The role of strangeness and baryon enhancement in heavy-quark hadronisation from pp to Pb$-$Pb collisions with ALICE

Deep within the heart of matter, where the building blocks of the universe reside, lies a process that remains one of nature's most stubborn puzzles. When quarks, the fundamental particles that make up protons and neutrons, are created in high-energy collisions, they do not stay alone. They must immediately combine with other particles to form stable groups called hadrons. This transformation, known as hadronisation, is governed by the strong force, the most powerful interaction in nature. However, the rules of this process are so complex that they cannot be calculated from first principles using current mathematics. For decades, physicists have relied on data from simple collisions, like those between electrons and positrons, to build a standard model of how these particles stick together. This model assumes that the way quarks form matter is universal, meaning it happens the same way regardless of the environment. Yet, recent observations suggest this assumption might be wrong, hinting that the environment itself can change the rules of how matter is assembled.

A researcher using the ALICE detector at the Large Hadron Collider has now taken a closer look at this phenomenon, specifically focusing on particles containing a heavy charm quark. By smashing protons together and colliding heavy lead ions, they created environments ranging from empty space to a dense, hot soup of free quarks and gluons. Their goal was to see if the presence of strange quarks and the density of the collision environment could alter how charm quarks decide which partners to join. The results reveal a clear shift in behavior: in the crowded conditions of heavy-ion collisions, charm quarks are far more likely to form complex, multi-particle groups than they are in simple collisions. This finding challenges the long-held belief of a universal mechanism and points toward a process where particles merge together from a shared pool of resources rather than breaking apart from a single source.

The study began by examining collisions between protons, the smallest units of matter used in these experiments, at an energy level of 13.6 trillion electron volts. The researcher measured the ratio of a specific type of charm baryon, a particle made of three quarks, to a charm meson, a particle made of two. In simple proton collisions, this ratio was found to be significantly higher than what was observed in electron-positron collisions and higher than what standard computer models predicted. These models, which assume particles form by breaking apart in a vacuum, failed to account for the abundance of these three-quark groups. The data suggests that in proton collisions, even at high energies, there are additional mechanisms at play, such as particles reconnecting or forming small, temporary droplets of plasma, which encourage the formation of these more complex structures.

To understand how this changes in a much denser environment, the researcher turned to collisions between lead ions, which create a state of matter known as a quark-gluon plasma. In these collisions, the energy density is so high that protons and neutrons melt, releasing their constituent quarks into a shared, chaotic medium. The researcher measured the production of a specific strange charm baryon in these heavy-ion collisions and found a dramatic increase. The production rate of these particles was roughly three times higher than what would be expected if the particles were simply passing through without interacting. This massive enhancement indicates that the dense environment is actively helping charm quarks find partners, specifically those containing strange quarks, to form new hadrons. The data aligns with models where particles coalesce, or merge, from the surrounding soup, rather than forming independently.

The investigation also looked at how the ratio of strange charm mesons to non-strange charm mesons changes as the number of particles produced in a collision increases. In simple proton collisions, this ratio remained steady regardless of how many particles were created, consistent with a scenario where particles form by fragmentation alone. However, in the lead-ion collisions, the ratio of strange to non-strange particles grew steadily as the collision became more crowded. This trend suggests that as the environment fills with more particles, the likelihood of a charm quark combining with a strange quark increases. At very high speeds, where particles move too fast to interact with the medium, the ratio returns to the level seen in simple collisions, indicating that the merging process only dominates when particles are moving slowly enough to interact with their surroundings.

To ensure these findings were not skewed by the decay of heavier, excited particles, the researcher also measured the production of a specific excited charm state. This measurement provided the first detailed look at how these unstable particles contribute to the final count of stable hadrons. The data showed that the production of these excited states follows a predictable pattern that helps explain the final ratios observed. When combined with the other measurements, the evidence strongly suggests that the formation of matter is not a fixed, universal rule but a dynamic process that responds to its environment. In the dense, strangeness-rich conditions created in heavy-ion collisions, the rules change, favoring the creation of complex, strange particles through a process of merging. This work provides a crucial piece of the puzzle in understanding how the fundamental building blocks of the universe assemble themselves under extreme conditions.