Jet-like dihadron correlations in OO and Ne--Ne collisions at $\sqrt{s_{\rm NN}} = 5.36$ TeV with ALICE

For decades, physicists have been smashing heavy atomic nuclei together at nearly the speed of light to recreate the conditions of the universe just moments after the Big Bang. In these violent collisions, the protons and neutrons that usually make up atomic nuclei melt away, forming a hot, dense soup of their fundamental building blocks: quarks and gluons. This state of matter is called the quark-gluon plasma. Scientists have long believed that this plasma behaves like a perfect fluid, flowing and expanding in ways that can be predicted by the laws of hydrodynamics, much like water flowing through a pipe. However, a puzzling discovery changed the conversation. Researchers found that even when they smashed together much smaller particles, like a single proton hitting a heavy lead nucleus, this fluid-like behavior still appeared. This raised a fundamental question: how small can a system be before it stops acting like a fluid? To answer this, scientists needed to study collisions that were smaller than the massive lead-on-lead crashes but larger than the tiny proton collisions, effectively bridging the gap between the known and the unknown.

To explore this middle ground, a team of researchers using the ALICE detector at the Large Hadron Collider turned their attention to collisions involving light atomic nuclei. Specifically, they looked at collisions between oxygen nuclei and oxygen nuclei, as well as neon nuclei and neon nuclei. These are not the heavy, slow-moving giants of previous experiments, but lighter, faster-moving projectiles. The team recorded these collisions at an energy level of 5.36 trillion electron volts per nucleon pair, a scale high enough to potentially create the same hot, dense matter seen in larger systems. Their goal was to see if the signatures of a fluid-like medium would appear here, and if so, how they compared to the well-understood behavior of the massive lead collisions. They focused on a specific phenomenon: what happens when a high-speed particle, born from the collision, tries to punch its way through this potential soup of matter.

In the world of high-energy physics, when two particles collide, they often produce a pair of high-speed particles that shoot off in opposite directions. Scientists call this a "jet." If the collision happens in a vacuum, these two particles fly away cleanly, balancing each other out. But if the collision happens inside a dense, fluid-like medium, the particle trying to move through the soup loses energy, much like a runner trying to sprint through deep water. This energy loss changes the pattern of the particles that emerge. The researchers looked for these changes by measuring the relationship between pairs of particles produced in the oxygen and neon collisions. They focused on two specific directions: the "near side," where particles fly out in roughly the same direction as the initial trigger, and the "away side," where particles fly in the opposite direction. By comparing the number of particles found in these directions to what is seen in standard proton-proton collisions, they could determine if the medium was interfering with the particles' paths.

The results revealed a subtle signal of interaction. In the oxygen collisions, the researchers found that at lower speeds, there were more particles than expected on both the near and far sides, suggesting that the environment was adding extra activity to the mix. However, as they looked at faster, higher-energy particles, a distinct pattern emerged. On the side opposite to the initial trigger, the number of particles dropped significantly below what was seen in the standard proton collisions. This suppression is qualitatively consistent with jet–medium interactions, suggesting that the particles traveling in the opposite direction were losing energy as they moved through the medium created by the collision. It is a sign that the oxygen nuclei, despite being much smaller than lead, were capable of creating an environment dense enough to slow down high-speed particles. The effect was not as strong as what is seen in the massive lead collisions, but it was there, suggesting that the fluid-like behavior extends to these smaller systems.

Interestingly, the researchers also observed that the cluster of particles flying in the same direction as the trigger became slightly wider as the number of particles in the collision increased. This broadening suggests that the energy of the jet is being redistributed, perhaps scattering slightly as it interacts with the surrounding matter. When they compared these findings to the massive lead collisions, they found that the behavior of the central oxygen collisions was surprisingly similar to the outer edges of the lead collisions. This similarity provides a crucial link, showing that the transition from small to large systems is not a sudden jump but a gradual change in how the matter behaves. The data suggests that even in these light-ion collisions, the quark-gluon plasma is forming, and it is interacting with the high-speed particles passing through it.

However, the story is not yet fully complete. While the suppression of particles on the far side strongly hints at the presence of a fluid medium, the researchers caution that other factors could be at play. Effects related to the initial state of the nuclei or the way the collisions are selected for study could also influence the results. The team acknowledges that the extent to which these alternative explanations contribute to the observed suppression remains an open question, as they were not addressed in this analysis. The current findings are a strong indication, a qualitative match to what is expected if a medium is present, but they are not a definitive proof. The next step for the scientific community is to combine these measurements with other types of observations, such as directly reconstructing the paths of the jets themselves, to build a more complete picture. For now, the study of oxygen and neon collisions has successfully opened a new window, showing that the mysterious, fluid-like state of matter created in the early universe can exist in systems much smaller than previously thought possible.