At Argonne National Laboratory in Illinois, physicists just captured something no one has ever seen before: the internal blueprint of a pion, a subatomic particle so tiny it helps glue matter itself together. Using the Polaris supercomputer, a team led by physicist Yong Zhao created high-resolution 3D images of quarks moving inside the pion—revealing the invisible architecture of one of nature's most fundamental building blocks.
This matters because pions are the mediators of the strong nuclear force, the fundamental force that binds protons and neutrons inside atomic nuclei. Understanding how pions work means understanding how visible matter forms from the most elementary particles of all. "By probing the pion's internal structure, we gain a deeper understanding of how quarks and gluons are confined to creating visible matter," Zhao explained in the research published in the Journal of High Energy Physics in 2025. It's a profound question: How do the invisible quantum particles we call quarks and gluons conspire to create the tangible universe around us?
For decades, scientists have wanted to map how quarks are distributed inside composite particles held together by the strong force. But pions presented a puzzle—there are few experimental measurements available, so researchers must rely on massive computer simulations to reveal what experiments cannot yet directly observe. That's where Polaris came in. The supercomputer performed calculations that would be impossible for conventional computers, capturing hundreds of snapshots of 4D spacetime represented on a lattice with millions of grid points. "Polaris allowed us to simulate how quarks move and correlate inside the pion, both along its direction of motion and across it," Zhao said.
The simulations produced a quark generalized parton distribution (GPD) of the pion—essentially a detailed map showing how quarks are arranged at different positions and momenta inside the particle. The results revealed something striking: the pion's transverse size actually decreases as the momentum in the direction of the pion increases. This same pattern appears in the proton, suggesting a fundamental property of how matter is structured at the quantum scale. Even more intriguingly, the pion turns out to be smaller than the proton at moderate parallel momentum values—a finding that reshapes our understanding of subatomic geometry.
Because no experimental measurements of pion GPD exist yet, these theoretical results provide crucial guidance for upcoming experiments. Physicists are preparing to test these predictions at the Thomas Jefferson National Accelerator Facility and at the future Electron-Ion Collider at Brookhaven National Laboratory. The computational window Zhao's team opened onto the pion's interior will now serve as a roadmap for experimentalists hunting to confirm these findings in the lab.
The work doesn't stop here. Zhao and his team are already preparing to use Argonne's even more powerful Aurora supercomputer to map the proton in three dimensions—a monumental task that could illuminate how all the atomic nuclei composing visible matter in our universe are actually constructed. Each layer of understanding brings us closer to a complete picture of matter itself.