Researchers at the United States Department of Energy’s Argonne National Laboratory have produced the most detailed picture yet of the internal structure of a pion particle, offering new insights into one of the fundamental building blocks involved in the formation of visible matter.
The study, carried out in collaboration with scientists from Brookhaven National Laboratory, used the Polaris supercomputer at Argonne Leadership Computing Facility (ALCF) to simulate the behaviour of quarks inside a pion.
The work generated high-resolution three-dimensional images that reveal how these elementary particles are arranged and interact.
The findings represent a significant advance in nuclear physics, helping researchers better understand the strong nuclear force that binds atomic nuclei together.
The results are also expected to support future experiments designed to probe the structure of matter at an even deeper level.
The study was published in the Journal of High Energy Physics and received support from the Department of Energy’s Innovative and Novel Computational Impact on Theory and Experiment (INCITE) programme.
Argonne team maps the pion in three dimensions
The pion particle occupies a central position in nuclear physics because it is closely linked to the strong nuclear force, one of the four fundamental forces of nature. This force holds protons and neutrons together inside atomic nuclei and is essential to the stability of matter.
Despite its importance, the pion remains one of the least understood composite particles. Experimental data on its internal structure are limited, making advanced computational modelling a crucial tool for researchers seeking to uncover how quarks and gluons behave inside it.
To address this challenge, the research team turned to large-scale simulations capable of recreating the complex dynamics of the strong force at the subatomic level.
Polaris simulations uncover quark distributions
Using the Polaris supercomputer, researchers simulated the movement and interactions of quarks within a moving pion.
The calculations relied on sophisticated theoretical methods combined with massive computing power to model millions of points across a four-dimensional spacetime lattice.
The simulations generated detailed visualisations showing how quarks are distributed throughout the particle. Researchers were able to observe not only how quarks are arranged along the pion’s direction of motion but also how they are distributed across its transverse dimensions.
The scale of the calculations required extensive parallel processing, highlighting the increasingly important role of supercomputing in modern particle physics research.
New measurements reveal changing particle size
A major outcome of the project was the determination of the pion’s generalised parton distribution (GPD). This framework provides a detailed description of how quarks are distributed in both space and momentum inside a particle.
By analysing the pion GPD, the team constructed a highly detailed three-dimensional map of the particle’s internal structure. The results showed that the pion’s transverse size becomes smaller as the momentum carried by its quarks increases.
Researchers also found that the pion appears smaller than the proton at moderate momentum values. These findings provide new evidence about how quarks are confined within composite particles and how the strong force shapes their behaviour.
Findings set the stage for future collider experiments
Because direct experimental measurements of pion GPD do not yet exist, the new calculations provide an important benchmark for the scientific community.
The results are expected to support future investigations at the Thomas Jefferson National Accelerator Facility and the forthcoming Electron-Ion Collider at Brookhaven National Laboratory.
Both facilities aim to explore how quarks and gluons combine to create the matter that makes up the observable Universe.
The research team is already looking ahead to the next phase of its work. Using Argonne’s Aurora supercomputer, scientists plan to create similarly detailed three-dimensional maps of the proton, which, together with neutrons, form the atomic nuclei found throughout the Universe.