that I don't want to mangle it in my paraphrased retelling of it.
The atomic nucleus is made up of protons and neutrons, particles that exist through the interaction of quarks bonded by gluons. It would seem, therefore, that it should not be difficult to reproduce all the properties of atomic nuclei hitherto observed in nuclear experiments using only quarks and gluons. However, it is only now that an international team of physicists has succeeded in doing this. . . .This long-standing deadlock has only now been broken, in a paper published in Physical Review Letters. Its main authors are scientists from the international nCTEQ collaboration on quark-gluon distributions."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," says Dr. Aleksander Kusina, one of the three theoreticians from the Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) participating in the research. . . .Experiments . . . show that when electrons have relatively low energies, atomic nuclei behave as if they were made of nucleons (i.e. protons and neutrons), whereas at high energies, partons (i.e. quarks and gluons) are "visible" inside the atomic nuclei.The results of colliding atomic nuclei with electrons have been reproduced quite well using models assuming the existence of nucleons alone to describe low-energy collisions, and partons alone for high-energy collisions. However, so far these two descriptions have not been able to be combined into a coherent picture.In their work, physicists from the IFJ PAN used data on high-energy collisions, including those collected at the LHC accelerator at CERN laboratory in Geneva. The main objective was to study the partonic structure of atomic nuclei at high energies, currently described by parton distribution functions (PDFs).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 of a specific particle being created in an electron or proton collision with the nucleus.From the theoretical point of view, the essence of the innovation proposed in this paper was the skillful extension of parton distribution functions, inspired by those nuclear models used to describe low-energy collisions, where protons and neutrons were assumed to combine into strongly interacting pairs of nucleons: proton-neutron, proton-proton and neutron-neutron.The novel approach allowed the researchers to determine, for the 18 atomic nuclei studied, parton distribution functions in atomic nuclei, parton distributions in correlated nucleon pairs and even the numbers of such correlated pairs.The results confirmed the observation known from low-energy experiments that most correlated pairs are proton-neutron pairs (this result is particularly interesting for heavy nuclei, e.g. gold or lead). Another advantage of the approach proposed in this paper is that it provides a better description of the experimental data than the traditional methods used to determine 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," explains Dr. Kusina.The agreement between theoretical predictions and experimental data means that, using the parton model and data from the high-energy region, it has been possible for the first time to reproduce the behavior of atomic nuclei so far explained solely by nucleonic description and data from low-energy collisions. The results of the described studies open up new perspectives for a better understanding of the structure of the atomic nucleus, unifying its high- and low-energy aspects.
From Phys.org. The paper and its abstract are as follows:
We extend the QCD Parton Model analysis using a factorized nuclear structure model incorporating individual nucleons and pairs of correlated nucleons. Our analysis of high-energy data from lepton deep-inelastic scattering, Drell-Yan, and 𝑊 and 𝑍 boson production simultaneously extracts the universal effective distribution of quarks and gluons inside correlated nucleon pairs, and their nucleus-specific fractions. Such successful extraction of these universal distributions marks a significant advance in our understanding of nuclear structure properties connecting nucleon- and parton-level quantities.
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