ATLAS Uses Proton-Oxygen Collisions to Recreate Cosmic-Ray Showers in the Lab

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.