Physicists zoom into the birth of cosmic rainstorms with new CERN study

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Jesse Liu, New York University

(THE CONVERSATION) Every second, particles zip through your body at nearly the speed of light. They rain down from a storm high in the sky, where cosmic rays, a powerful type of interstellar matter, are constantly striking atoms in Earth’s atmosphere. The impacts break them apart into a shower of particles that rain to the ground.

Scientists have studied these cosmic rainstorms for over a century, but they lack a precise understanding of how they form. Many researchers are searching for a clearer picture, and they don’t even need to look to the sky to study them. They can instead use a particle collider, CERN’s Large Hadron Collider in Switzerland.

With a team of particle physicists, I study how these cosmic rainstorms are born. We used the world’s first laboratory collisions of oxygen atoms with protons to simulate cosmic rays in the lab. Our results are now published in Physical Review Letters.

What are cosmic rays?

In the early 20th century, physicist Victor Hess discovered cosmic rays aboard hot air balloons. While ascending several kilometers, his instruments found radiation levels unexpectedly kept rising. He deduced these rays must come from space and received the 1936 Nobel Prize in physics for this discovery.

The surprises did not stop there. Experiments later found this cosmic radiation harbored new kinds of matter called antimatter. These new kinds of matter included muons, pions and kaons. These particles are similar to the electrons, protons and neutrons inside atoms, but they exist fleetingly and are harder to see. These building blocks of matter helped fill out the modern theory of particle physics.

Today, scientists understand cosmic rays as fast-moving nuclei – the heavy centers of atoms – from mostly hydrogen. But open questions remain. Where do they come from? How do they reach such high energies?

Scientists are uncovering clues that suggest cosmic rays come from extremely hot and faraway areas of the universe, ejected by exploding stars and powerful objects called supermassive black holes.

Cosmic rays are also helpful for other fields. In the 1950s, scientists discovered they could use them for carbon dating of ancient artifacts, revolutionizing archaeology. More recently, cosmic rays have helped geologists see hidden chambers of active volcanoes, and archaeologists use them to reveal hidden tombs in ancient Egyptian pyramids. This imaging works by measuring how rock deflects the cosmic particles flying at these objects.

How are cosmic showers born?

The usual way to study cosmic rainstorms is by placing hundreds of specialized cameras across several miles of land. Together, these cameras act as one telescope. One of the biggest is the Telescope Array project in the high desert of Utah. This experiment sees the highest energy particles from the cosmos.

To decode information about the cosmic matter bombarding Earth, scientists must use computers to simulate the rainstorm of particles in air. But there is a long-standing problem: Different computer models vary in their predictions for how particle showers form in the sky.

Scientists need to know which model, if any, describes real collisions in the sky. To find out, they need data. No previous experiment has zoomed into the birth of these particle showers, so for several years I collaborated with physicist Lydia Beresford to make the case for reconfiguring our existing instruments at the CERN particle collider to study cosmic rays.

Oxygen beams to recreate cosmic rays in the lab

On July 1, 2025, scientists at CERN pioneered this experiment. They made oxygen atoms collide with protons for the first time using the Large Hadron Collider, the most powerful particle accelerator in the world. For a few days, they recreated cosmic rainstorms in the lab.

In this new experiment, a proton beam acted as the cosmic ray, while the oxygen beam played the role of Earth’s atmosphere. The energy in each collision converted into a spray of particles, recreating the first moments of a cosmic rainstorm.

At the Large Hadron Collider, several particle cameras take photos of collisions. One of these cameras that I work on is called the ATLAS experiment. This giant instrument is the size of a football field, which thousands of scientists work together to run.

The high-speed camera at the heart of ATLAS can take over 200 million photos in a day. It uses a large set of silicon sensors to take close-up portraits of particles, similar to the sensors in your phone camera.

First close-up photos of cosmic rainstorms

Along with a small team of physicists, we analyzed these images and used the photos to measure both how many particles the collisions created and at what energies they flew out from the collisions. We were able to measure the properties of these particles over 10 times more precisely than what computer models predicted.

Our results will help pinpoint how many high-energy cosmic rays are made of hydrogen, compared to heavier atoms. Understanding their composition could provide important clues about their origins. We were excited to share such valuable data. So we promptly sought feedback from experts in the ATLAS collaboration, then received the green light to go publicwith our results.

Our study renews the links between particle physics and high-energy astrophysics. Both fields have lots of overlap and shared history, but they remain separate fields, with much to learn from collaborating with each other. My colleagues and I hope these results will improve scientists’ knowledge of cosmic rainstorms and help unravel the most extreme events in the universe.

This article is republished from The Conversation under a Creative Commons license. Read the original article here: https://theconversation.com/physicists-zoom-into-the-birth-of-cosmic-rainstorms-with-new-cern-study-280890.