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ATLAS turns into a cosmic-ray laboratory with proton-oxygen collisions

15 April 2026 | By

The ATLAS Collaboration reports its first measurement of proton-oxygen collisions at the Large Hadron Collider (LHC). These results transform the ATLAS experiment into a cosmic-ray laboratory, helping to unravel the nature of high-energy particle showers in the sky.

Tens of kilometres above ground, high-energy particles from outer space constantly bombard the Earth. When they strike the atmosphere, these “cosmic rays” create showers of energetic secondary particles that rain down from the sky. Approximately one of these particles passes through your head every second.

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 depend on computer simulations of the showers to understand the data. However, these simulations require knowledge of the strong force that is difficult to model accurately. Current models disagree with one another, making it difficult for astrophysicists to interpret their measurements.

To solve this problem, physicists at the ATLAS experiment set out to recreate cosmic-ray collisions in the laboratory – effectively putting them under a microscope. In July 2025, for a few days, they transformed the LHC into a giant cosmic-ray machine. For the first time in its history, the LHC was configured to collide protons with oxygen ions. The beam of protons acted as the cosmic ray, while the beam of oxygen ions played the role of the atmosphere.

These results establish a novel way to use the LHC as a cosmic-ray laboratory, opening the path for detailed experimental studies of proton-oxygen interactions.

Physics,ATLAS — Figure 1: Graph showing how often proton-oxygen collision events are measured for the number of charged particles on the horizontal axis. Black circles show data with the shaded band being its uncertainty. Lines show predictions from various computer simulation models. The lower panel shows how different the models are from data with ATLAS-measurement uncertainties given by the shaded band. (Image: ATLAS Collaboration/CERN)

ATLAS physicists analysed these collisions by measuring the tracks left in the experiment from electrically charged particles (yellow lines in the event display above). They measured key properties of the collision, including how often the particles were created, how many were created, and the energies and angles at which they flew out.

Figure 1 shows the measured number of charged particles compared to simulations typically used to interpret data from cosmic-ray observatories. These simulations, which are tuned to reproduce previous data from collisions of protons and heavier nuclei, disagree with one another. The new measurements achieve a precision at the level of a few percent, significantly improving the knowledge of proton-oxygen collisions. ATLAS physicists now pass the baton to theorists, who can use this input to refine their models to better match experimental measurements.

These results establish a novel way to use the LHC as a cosmic-ray laboratory, opening the path for detailed experimental studies of proton-oxygen interactions. This can help shed light on the mysterious high-energy particles arriving from the cosmos.

About the banner image: Event display showing nineteen charged-particle tracks (yellow lines) recorded by the ATLAS experiment during proton-oxygen collisions in July 2025. (Image: ATLAS Collaboration/CERN)