Full Breakdown
CERN Recreates Cosmic-Ray Showers with Oxygen-Proton Collisions
By Drooid · · How we work
The breakthrough experiment
On July 1 2025, scientists at the European Organization for Nuclear Research (CERN) used the Large Hadron Collider (LHC) to collide oxygen atoms with protons for the first time. In the laboratory set-up, the proton beam simulated an incoming cosmic ray, while the oxygen beam represented Earth’s atmospheric nuclei. The resulting collisions generated a spray of secondary particles that mimics the earliest moments of a natural cosmic-ray air shower. Images of these events were captured by the ATLAS detector, a particle-tracking system the size of a football field that can record more than 200 million collision “photos” each day.
Why laboratory studies matter
Cosmic rays—high-energy nuclei that constantly bombard Earth—have been observed for over a century, beginning with Victor Hess’s balloon experiments in the early 20th century. While ground-based arrays such as the Telescope Array in Utah record the final particle cascade, researchers have long relied on computer simulations to infer the shower development. Different models have produced divergent predictions, creating uncertainty about the composition and origin of the primary cosmic rays. By reproducing the first interaction in a controlled collider environment, the CERN team provided direct data to benchmark those simulations.
Findings and precision gains
Analyzing ATLAS images, the collaboration measured both the number of particles produced and their energy distribution. The study reports that these measurements are over ten times more precise than the values predicted by existing atmospheric-shower models. This heightened accuracy stems from the detector’s silicon-sensor camera system, which offers fine-grained, high-speed tracking of each particle’s trajectory. The results also help differentiate how many high-energy cosmic rays consist of hydrogen versus heavier nuclei, a key clue to their astrophysical sources.
Broader implications
The experiment bridges particle physics and high-energy astrophysics, two fields that share a common heritage but have operated largely independently. By supplying empirical data on the initial stage of cosmic-ray showers, the work is expected to refine models used in interpreting observations from ground-based telescopes and satellite detectors. Improved composition estimates could, in turn, narrow down the astrophysical environments—such as supernova remnants or jets from supermassive black holes—that accelerate particles to extreme energies. The researchers hope that continued collaboration between collider and astrophysics communities will further unravel the most energetic phenomena in the universe.