Cosmic rays are particles that arrive from deep space. When such a particle hits the atmosphere, it triggers a whole shower of new particles, like a billiard ball breaking a rack, sending balls scattering in all directions. To predict what such a shower will look like, we need to know how the primary particle interacts with the nuclei of air atoms. Air is mostly nitrogen and oxygen, so scientists decided to measure proton collisions with oxygen nuclei.

At the Large Hadron Collider at CERN, a proton beam was accelerated and directed at an oxygen target. The collision energy was enormous—9.62 teraelectronvolts in the center-of-mass system, thousands of times greater than the energy released in the fission of a single uranium nucleus. The ATLAS detector recorded 246 million events in which at least one charged particle was produced with transverse momentum greater than 500 megaelectronvolts and emission angle within 2.5 units of pseudorapidity. It's as if you threw a stone into a pond 246 million times and each time measured the resulting splash.

The results will help astrophysicists calculate cosmic showers more precisely. This is important for observatories that detect ultra-high-energy cosmic rays, such as the Pierre Auger Observatory in Argentina. The better we understand how a particle interacts with air, the more accurately we can reconstruct its energy and direction. This, in turn, brings us closer to solving the mystery of where the most energetic particles in the universe come from.