Evidence of nuclear geometry-driven anisotropic flow in OO and Ne$-$Ne collisions at $\mathbf{\sqrt{{\textit s}_{\rm\mathbf {NN}}}}$ = 5.36 TeV

In the earliest moments of the universe, just after the Big Bang, matter existed in a state unlike anything we see today. It was a seething, super-hot soup of fundamental particles called quarks and gluons, which normally bind together to form protons and neutrons. Physicists call this state the quark-gluon plasma. To understand how this primordial soup behaved, scientists smash heavy atomic nuclei together at nearly the speed of light, recreating these extreme conditions for a fleeting instant. When these collisions happen, the particles fly out in specific patterns. If the particles move together in a coordinated way, flowing like a liquid rather than bouncing off each other like billiard balls, it suggests that a tiny droplet of this plasma has formed. This coordinated movement is known as "flow," and it is one of the strongest pieces of evidence that the matter created in these collisions behaves like a nearly perfect fluid.

For decades, researchers focused on smashing together the heaviest atoms available, such as lead or gold, to create the largest possible droplets of this plasma. However, a lingering question remained: could such a fluid form in much smaller collisions, involving lighter atoms? If the droplet is too small, it might not have enough time or space to develop this fluid-like behavior. To answer this, scientists turned to two specific light atoms: oxygen and neon. While both are small, they have different internal shapes. Oxygen atoms are arranged in a specific, rigid geometric pattern, while neon atoms are shaped somewhat like a bowling pin. By colliding these atoms against themselves, researchers could test whether the specific shape of the starting atoms leaves a fingerprint on the final flow of particles, proving that even in these tiny systems, the matter behaves collectively.

The ALICE collaboration at the Large Hadron Collider recently performed the first measurements of this phenomenon in collisions of oxygen and neon nuclei. They gathered data from billions of oxygen-oxygen collisions and hundreds of millions of neon-neon collisions, all occurring at a center-of-mass energy per nucleon pair of 5.36 TeV. The team focused on measuring how the particles spread out in different directions after the crash. They looked for two specific patterns: an elliptical flow, where particles prefer to move in an oval shape, and a triangular flow, where they move in a three-pointed star shape. These patterns are not random; they are driven by the initial shape of the overlapping nuclei at the moment of impact. If the starting atoms are shaped like a bowling pin, the resulting fluid should expand in a way that reflects that pin shape.

The results showed a clear signal of this collective behavior. In both oxygen and neon collisions, the particles exhibited the expected elliptical and triangular flows. Crucially, the strength of this flow changed depending on how central the collision was—meaning how directly the two nuclei hit each other. In the most direct, central collisions, the neon-neon collisions produced a stronger elliptical flow than the oxygen-oxygen collisions. This difference aligns perfectly with the known shapes of the two atoms. Because neon has a more pronounced elongated shape compared to the more compact oxygen, the fluid created in neon collisions expands more strongly in that direction. This observation confirms that the initial geometry of the colliding nuclei drives the flow, even in these very small systems.

To verify these findings, the researchers compared their measurements with sophisticated computer simulations that included detailed models of the internal structure of oxygen and neon atoms. These models, derived from advanced nuclear physics theories, predicted that oxygen atoms have a tetrahedral arrangement of their internal parts, while neon atoms form a structure resembling a bowling pin. The simulations, which treated the collision as a hydrodynamic expansion of a fluid, matched the experimental data remarkably well. The agreement was so precise that it surpassed the accuracy of previous studies on heavier nuclei. This match suggests that the matter created in these light-ion collisions does indeed behave as a fluid governed by the laws of hydrodynamics, responding directly to the geometric shapes of the colliding atoms.

The study also revealed that the triangular flow, which arises from random fluctuations in how the particles are arranged inside the nuclei, behaved differently than the elliptical flow. In central collisions, the triangular flow was weaker in neon than in oxygen, a result that the simulations also captured. This difference highlights the unique internal structure of the oxygen atom, which creates a larger triangular distortion in the initial collision zone. By comparing the two systems directly, the researchers were able to cancel out many of the confusing factors that usually complicate these measurements, isolating the effect of the nuclear shape itself.

These findings provide strong evidence that the quark-gluon plasma can form in systems much smaller than previously thought, provided the collision energy is high enough. The fact that the flow patterns are so sensitive to the specific shapes of oxygen and neon suggests that these collisions offer a new, powerful way to study the structure of atomic nuclei. Instead of just looking at how nuclei behave at low energies, scientists can now use high-energy collisions to "image" the arrangement of particles inside them. The data confirms that the initial geometry of the nuclei is the primary driver of the flow, and that the resulting matter expands as a nearly perfect fluid. This work bridges the gap between the study of heavy ions and smaller systems, showing that the emergence of collective fluid behavior is a robust feature of quantum chromodynamics, the theory that describes the strong force holding matter together.