Scientists observe collective behavior of femtoscopic droplets at CERN

At CERN’s Large Hadron Collider (LHC), lead atom nuclei, accelerated in opposite directions, collide at speeds close to the speed of light. In such scattering processes, the quarks and gluons that make up these nuclei collide, creating other quarks and gluons, produced by the fundamental interaction known as the “strong interaction.” The number of particles created is around one hundred times greater than the initial number.

As the particles created are numerous and interact strongly with one another, emergent phenomena arise: the whole is more than the sum of its parts. More precisely, the 30,000 or so created particles form a fluid (with droplets of femtoscopic size, 10^-14 m), where their individuality disappears.

This description has the advantage of simplicity, as the fluid is characterized by a handful of parameters: temperature (about 2,500 billion degrees) and velocity.

In 2024, theorists at the Institut de Physique Théorique of CEA Paris Saclay proposed a new method for observing this so-called “collective” behavior.

Their method has just been implemented by two of the LHC’s four major experimental collaborations, ATLAS and ALICE, whose results were presented at the major international conference Quark Matter 2025, which brought together almost 1,000 scientists in Frankfurt in early April. The papers are available on the arXiv preprint server.

The principle of the method is to exploit the small temperature variations, of the order of a percent, from one collision to the next. The temperature is not exactly the same in all collisions, but it is homogeneous in each collision. This implies that particles emitted at different angles to the beam direction come from a fluid at the same temperature.

In other words, temperatures measured in different directions are correlated. The ATLAS collaboration, whose detector has excellent angular coverage, verified that this correlation was independent of the relative angle between the emitted particles, confirming the hypothesis of a homogeneous temperature.

The experiments also measured the variation in the “spectrum” of emitted particles (the probability law of momentum) resulting from a small increase in temperature. A warmer fluid produces fewer slow-moving particles and more fast-moving ones.

The results are in quantitative agreement with hydrodynamic predictions, except for the very fast particles, which represent only a tiny fraction of the whole, and which are thought to be emitted in the very first moments of the collision, before the formation of the fluid.