LHC reveals oxygen and neon nuclei have dynamic, shifting shapes

The Large Hadron Collider at CERN is famous for chasing exotic particles and testing the boundaries of physics, but a new study published in Physical Review Letters shows it can also reveal something unexpected about atoms we thought we already understood. When researchers smashed oxygen and neon nuclei together at nearly the speed of light, the collision patterns told them something textbooks have gotten wrong: the nuclei of these familiar elements don't have fixed, well-defined shapes. They shift and change, their geometry determined by energy states and probability.

When atomic nuclei collide at extreme speeds, they create a fleeting state of matter called quark-gluon plasma—an extraordinarily hot soup of fundamental particles that behaves like a liquid. The particles streaming out of these collisions don't scatter randomly. Instead, they flow in coordinated directions, exhibiting collective behavior that mirrors the initial geometry of the collision itself. By studying this flow pattern, physicists can work backward to infer what the nuclei actually looked like at the moment of impact.

Previous experiments had used this technique on heavier nuclei like uranium, and hints of similar flow patterns had appeared in proton collisions too. But oxygen and neon offered something cleaner: a more direct window into nuclear structure. Unlike proton-based collisions, where the internal structure of the proton itself muddies the picture, collisions between two identical light ions are governed primarily by how the nucleons—the protons and neutrons—are spatially arranged. This makes them ideal laboratories for testing whether nuclei really do have the shapes that theoretical calculations predict.

Theory had proposed that oxygen-16 has a tetrahedral arrangement of nucleons, while neon-20 is more elongated, resembling a bowling pin. If these predictions were correct, the two nuclei should behave differently when they collide, even though they have similar mass numbers. The research team, analyzing data from the CMS detector at the LHC with collisions at 5.36 TeV per nucleon pair, found exactly that. In head-on collisions, neon produced a stronger elliptic-flow signal than oxygen—a pattern consistent with neon having a less spherical, more deformed internal structure.

The triangular flow measurements, however, told a more complicated story. Instead of matching theoretical predictions, the triangular-flow ratio moved in the opposite direction as collisions became more head-on. This mismatch suggests that current theoretical models don't fully capture what happens when these nuclei collide. The researchers acknowledge that while their results qualitatively support the idea that neon-20 is indeed deformed compared to oxygen-16, they haven't yet pinned down the exact shape. Translating flow measurements into precise nuclear geometry requires more work, and better theoretical treatment of how event-by-event fluctuations affect small collision systems.

What emerges from this work is a portrait of atomic nuclei as dynamic, fluid-like objects whose structure reveals itself only through violent collision. The nucleus is not a static thing drawn in a textbook. It is a probability cloud, a shape that exists in superposition until the moment of impact forces it to declare itself through the spray of particles it releases. The LHC, in smashing these familiar atoms together, has shown us that even the most elementary building blocks of ordinary matter still hold surprises.