Physicists at CERN have directly measured proton-oxygen collisions at 9.62 TeV per nucleon pair, closely recreating an important type of interaction that occurs when extremely energetic cosmic rays strike Earth’s atmosphere. The ATLAS experiment analyzed 246 million selected collisions containing more than 5.1 billion reconstructed tracks and found that no major cosmic-ray interaction model successfully reproduced all the observed particle behavior. The measurements could improve simulations used to determine the energies, masses and possible origins of cosmic rays that are too energetic to measure directly with instruments in space. Earth is constantly being bombarded by particles from space, but the most energetic arrivals are difficult to understand because scientists rarely observe them directly. Instead, researchers usually detect the vast showers of secondary particles created when a cosmic ray smashes into a molecule high in the atmosphere. Working backward from those showers requires computer models that attempt to reconstruct the original particle and the collision that started everything. There is a problem. Different models can produce substantially different answers. Physicists working with CERN’s ATLAS experiment have …