The small but ubiquitous proton serves as a foundation for the bulk of the visible matter in the universe. It abides at the very heart of matter, giving rise to everything we see around us as it anchors the nuclei of atoms. Yet, its structure is amazingly complex, and the quest to understand these details has occupied theorists and experimenters alike since its discovery over a century ago.

“A large part of the visible matter in the universe is made of protons,” said Kyungseon Joo, a physics professor at the University of Connecticut. “And so, if you want to understand the universe, it is important to understand the proton.”

Currently, proton structure is only well understood in processes where they are probed at high energy and where a lot of momentum is transferred to the proton. In such cases, the probes interact with the quarks and gluons (together called “partons”) that form the proton so quickly that they react like a tightly set rack of billiard balls hit by a well-struck cue ball.