Exploring the limit of partonic collectivity in small collision systems at the LHC

To understand the work described in this new study, one must first picture the most extreme state of matter known to physics. When scientists smash heavy atomic nuclei together at nearly the speed of light, the protons and neutrons that usually make up those nuclei melt. For a fleeting instant, the quarks and gluons that are normally locked inside them are freed, forming a hot, dense soup called a quark-gluon plasma. In the largest collisions, involving heavy atoms like lead, this soup behaves like a perfect fluid, flowing together in a coordinated way. Scientists have long known that this fluid expands and pushes particles outward, creating a specific pattern in how those particles fly apart. This pattern, known as elliptic flow, is a signature that the particles are moving as a collective group rather than as independent individuals.

For years, a major question has lingered: does this collective behavior exist only in the massive collisions of heavy atoms, or can it also happen in much smaller systems? Researchers have been testing this by smashing protons into lead atoms, or even just protons into other protons. These are tiny collisions compared to the heavy-atom ones, yet they have shown hints of the same fluid-like behavior. The central mystery is where the line is drawn. At what point does a collision become too small to generate this collective flow? The new research from the ALICE collaboration at the Large Hadron Collider seeks to find that boundary by looking at collisions with varying numbers of particles produced, pushing the investigation into the smallest, most sparse events possible.

The researchers focused on two types of collisions: protons hitting lead nuclei, and protons hitting other protons. They sorted these collisions into groups based on how many charged particles were created in the aftermath. They looked at the most crowded events, where fifty or more particles were produced, as well as medium-sized groups with between twenty-five and fifty particles, and finally, the sparsest events with between fifteen and twenty-five particles. In each group, they tracked specific types of particles, distinguishing between lighter ones like pions and heavier ones like protons and lambda particles. By measuring how these different particles moved, they could see if the lighter and heavier particles were behaving in a coordinated, fluid-like manner.

In the collisions that produced the most particles, the results were clear and consistent with what is seen in the largest heavy-atom collisions. The lighter particles moved with a stronger collective flow than the heavier ones at lower speeds, a pattern known as mass ordering. As the particles moved faster, the heavier ones began to catch up, and the two groups of particles started to separate in a distinct way. This separation, where the heavier particles show a different flow pattern than the lighter ones, is a strong sign that the particles are formed from a common pool of quarks that are moving together before they combine into the final particles. This behavior suggests that even in these smaller proton-lead and proton-proton collisions, a tiny droplet of quark-gluon plasma is forming and flowing.

The study then moved to the smaller groups of particles, where the question of whether this flow exists becomes much harder to answer. In the medium-sized groups, the same patterns held true. The lighter and heavier particles showed the same coordinated movement and separation seen in the larger events. This confirmed that the collective behavior persists even when the number of particles produced drops significantly. However, when the researchers looked at the smallest group, with only fifteen to twenty-five particles, the picture became less sharp. In the proton-lead collisions, a hint of the separation between heavy and light particles remained, but the evidence was weaker, showing a significance of about two standard deviations. This means the signal is present but not yet strong enough to be considered a definitive discovery on its own.

The situation was even more ambiguous in the smallest proton-proton collisions. In these events, the separation between the heavy and light particles was barely visible, with a significance of less than one standard deviation. The data was too noisy to say with certainty whether the collective flow had disappeared or was simply hidden by the limitations of the measurement. The researchers noted that while the patterns in the larger groups strongly suggest the presence of a partonic fluid, the smallest events do not yet provide a clear answer. The results indicate that the transition from a system with clear collective flow to one without it likely happens in this low-multiplicity region, but the current data cannot pinpoint the exact moment it occurs.

This work represents a crucial step in mapping the limits of how small a system can be while still behaving like a fluid. The findings confirm that the collective flow observed in massive heavy-ion collisions is not unique to them; it appears in high-multiplicity proton collisions as well. However, as the number of particles decreases, the signal of this flow fades into the background noise. The study does not prove that the flow vanishes completely in the smallest events, nor does it rule it out. Instead, it highlights the need for more precise measurements. The author suggests that future data from the next phase of the Large Hadron Collider's operations will be essential to determine whether this tiny fluid exists in the sparsest collisions or if there is a hard limit below which the collective behavior cannot form. Until then, the boundary between a chaotic spray of particles and a flowing fluid remains an open question, waiting for clearer data to define it.