Multiplicity dependence of K$^*(892)^{\pm}$ production in pp collisions at $\sqrt{s}$ = 13 TeV

In the heart of the Large Hadron Collider, where protons smash together at speeds close to the speed of light, physicists are trying to understand how the universe builds itself from the ground up. When these tiny particles collide, they create a fleeting, super-hot soup of fundamental building blocks called quarks and gluons. As this soup cools down, it freezes into the stable particles that make up the world around us, such as protons and neutrons. However, the transition from this hot soup to solid matter is not instantaneous. There is a brief, chaotic middle stage where particles bounce off one another, trading energy and changing direction before finally settling down. To study this middle stage, scientists look for specific, short-lived particles that act like messengers. These messengers are born, travel for a tiny fraction of a second, and then decay into other particles. By measuring how many of these messengers survive the journey, researchers can deduce how long the chaotic middle stage lasted and how crowded it was.

A researcher using the ALICE detector at CERN has now taken a closer look at one such messenger, a particle known as the K-star, in collisions between protons. While protons are much smaller than the heavy atomic nuclei usually smashed together in these experiments, recent observations suggest that even in these tiny collisions, a similar, albeit smaller, version of that hot soup might form. The team focused on how the production of these K-star particles changes when the collisions are more crowded versus when they are less crowded. They found that in the most crowded proton collisions, the number of these short-lived messengers drops significantly compared to a more stable particle called the K-zero. This drop is a strong signal that the decay products of the K-star are getting scattered by other particles in the collision zone, preventing the K-star from being reconstructed later. This observation provides the first clear evidence of a significant suppression of the K-star to K-zero ratio in small collision systems, a finding that adds to the ongoing investigation into whether such complex behavior, typically associated with massive heavy-ion collisions, can also occur in smaller systems.

The researcher analyzed data from billions of proton collisions that occurred at a center-of-mass energy of 13 tera-electronvolts. They sorted these events into different groups based on how many charged particles were produced, ranging from sparse collisions with very few particles to extremely crowded ones with many. For each group, they reconstructed the K-star particles by looking for their decay products: a charged pion and a neutral K-zero particle. Because the K-star lives for such a short time, it cannot be seen directly; instead, the scientist had to piece it back together from the tracks left by its children. They also measured the production of the stable K-zero particles in the same events to serve as a reference point. By comparing the ratio of K-stars to K-zeros across the different levels of crowding, they could see how the environment affected the short-lived particles.

The results showed a clear and dramatic trend. As the collisions became more crowded, the ratio of K-stars to K-zeros decreased. In the most crowded events, this ratio was significantly lower than in the least crowded ones. The researcher calculated that this drop was not a random fluctuation but a real physical effect, with a statistical certainty corresponding to a 7σ significance level, meaning the probability of this occurring by chance is vanishingly small. This suppression of the K-star signal suggests that in the crowded events, the decay products of the K-star are colliding with other particles in the medium before the K-star can be identified. These collisions scramble the information needed to reconstruct the original particle, effectively hiding it from the detectors. This process is known as rescattering. The fact that this happens in proton collisions implies that even in these small systems, there is a dense enough environment for these interactions to occur, creating a short-lived hadronic phase where particles interact strongly with one another.

To understand if this was a new discovery or just a known effect, the researcher compared their findings with several computer models used to simulate particle collisions. Some of these models, which rely on standard theories of how particles interact, predicted that the ratio should remain relatively flat or overestimate the values at high multiplicities, failing to match the observed drop. These models failed to match the data. However, a model called EPOS-LHC, which includes a mechanism where strings of energy overlap and interact in a dense core, successfully reproduced the observed drop in the ratio. This model does not require a special "afterburner" to simulate the later stages of particle interactions; the suppression emerges naturally from the way the model handles the dense core of the collision. This suggests that the suppression is a natural consequence of the high density of particles in the collision zone.

The researcher also looked at how this effect changed with the speed of the particles. They found that the suppression was strongest for particles moving slowly. Fast-moving particles were less affected, likely because they zip through the dense medium too quickly to be scattered. This detail aligns with the idea that the rescattering process takes time; slow particles spend more time in the crowded environment, giving them more opportunities to collide with others and lose their identity. The researcher also noted that while the overall number of particles produced increased with the density of the collision, the average speed of the particles also increased, a phenomenon often seen in larger, heavy-ion collisions and usually attributed to a collective expansion of the system.

This study marks a significant step forward in understanding the nature of matter created in high-energy collisions. By demonstrating a clear suppression of short-lived resonances in proton-proton collisions, the ALICE collaboration has provided strong evidence that the complex, fluid-like behavior of matter is not exclusive to the massive collisions of heavy atomic nuclei. The results suggest that even in the smallest collision systems, a dense, interacting phase of matter can form, lasting just long enough to scatter the decay products of unstable particles. While the data strongly supports the existence of this phase, the researcher acknowledges that the exact nature of the medium and the duration of this phase remain subjects of ongoing investigation. The ability of certain computer models to reproduce these results without explicitly adding a hadronic phase indicates that the underlying physics is complex, and the question of whether a true quark-gluon plasma forms in these small systems remains open. Nevertheless, the observation of this suppression at such a high level of confidence offers a new, precise tool for probing the earliest moments of matter creation in the universe.