When a proton slams into a lead nucleus at the Large Hadron Collider, a swarm of particles is born. They fly off in all directions, but not entirely randomly: their motion resembles ripples on water from a thrown stone. Physicists call this orderly dispersal a "flow."

In the new study, scientists tested how stable this flow is. It turns out that from collision to collision it varies slightly, like ripples on water depend on exactly how the stone fell. Previously, such fluctuations were only seen in collisions of two large lead nuclei, but now they have also been observed in the small system of "proton + lead nucleus."

To ensure the effect was not due to background noise (for example, from individual particle jets), physicists applied a special data-cleaning method. The background could not explain the observed fluctuations. So the flow is indeed not the same in every event.

This discovery helps better understand how coordinated motion emerges from many individual particles. Such studies at the Large Hadron Collider provide clues about the earliest stage of collisions, when matter behaves like a liquid.