{
  "id": 8867609,
  "title": "Proton–oxygen collisions put cosmic-ray models to the test",
  "url": "https://urgent.news/2026/09/21/proton-oxygen-collisions-put-cosmic-ray-models-to-the-test",
  "topic": "science",
  "section": "Science",
  "published": "2026-09-21T07:15:56.000Z",
  "source": {
    "name": "Physics World",
    "slug": "physics-world",
    "url": "https://physicsworld.com/a/proton-oxygen-collisions-put-cosmic-ray-models-to-the-test/"
  },
  "original_language": "en",
  "account": "Physicists at CERN have recreated the collisions that create cosmic-ray air showers in Earth's atmosphere. The results from the ATLAS experiment on the Large Hadron Collider (LHC) show that none of the computer models used to simulate these showers accurately capture every detail. Measurements of particle production in proton–oxygen collisions are over ten times more precise than the differences between the models, potentially improving our understanding of the most energetic particles in the universe. Cosmic rays are high-speed particles from outer space that strike Earth's atmosphere, generating cascades of secondary particles. Jesse Liu, the lead author of a paper describing the study, notes that cosmic rays originate from extreme regions of the universe, such as exploding stars and supermassive black holes in distant galaxies. However, the origin of the most energetic cosmic rays remains one of the biggest mysteries in astrophysics. These particles carry energies of a hundred billion-billion electron volts or more, far exceeding the million-million electron volts achieved by the LHC. Their detection depends on observing the extensive air showers they produce. Interpreting these showers relies on computer simulations, but calculating the role of the strong nuclear force governing these collisions is notoriously challenging. Existing models widely disagree on how these showers form, and the LHC's previous beams consisted only of protons or heavy nuclei, unlike the lighter nitrogen and oxygen nuclei found in air. In July 2025, the LHC was reconfigured to collide protons with oxygen nuclei, simulating the conditions of cosmic-ray collisions. ATLAS physicists studied the number of particles created, their energies, and angles, as well as the cross section, which indicates how often these collisions occur. Their results show that none of the models accurately predict the number of particles created in these collisions, with some models misjudging rare, particle-rich collisions by a factor of ten. The blind spot in current measurements is the region where most of the collision energy remains unseen by ATLAS and CMS but strongly influences air shower features. Future specialized experiments like LHCf aim to fill this gap. The next step is to apply these data to improve the computer models of cosmic-ray showers, potentially resolving the \"muon puzzle\" where facilities like the Pierre Auger Observatory see more muons than simulations predict. This data is particularly important because existing models suggest the most energetic cosmic rays are mostly made of heavier nuclei like carbon, nitrogen, oxygen, and silicon. The findings could help unify particle physics and high-energy astrophysics, with significant implications for understanding cosmic ray origins.",
  "summary": "CERN’s ATLAS experiment probes the extreme universe The post Proton–oxygen collisions put cosmic-ray models to the test appeared first on Physics World .",
  "key_points": [
    "Proton–oxygen collisions at LHC tested against cosmic-ray models",
    "ATLAS measurements 10x more precise than existing models' differences",
    "Results reveal models inaccurately predict particle production in cosmic-ray collisions"
  ],
  "editors_take": null,
  "illustration": null,
  "coverage": {
    "outlets": 1,
    "also_reported_by": []
  },
  "ai_generated": true,
  "disclaimer": "Summaries, key points and the editor’s take are written by software from other outlets’ reporting and may contain errors — always check the linked original."
}