The LHC takes on a problem from the sky
Cosmic rays constantly strike Earth’s atmosphere, setting off cascades of secondary particles that spread through the sky and pass through detectors on the ground. Those showers are a key source of information about some of the highest-energy particles in the universe, but they are difficult to interpret because the underlying collision physics is hard to model accurately. Now the ATLAS Collaboration says its first measurement of proton-oxygen collisions at the Large Hadron Collider could help close that gap.
The new result comes from a mode the LHC ran for the first time in July 2025, when it collided beams of protons with beams of oxygen ions. In that setup, the proton beam acts like a cosmic ray, while the oxygen beam stands in for part of Earth’s atmosphere, which is composed primarily of nitrogen and oxygen. That makes the experiment a controlled way to recreate one of the fundamental interactions that powers atmospheric particle showers.
Why cosmic-ray data are so hard to decode
Modern cosmic-ray observatories infer the nature of incoming particles by detecting the showers they produce after hitting the atmosphere. But those shower patterns depend on the strong force, one of nature’s fundamental interactions and one that remains notoriously hard to model in the high-energy, many-particle environments relevant to cosmic rays.
As CERN notes, current simulations do not agree with one another. That disagreement limits what astrophysicists can confidently conclude from measurements on the ground. If the simulation framework is off, then inferences about the energy, composition or origin of cosmic rays can also be distorted.
This is where collider data become useful. A laboratory collision does not reproduce every feature of a natural cosmic-ray event, but it can provide direct measurements of particle production under more controlled conditions. Those measurements can then be used to test and tune the simulation tools that observatories depend on.
What ATLAS actually measured
According to the collaboration’s preprint, physicists analyzed proton-oxygen collisions by tracking the electrically charged particles produced in the interactions. They measured how often such particles were created, how many were produced, and the energies and angles at which they emerged from the collision region.
That type of information is exactly what shower models need. The early stages of a cosmic-ray cascade are determined by how an incoming high-energy particle transfers energy into a spray of secondaries. Differences in multiplicity, angular spread and energy distribution can propagate through the entire simulated shower.
ATLAS then compared the measured charged-particle distributions with the predictions made by several simulations commonly used to interpret data from cosmic-ray observatories. The point was not only to publish a first measurement, but to identify where the models match and where they fail.
A collider becomes a cosmic-ray laboratory
The unusual strength of the result is conceptual. The LHC is usually associated with fundamental particle physics questions such as the Higgs boson or searches for new particles. Here, ATLAS is operating in a different role: as a calibration laboratory for astrophysics. It is recreating, in a cleaner environment, a class of collisions that naturally happens tens of kilometers above Earth.
That bridge between particle physics and cosmic-ray science is especially valuable because direct measurements of primary cosmic rays at the highest energies are rare and difficult. By improving the models used to interpret atmospheric showers, collider data can indirectly sharpen the conclusions drawn by observatories that watch those showers unfold.
The work also highlights a practical point about the atmosphere itself. Oxygen is a major constituent of air, so proton-oxygen data are more directly relevant to cosmic-ray interactions than many standard proton-proton collider datasets. That makes this measurement a targeted input rather than a generic one.
What changes next
The current result is based on the first proton-oxygen collision run and is described in a paper posted to arXiv, so it represents an early step rather than a final answer. But it establishes a new dataset that can be used to benchmark and improve the hadronic interaction models central to cosmic-ray research.
Better models should eventually mean better reconstructions of what cosmic rays are and where they come from. That is the long-term scientific payoff. If observatories can trust their shower simulations more, then disagreements in interpretation become less about the modeling itself and more about the astrophysics of the sources.
ATLAS has not solved the cosmic-ray puzzle in one stroke. What it has done is provide a new experimental foothold on one of its most stubborn uncertainties. By measuring proton-oxygen collisions directly, the collaboration has turned a particle collider into a tool for understanding phenomena that begin far above the planet and end, every second, in the air around us.
This article is based on reporting by Phys.org. Read the original article.
Originally published on phys.org
