Multiplicity dependence of two-particle angular correlations of identified particles in pp collisions at $\mathbf{\sqrt{s} = 13}$ TeV

In the heart of the universe, where matter is stripped down to its most fundamental building blocks, scientists study how particles are born and how they behave when they crash into one another. When two protons collide at nearly the speed of light, they do not simply bounce off; they shatter into a shower of new particles. These particles, which include pions, kaons, and protons, fly out in all directions. By measuring the angles at which these particles emerge relative to one another, physicists can reconstruct the invisible forces and rules that governed their creation. It is a bit like watching the debris from a shattered vase to understand how the vase was put together, but on a scale so small that the laws of quantum mechanics and the conservation of fundamental quantities like energy and electric charge dictate the outcome. For decades, researchers have used these collisions to test their theories about how the universe works, yet a specific pattern involving heavy particles called baryons has remained stubbornly difficult to explain.

A new study by the ALICE Collaboration at CERN's Large Hadron Collider has taken a fresh look at these particle collisions, this time focusing on how the number of particles produced in a single crash changes the way they relate to each other. The researchers analyzed data from proton-proton collisions at an energy of 13 tera-electronvolts, a record-breaking level of power that allows them to probe the subatomic world with unprecedented detail. They looked at billions of collisions, sorting them into groups based on how many charged particles were produced in each event, ranging from very quiet collisions with few particles to extremely busy ones with many. For each group, they measured the angular distance between pairs of particles, specifically looking at how often particles of the same type, such as two protons, appeared close together versus far apart.

The most striking discovery concerns pairs of protons and other heavy particles known as baryons. In previous experiments, scientists noticed that when two protons are produced in the same collision, they tend to avoid appearing close to each other in the direction of the collision. Instead of clustering together, they show a distinct "anti-correlation," meaning they are less likely to be found near one another than random chance would suggest. This paper confirms that this puzzling avoidance happens across all types of collisions, from the quietest to the most violent. What is more, the researchers found that this avoidance becomes even more pronounced when the collision produces a higher density of particles. This behavior challenges the current computer models used to simulate these events, which generally predict that particles should either cluster together or show no such strong preference to stay apart.

To understand why this matters, one must look at the different types of particles involved. The study examined pions, kaons, and protons, which are the primary products of these high-energy crashes. When the researchers looked at pairs of pions or kaons, the results matched existing theories quite well. These lighter particles showed a tendency to bunch together in the direction of the collision, a behavior driven by the creation of tiny jets of particles and the quantum nature of the particles themselves. However, the protons told a different story. While the lighter particles behaved as expected, the protons consistently refused to sit near each other. This "near-side" avoidance was observed in every multiplicity class, suggesting that the mechanism preventing protons from being close is a fundamental feature of how matter is created in these collisions, rather than a fluke of a specific type of event.

The researchers also introduced a new way of analyzing the data to strip away a simple mathematical effect that can hide the true physics. In previous studies, the way correlations were measured made it look like the effect was stronger in collisions with fewer particles. By adjusting their method to remove this scaling factor, the team revealed that the true strength of the proton avoidance actually grows as the number of particles in the collision increases. This finding is crucial because it points toward a complex environment where the creation of one proton influences the creation of another nearby. The data suggests that the current models, which rely on established theories of how strings of energy break apart to form particles, are missing a key ingredient. These models fail to reproduce the strong avoidance seen in the data, indicating that our understanding of how baryons are formed and how they interact is incomplete.

When the researchers compared their measurements to the predictions of four different computer models, the results were mixed. The models did a decent job of describing the behavior of the lighter pions and kaons, capturing the general shapes of the correlations. However, none of the models could accurately reproduce the specific pattern of proton avoidance observed in the experiment. Some models predicted that protons would cluster together, while others showed no strong pattern at all. The most sophisticated models, which attempt to account for the complex interactions of the particles, still fell short of matching the experimental data. This gap between the simulation and reality highlights a significant blind spot in our theoretical understanding of particle production.

The study also looked at how these correlations change when the particles are of opposite types, such as a proton and an antiproton. In these cases, the particles showed a different behavior, often appearing closer together, which is consistent with the idea that they are produced in pairs to conserve certain physical properties. This contrast further emphasizes that the avoidance seen in proton-proton pairs is a unique phenomenon specific to identical baryons. The researchers noted that this behavior persists even when the collision energy is high enough to create a dense environment where many particles are produced simultaneously. This suggests that the rules governing these interactions are robust and do not disappear even in the most chaotic conditions.

Ultimately, this work serves as a vital guide for theorists who are trying to refine their models of the subatomic world. The data provides a clear target: any successful theory must be able to explain why protons avoid each other in these collisions and why this effect strengthens as the collision becomes more crowded. The findings suggest that the current methods for simulating particle creation, which have been successful for decades, need to be updated to include new mechanisms that account for this baryon avoidance. Whether this involves new rules for how energy strings break apart or a deeper understanding of the quantum interactions between quarks, the path forward is now illuminated by these precise measurements. The study does not claim to have solved the mystery of baryon production, but it has firmly established the nature of the problem, providing the experimental community with the tools needed to push the boundaries of our knowledge further.