CERN Recreates Cosmic-Ray Collisions on Earth to Solve a Mystery From Deep Space

Scientists at CERN have turned the Large Hadron Collider into an artificial cosmic-ray laboratory, recreating the type of particle collisions that occur when high-energy particles from deep space smash into Earth’s atmosphere.

The experiment, carried out by researchers working with the ATLAS detector, marks the first measurement at the LHC of collisions between protons and oxygen ions for this purpose.

The result could help physicists understand one of the most energetic natural phenomena constantly occurring above our heads: cosmic-ray particle showers.

These showers begin when extremely energetic particles from space strike nuclei in Earth’s upper atmosphere, producing cascades of secondary particles that eventually travel toward the ground.

Scientists can detect those secondary particles, but interpreting them accurately depends on computer simulations.

And there is a problem.

Different simulations do not always agree.

The new CERN measurements give physicists experimental data that can be used to improve those models.

What Are Cosmic Rays?

Cosmic rays are high-energy particles that travel through space and constantly bombard Earth.

Many are protons, while others are heavier atomic nuclei.

When one of these particles reaches Earth’s atmosphere, it can collide with a nucleus of an atmospheric atom.

That collision produces a huge cascade of secondary particles.

The result is known as an air shower.

Although the original cosmic ray may have traveled enormous distances through space, scientists on Earth can study the secondary particles created by its interaction with the atmosphere.

According to CERN, approximately one particle from cosmic-ray showers passes through a person’s head every second.

That means cosmic radiation is not something that happens only in distant galaxies.

It is happening around us continuously.

CERN Has Now Recreated Part of That Process

The ATLAS Collaboration wanted to reproduce these interactions under controlled laboratory conditions.

The solution was to use the world’s largest particle accelerator.

In the experiment, a beam of protons represented the incoming cosmic ray.

A beam of oxygen ions represented part of Earth’s atmosphere.

When the two beams collided inside the Large Hadron Collider, the resulting interactions produced particles that could be measured in detail.

It effectively allowed scientists to bring a tiny version of a cosmic-ray collision into the laboratory.

Why Use Oxygen?

Earth’s atmosphere is made mostly of nitrogen and oxygen.

Cosmic rays therefore do not simply collide with isolated protons.

They interact with much larger atomic nuclei.

That matters because the physics of the resulting particle shower depends on the type of nucleus involved.

By colliding protons with oxygen ions, researchers can study an interaction that more closely resembles what happens when cosmic rays encounter Earth’s atmosphere.

The experiment therefore provides a bridge between particle physics performed at CERN and cosmic-ray observations made by detectors on Earth.

The Large Hadron Collider Became a Cosmic-Ray Machine

The Large Hadron Collider is normally associated with collisions between beams of particles accelerated to enormous energies.

Its best-known discoveries include the Higgs boson.

But the machine can be configured for different types of collisions.

In July 2025, ATLAS researchers used the LHC to collide protons with oxygen ions for the first time in this configuration.

The new analysis measures what happened during those collisions and compares the results with the computer simulations used by cosmic-ray researchers.

Why Do Scientists Need Simulations?

Astronomers cannot simply catch the original cosmic ray after it hits the atmosphere.

The initial particle interacts with the atmosphere and creates a cascade.

Scientists then observe the particles that reach detectors on or near the ground.

To reconstruct what happened at the top of the atmosphere, researchers use computer models.

Those models simulate the complicated chain of particle interactions.

The problem is that the underlying physics involves the strong nuclear force, which is extremely difficult to calculate at the energies involved.

Different simulation models can therefore produce different predictions.

That uncertainty makes it harder to determine the properties of the original cosmic rays.

CERN’s New Data Can Test Those Models

This is where the new ATLAS measurement becomes important.

Researchers measured several properties of the particles created in the proton-oxygen collisions.

They examined:

• How many charged particles were produced
• How frequently different collision outcomes occurred
• The energies of the particles
• The angles at which particles emerged
• The tracks left inside the detector

They then compared the experimental results with predictions from existing cosmic-ray simulation models.

The models did not all agree with each other.

The new measurements therefore give theorists a much stronger experimental reference point.

The Measurements Are Precise to a Few Percent

ATLAS reports that the new measurements achieve precision at the level of a few percent.

That is significant because small differences in the behavior of particle collisions can become important when researchers simulate enormous atmospheric particle showers.

A model that is slightly wrong at the collision level can produce increasingly different predictions as the simulated shower develops.

Improving the initial physics therefore helps improve the interpretation of observations made many kilometers away.

Why Cosmic Rays Matter

Cosmic rays are more than a curiosity.

They provide information about some of the most energetic processes in the universe.

Scientists believe cosmic rays can originate from extreme astrophysical environments, including exploding stars and other powerful cosmic accelerators.

Some cosmic rays reach energies far beyond what conventional human-made accelerators can produce.

That makes them natural probes of physics under extreme conditions.

But scientists need to understand how those particles interact with matter before they can fully interpret the signals detected on Earth.

A Cosmic Ray Can Start a Particle Avalanche

Imagine one extremely energetic proton traveling toward Earth.

It enters the atmosphere and collides with an atomic nucleus.

That collision creates new particles.

Those particles collide with additional atmospheric nuclei.

More particles are produced.

Those particles create still more interactions.

The result is a rapidly expanding cascade of particles.

Eventually, some of those particles reach the ground.

Large observatories can detect them.

By measuring the shower, scientists attempt to reconstruct the properties of the original cosmic ray.

The process is similar to investigating an explosion by examining the debris — except the explosion occurs high above Earth and involves subatomic particles.

The Atmosphere Becomes Part of the Experiment

One of the fascinating aspects of cosmic-ray research is that Earth itself acts as part of the detector system.

The atmosphere provides the material that cosmic rays collide with.

Those collisions create secondary particles.

Scientists then use detectors on the ground to observe the resulting shower.

But because the atmosphere is complicated, researchers need accurate physics models to interpret what they see.

CERN’s proton-oxygen experiment gives them a laboratory-controlled way to test an important part of that chain.

What Did ATLAS Actually Discover?

It is important not to overstate the result.

The experiment did not discover a new fundamental particle.

It did not identify the source of cosmic rays.

And it did not solve the entire mystery of ultra-high-energy cosmic rays.

Instead, ATLAS produced a new, precise measurement of proton-oxygen collisions that occur in the laboratory and can be used to improve the simulations of cosmic-ray air showers.

That may sound less dramatic than a new particle discovery, but it is scientifically important because better models can improve the interpretation of future cosmic-ray observations.

Why the Result Matters for Astrophysics

Suppose an observatory detects an enormous particle shower.

Researchers want to know how energetic the original cosmic ray was and where it came from.

To answer those questions, they need to compare the observed shower with simulations.

If the simulation is inaccurate, the inferred properties of the cosmic ray can also be inaccurate.

Better measurements of proton-oxygen interactions can therefore reduce uncertainties in those calculations.

That could help researchers better understand the extreme objects and environments that accelerate cosmic rays.

The Experiment Connects Two Worlds of Physics

The project brings together two areas that are often studied separately.

The first is particle physics.

Scientists use accelerators to study the fundamental interactions between particles.

The second is astroparticle physics.

Scientists observe particles arriving from space and attempt to determine where they came from and how they were produced.

The CERN experiment creates a direct connection between the two.

Laboratory measurements can improve astrophysical simulations, while observations of cosmic rays can reveal questions that particle physicists need to investigate.

What Happens When a Cosmic Ray Hits Earth?

A simplified chain looks like this:

Cosmic ray → atmospheric collision → secondary particles → particle shower → ground-based detection

The original cosmic ray may have crossed enormous distances before reaching Earth.

The first collision happens high in the atmosphere.

That collision produces a large number of secondary particles.

Some of those particles generate additional collisions.

Eventually, a fraction of the resulting particles reach the Earth’s surface.

Scientists detect those particles and use the data to reconstruct the original event.

Why Oxygen Collisions Were Difficult to Study

Particle collisions involving heavier nuclei are more complicated than simple proton-proton collisions.

An oxygen nucleus contains many protons and neutrons.

When a high-energy proton strikes it, the resulting interaction can involve many particles and complicated internal processes.

That makes the theoretical modeling challenging.

The LHC provides a way to measure these interactions directly under controlled conditions.

The experimental data can then be compared with theoretical predictions.

The New Results Challenge Existing Models

ATLAS researchers compared their measurements with several simulation models currently used in cosmic-ray studies.

The predictions showed noticeable differences.

That means scientists still have work to do before all models can accurately describe these collisions.

The new measurements provide a benchmark.

Theorists can now adjust their models to reproduce the experimental data more accurately.

Could This Help Explain the Most Powerful Cosmic Rays?

Potentially.

The universe produces cosmic rays with extraordinary energies.

Some are far more energetic than the particles scientists can accelerate directly in laboratories.

Understanding how cosmic rays interact with atmospheric nuclei is essential for interpreting observations of these extreme events.

The CERN measurements are only one part of that effort, but they improve an important piece of the puzzle.

What Scientists Want to Learn Next

Researchers can now use the proton-oxygen data to refine the models used in cosmic-ray physics.

Future experiments could investigate additional collision configurations and energies.

More accurate models could then be incorporated into simulations used by large cosmic-ray observatories.

Over time, this could improve measurements of the energy and composition of cosmic rays arriving at Earth.

Scientists could then use those improved measurements to investigate their astrophysical origins.

A Telescope Cannot Do This Experiment

A telescope observes what naturally happens in the universe.

The LHC does something different.

It creates controlled conditions that allow scientists to isolate a particular physical interaction.

This is why accelerator experiments remain valuable even when astronomers can observe cosmic rays directly.

The universe provides the extreme conditions.

CERN provides a laboratory where parts of those conditions can be reproduced and measured precisely.

Why This Matters Beyond CERN

The research could ultimately benefit observatories located far from Switzerland.

Cosmic-ray observatories cover huge areas because particle showers can spread across large distances.

Their measurements depend on models of how those showers develop.

Improving the underlying collision physics can therefore improve the interpretation of data from experiments around the world.

The result is a good example of how a particle accelerator on Earth can help scientists understand events that begin billions of kilometers away.

Key Facts

• Laboratory: CERN
• Experiment: ATLAS
• Accelerator: Large Hadron Collider
• Collision: Proton–oxygen
• Purpose: Study interactions relevant to cosmic-ray air showers
• First LHC proton-oxygen measurement: 2025
• Results reported: 2026
• Precision: A few percent
• Main problem addressed: Differences between cosmic-ray simulation models
• Potential benefit: Better interpretation of high-energy cosmic rays
• Cosmic-ray showers: Produced when energetic particles from space strike Earth’s atmosphere

Frequently Asked Questions

What did CERN recreate?

CERN’s ATLAS experiment recreated proton-oxygen collisions that help scientists model what happens when high-energy cosmic rays interact with Earth’s atmosphere.

Did CERN create cosmic rays?

No. Researchers created controlled particle collisions that mimic an important type of interaction involving cosmic rays.

What are cosmic rays?

Cosmic rays are high-energy particles arriving from outer space. When they strike Earth’s atmosphere, they generate showers of secondary particles.

Why did scientists use oxygen?

Earth’s atmosphere contains large amounts of oxygen and nitrogen. Using oxygen ions provides a laboratory system that is more representative of atmospheric nuclei than simple proton-proton collisions.

Did CERN discover a new particle?

No. The experiment produced a new measurement of proton-oxygen collisions rather than a new fundamental particle.

Why are cosmic rays important?

They provide information about extremely energetic processes in the universe and can reach energies beyond those routinely produced by human-made accelerators.

How does this help scientists?

The measurements can be used to improve computer simulations of cosmic-ray air showers, potentially making it easier to determine the properties and origins of cosmic rays detected on Earth.

The Bigger Picture

Every second, high-energy particles from space strike Earth’s atmosphere.

Most people never notice.

There is no visible explosion and no sound.

But each collision can trigger a cascade involving enormous numbers of subatomic particles.

For scientists, those invisible showers contain information about some of the most energetic environments in the universe.

The challenge has always been figuring out exactly what the showers are telling us.

CERN’s latest work provides a new piece of that puzzle.

By recreating proton-oxygen collisions inside the Large Hadron Collider, ATLAS researchers have created a controlled laboratory version of an interaction that normally happens high above our planet.

The measurements can now be used to improve the simulations that connect cosmic-ray observations to the physics that produced them.

The result is not a new particle or a dramatic discovery of an unknown cosmic object.

It is something more fundamental:

a better experimental understanding of how particles from deep space interact with our atmosphere.

And that improved understanding could ultimately help scientists trace some of the most energetic particles in nature back to their mysterious cosmic sources.

Sources & References

Primary source: CERN ATLAS Collaboration — ATLAS turns into a cosmic-ray laboratory with proton-oxygen collisions.

CERN overview: CERN — ATLAS acts as a cosmic-ray laboratory.

Background: CERN — LHCf: particles from the LHC used to simulate cosmic rays.

Editorial note: This article distinguishes between laboratory measurements and cosmic-ray observations. The CERN experiment does not recreate an entire cosmic-ray shower or identify the astrophysical source of individual cosmic rays; it measures proton-oxygen interactions that are important inputs for air-shower modeling.