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.

In the early 20th century, physicist Victor Hess discovered cosmic rays aboard hot air balloons. While ascending several kilometres, 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. Later experiments found this cosmic radiation harboured new kinds of matter called antimatter, including muons, pions, and kaons, which helped build the modern theory of particle physics.

Today, scientists understand cosmic rays as fast-moving atomic nuclei, mostly hydrogen. Evidence suggests they originate from extremely hot, distant areas of the universe, ejected by exploding stars and supermassive black holes. Cosmic rays are also helpful for other fields. In the 1950s, scientists discovered they could use them for radiocarbon dating of ancient artefacts, revolutionising archaeology. More recently, geologists have used them to scan hidden chambers inside active volcanoes, while archaeologists use them to reveal hidden tombs in ancient Egyptian pyramids by measuring how rock deflects flying particles.

To decode information about the cosmic matter bombarding Earth, scientists must use computers to simulate the particle rainstorm. But a long-standing problem remains: different computer models vary significantly in their predictions for how particle showers form in the sky. To know which model describes real collisions, scientists need precise data.

No previous experiment had zoomed into the birth of these particle showers. For several years, I collaborated with physicist Lydia Beresford to make the case for reconfiguring existing instruments at CERN to study cosmic rays. In July 2025, scientists at CERN pioneered this experiment, colliding oxygen atoms with protons for the first time.

A proton beam acted as the cosmic ray, while an 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. Using the ATLAS experiment — a giant detector the size of a football field — we analysed images captured by high-speed silicon sensors.