ATLAS Collaboration Publishes First Measurement of Proton-Oxygen Collisions

The ATLAS Collaboration at CERN has released its first measurement of proton-oxygen collisions, conducted at the Large Hadron Collider (LHC) in July 2025. This analysis provides high-precision data on the properties of charged-particle tracks produced in these unique collisions.

The results, achieving a precision level of a few percent, are intended to help improve simulations of cosmic-ray interactions with Earth's atmosphere.

The Cosmic Ray Connection

The study is deeply connected to a century-old mystery: cosmic rays. These high-energy particles from outer space constantly strike Earth's atmosphere, creating vast showers of secondary particles. Astrophysicists rely on ground-based detectors and complex computer simulations to understand these showers and trace the cosmic rays back to their origins.

A significant challenge has been that current simulations of these atmospheric interactions, which depend on modeling the strong nuclear force, show substantial disagreements with each other. These discrepancies make interpreting data from cosmic-ray observatories particularly difficult.

Purpose of the Proton-Oxygen Experiment

To address this, physicists configured the LHC to mimic a key cosmic-ray interaction.

• The proton beam represented an incoming high-energy cosmic ray.
• The oxygen ion beam represented the nuclei found in Earth's atmosphere, which is primarily composed of nitrogen and oxygen.

The ATLAS analysis measured detailed properties of the resulting particle showers, including production rates, multiplicities, energies, and angular distributions. These measurements were then compared to predictions from several existing simulation models used by the cosmic-ray research community.

The newly published paper states that this precise dataset can now be used by theorists to test and refine their models of high-energy particle interactions, ultimately leading to more accurate simulations of cosmic-ray air showers.