How ATLAS Turned the LHC Into a Cosmic-Ray Laboratory

In July 2025, the Large Hadron Collider (LHC) was configured for proton–oxygen collisions for the first time. For ATLAS, the setup provided a controlled laboratory analogue of the first interaction that occurs when a cosmic ray strikes Earth’s atmosphere. 69

A proton stands in for a cosmic ray, oxygen for the atmosphere

In this comparison, the proton beam represents the incoming particle from space, while oxygen ions represent atmospheric nuclei. Earth’s atmosphere is primarily nitrogen and oxygen, making proton–oxygen collisions a direct way to investigate a crucial opening stage of an atmospheric particle shower. 9

The collisions took place at 9.62 TeV. ATLAS did not attempt to recreate an entire shower, which can spread through many kilometres of atmosphere. Instead, it precisely measured the microscopic first collision that sets the shower in motion. 6

What ATLAS measured

The analysis used 246 million selected events, each containing at least one charged-particle track with transverse momentum above 500 MeV and within the detector range of |η| < 2.5. 9

Using those charged-particle tracks, the collaboration measured:

• particle-production rates and multiplicities;
• transverse-momentum distributions;
• pseudorapidity distributions, which describe emission angles relative to the beam direction; and
• interaction cross sections. 69

The measured proton–oxygen cross section within the experiment’s defined region was 396 ± 6 (experimental) ± 9 (luminosity) mb. After a theoretical extrapolation to proton–air collisions, the inelastic cross section was estimated at 406 ± 6 (experimental) ± 9 (luminosity) ± 28 (theory) mb. 9

According to the ATLAS summary, uncertainties on the multiplicity, transverse-momentum and pseudorapidity measurements are only a few percent—around an order of magnitude smaller than the differences among the hadronic-interaction models used in air-shower simulations. 69

Why these results matter

Ground-based cosmic-ray observatories generally do not see the original particle directly. They observe the secondary particles it creates in the atmosphere, then infer the primary particle’s energy and, potentially, its composition. That inference relies on simulations of strong-force, or hadronic, interactions—and existing models do not fully agree. 6

The proton–oxygen data give model builders laboratory measurements with which to tune interaction probabilities, particle yields, momentum distributions, emission angles and cross sections. The extrapolation to proton–air interactions connects the result even more closely to the real atmosphere. 69

What this could reveal about cosmic rays

A primary proton and a heavier atomic nucleus produce showers that develop differently and fluctuate in different ways. Better-calibrated simulations could therefore reduce a major systematic uncertainty in determining whether the highest-energy cosmic rays are mostly hydrogen nuclei or heavier elements.

That is a potential benefit of the new ATLAS data—not a direct conclusion of the measurement itself. 69

Cosmic-ray composition also matters for interpreting possible astrophysical accelerators. Magnetic deflection and the maximum energy a particle can gain both depend on nuclear properties. More reliable composition measurements could therefore strengthen tests of source scenarios involving, for example, explosive stellar environments or regions around supermassive black holes. But this ATLAS measurement does not identify cosmic-ray sources or prove a particular acceleration mechanism. 69

The key limitation

The LHC collisions are a calibration of the first, decisive interaction in an air shower—not a full reproduction of the cascade through the atmosphere. Their importance is that they replace part of the theoretical uncertainty with controlled collision data, improving the physics inputs used to simulate cosmic rays. 69