Centrality dependence of charged-particle pseudorapidity density at midrapidity in Pb-Pb collisions at $\mathbf{\sqrt{\textit{s}_{\rm NN}} = 5.36}$ TeV

In the heart of matter, where atoms are stripped of their shells and protons and neutrons are forced to collide, lies a state of existence that has not been seen since the first moments of the universe. When heavy atomic nuclei smash into one another at speeds approaching the speed of light, they create a tiny, fleeting drop of super-hot fluid known as a quark-gluon plasma. In this extreme environment, the fundamental building blocks of matter, which are usually locked tightly inside protons and neutrons, melt free and roam together. Physicists study these collisions to understand how the universe cooled down and formed the structure we see today. To do this, they need to count the particles that fly out of the crash. The number of particles produced tells them how much energy was packed into the collision and how the matter behaved in those first instants. It is a way of measuring the size and shape of the fireball created by the impact.

A team of scientists working with the ALICE detector at the Large Hadron Collider has now taken a fresh look at these collisions using a newly upgraded machine. They smashed lead nuclei together at a record-breaking energy level, creating a system so dense that it required a complete overhaul of the detector's ability to keep up. The researchers focused on counting the charged particles that emerged from the center of the crash. By analyzing billions of these events, they mapped out exactly how many particles are produced depending on how head-on the collision was. Their work confirms that the rules governing these tiny explosions hold true even at these new, higher energies, while also revealing that the amount of matter created grows faster in these heavy crashes than in collisions between single protons.

The experiment took place in 2023, when the Large Hadron Collider was running with lead ions at a center-of-mass energy per nucleon pair of 5.36 tera-electronvolts. This is the highest energy ever achieved for such heavy-ion collisions. To capture the data, the ALICE team relied on two major upgrades installed during a long shutdown of the accelerator. The first was a new inner tracking system made of layers of highly sensitive silicon sensors, and the second was a major overhaul of the large time projection chamber, which acts as a giant 3D camera for the particles. Unlike previous runs where the detector waited for a signal to start recording, the new system operates in a continuous mode, reading data non-stop. This was necessary because the collisions happened so frequently that the detector would have missed most of them if it had to pause between events. The team had to develop new software to sort through the continuous stream of information and match every particle track to the correct collision vertex.

The researchers analyzed a sample of 1.1 billion minimum-bias collisions, which includes all types of impacts from glancing blows to direct hits. They sorted these events into ten groups based on how central the collision was, using signals from forward detectors to estimate how many nucleons from the two lead nuclei actually interacted. For the most central collisions, where the nuclei hit almost dead center, the team found that the average number of charged particles produced at the center of the collision was 2,010, with a small margin of error. As the collisions became more peripheral, or glancing, the number of particles dropped significantly, down to about 21.5 for the most grazing impacts. This variation allowed the scientists to study how the geometry of the collision influences the production of matter.

To make sense of these numbers, the team compared the particle count to the number of participating nucleons, which represents the total number of protons and neutrons that took part in the crash. They found that when they normalized the data by dividing the particle count by the number of participating nucleon pairs, the result was 10.5 for the most central collisions. This value is about 4 percent higher than what was measured in similar collisions at a slightly lower energy of 5.02 tera-electronvolts. This increase follows a predictable trend seen in previous experiments at lower energies, confirming that the production of particles in heavy-ion collisions scales with energy in a specific way. The rate at which particle production increases with energy is faster in these heavy lead collisions than in collisions between single protons, suggesting that the collective behavior of the many nucleons plays a crucial role.

The paper also tested several theoretical models that attempt to describe how these particles are formed. Some models assume that the matter forms a fluid that expands and cools according to the laws of hydrodynamics, while others treat the collision as a series of individual interactions between nucleons without any collective fluid behavior. The data showed that while some of these models could capture the general shape of the results, they all systematically underestimated the actual number of particles produced across the range of collision centrality. One model that included a simulation of how particles interact after the initial crash came closer to describing the data for the most central collisions, but no single model successfully reproduced the results for all collision types. This indicates that our current understanding of the mechanisms driving particle production in these extreme conditions is still incomplete.

The findings serve as a major validation of the upgraded ALICE detector and its new continuous readout capabilities. The ability to reconstruct billions of events with high precision demonstrates that the new hardware and software framework can handle the extreme conditions of the Large Hadron Collider's current run. By providing a precise measurement of particle production at the highest energy yet achieved, the study sets a new reference point for future investigations. It confirms that the fundamental trends observed in the quark-gluon plasma are robust, even as the energy of the collisions increases, and highlights the need for more sophisticated theoretical tools to fully explain the complex dynamics of these subatomic explosions.