Oxygen Collisions Reveal Partial Thermal Equilibrium in Quark-Gluon Plasma

When two oxygen nuclei collide at nearly the speed of light inside the Large Hadron Collider at CERN, they create a fleeting state of matter that hasn't existed naturally since the first microseconds after the Big Bang. This is quark-gluon plasma—a soup so hot that the fundamental building blocks of atoms break free from their usual confinement. For years, physicists have watched these collisions and wondered: does the matter produced actually settle into a stable, equilibrated state that behaves like a fluid, or does it remain a chaotic tangle of particles?

A research team led by Professor Tetsufumi Hirano at Sophia University in Tokyo set out to answer that question with precision. Working with graduate student Naoya Ito, Hirano used a mathematical framework called the dynamical core-corona initialization model to separate the collision debris into two categories: matter that reaches thermal equilibrium, which they called the "core," and particles that remain out of equilibrium, labeled the "corona." The distinction matters because only equilibrated matter behaves like a fluid and can be described using the equations of relativistic hydrodynamics. Their results, published in Physical Review C on August 12, 2026, reveal something unexpected: oxygen collisions exist in a middle ground.

The team modeled collisions at an energy of 5.36 teraelectronvolts and tracked how the balance between core and corona shifted as the collision became more violent, measured by counting charged particles produced near the collision's center. The findings showed a clear threshold. When the number of charged particles exceeded about 20, the equilibrated core began to dominate over the non-equilibrated corona. But here is the crucial detail: even in the most violent, head-on collisions, the corona never disappeared. It persisted, accounting for roughly 30 percent of all the particles produced. This means that even under the most extreme conditions oxygen collisions can create, a substantial fraction of the matter remains out of equilibrium.

The researchers dug deeper into the particle data and found that the core contribution was strongest among slower-moving particles, while faster particles came disproportionately from the corona. For heavier particles, this transition occurred at even higher speeds, a signature of the collective push they received from the expanding equilibrated core. When the team examined the production of strange baryons—exotic particles containing a strange quark—they found another clue. The ratios of these particles to ordinary pions increased with collision intensity but fell short of what complete chemical equilibrium would predict. This gap provided independent confirmation that a non-equilibrated component persists throughout.

The implications reach beyond abstract physics. Hirano explained that understanding whether oxygen collisions produce equilibrated matter is essential because scientists want to use the particle distributions from these collisions as a window into the internal structure of oxygen nuclei themselves—specifically, whether oxygen nuclei might be organized into clusters of alpha particles. If the produced matter does not behave like a fluid, those interpretations become unreliable. The new work provides a quantitative foundation for making sense of oxygen collision data and clarifies the conditions under which quark-gluon plasma actually forms in smaller collision systems.

The findings also expose a limitation in current theory. Relativistic hydrodynamics—the mathematical framework that describes fluid behavior—cannot fully account for oxygen collisions on its own. The persistent non-equilibrated component means that any complete description must include both the equilibrated core and the corona. This opens a path forward. The team plans to extend their analysis to neon-neon collisions and other intermediate systems, mapping out how nuclear structure and the degree of equilibration shape the matter produced across a range of collision sizes. For the first time, physicists have a quantitative measure of how far oxygen collisions approach the fluid-like behavior seen in larger systems, and how far they fall short.