Tens of kilometers above Earth’s surface, high-energy particles from outer space constantly strike the atmosphere, creating showers of energetic secondary particles that rain down from the sky. Approximately one of these particles passes through your head every second, but the “cosmic rays” that produce them are still not fully understood. In a recent paper posted to the arXiv preprint server, the ATLAS Collaboration describes how its first measurement of proton–oxygen collisions at the LHC could help us learn more about them.

Cosmic rays were discovered over a century ago by physicist Victor Hess in experiments conducted aboard hot-air balloons. Today, astrophysicists use detectors on the ground to image cosmic-ray showers and computer simulations of the showers to understand that data.

However, these simulations depend on properties of the strong force—one of the fundamental forces of the universe—which is difficult to accurately model. Current simulations disagree with one another, making it difficult for astrophysicists to interpret their measurements of cosmic rays.