In a first-of-its-kind observation, physicists have detected triplets of particles born from high-energy proton collisions that move in a correlated, coherent manner. The breakthrough, achieved at the Large Hadron Collider (LHC), provides unprecedented insight into the process that forms the vast majority of matter in the universe today.
The result offers the strongest validation yet of the core-halo model, a framework that describes hadronization, the fundamental process by which free quarks and gluons—the building blocks of matter—bind together to create composite particles like protons and neutrons. By analyzing the subtle quantum relationships between three particles at once, researchers at the LHCb experiment have painted a more detailed picture of the space-time structure of the particle-emitting source created in the LHC’s violent collisions.
Re-creating the Primordial Soup
Deep within the 27-kilometer ring of the Large Hadron Collider at CERN, bunches of protons traveling at nearly the speed of light are smashed into each other. The immense energy released in these collisions melts the protons themselves, momentarily liberating their constituent quarks and the gluons that bind them. This creates a fleeting, ultra-hot and dense state of matter known as quark-gluon plasma, a primordial soup that is believed to have filled the entire universe for a few microseconds after the Big Bang.
This exotic plasma is not stable. As it rapidly expands and cools in a fraction of a second, the quarks and gluons are once again confined, binding together into the familiar hadrons that constitute the matter we see today. This transition is hadronization, a crucial but poorly understood phenomenon in particle physics. Because it occurs over infinitesimally small distances and timescales, the process cannot be observed directly. Instead, physicists must find ingenious ways to probe it by studying the particles that emerge from the collision.
A New Window into Hadronization
To measure the geometry of this microscopic fireball, scientists employ a technique analogous to the stellar interferometry first used in astronomy. By studying quantum statistical correlations between identical particles (a phenomenon known as the Bose-Einstein effect for bosons like pions), they can deduce the size and shape of the source that emitted them. For decades, these studies have primarily focused on correlations between pairs of particles, providing a one-dimensional measurement of the source’s radius.
The new analysis, pioneered by physicists from the Institute of Nuclear Physics of the Polish Academy of Sciences (IFJ PAN) working with the LHCb collaboration, extends this technique to three-particle correlations. This more complex analysis provides a much richer, multi-dimensional view of the hadronization process. It allows researchers not just to measure the source’s overall size, but to investigate its internal structure and the timing of particle emission in greater detail than ever before.
The Core-Halo Model Confirmed
The Two-Component Source
For years, physicists have theorized that the particle source in high-energy collisions is not a simple, uniform sphere. The core-halo model posits that it has two distinct components. The “core” is a compact, dense region where particles are formed directly from the cooling quark-gluon plasma. Surrounding this is a larger, more diffuse “halo.” This halo is created by the decay of heavier, unstable particles called resonances. These resonances travel a short distance from the initial collision point before decaying, effectively creating a second, larger and time-delayed source of secondary particles.
Triplets Provide the Proof
While two-particle correlations have provided hints of this structure, they have struggled to definitively prove it, particularly in the complex environment of proton-proton collisions. The new three-body correlation analysis overcomes this limitation. By tracking the quantum statistical links between three identical pions simultaneously, the LHCb team was able to clearly distinguish particles originating from the prompt, compact core from those emitted by the delayed, extended halo. The data strongly supports the core-halo structure, confirming a key theoretical prediction about how matter is organized as it emerges from a state of plasma.
The LHCb Experiment’s Unique Role
The result was made possible by the unique design of the LHCb detector. While other large LHC experiments like ATLAS and CMS focus on particles flying out perpendicular to the beamline, LHCb is designed to capture particles that are produced at very small angles, continuing in the “forward” direction close to the path of the original protons. This specific kinematic region provides a different and complementary view of the collision, expanding the overall knowledge gathered at the collider. This new analysis of triplet correlations leverages the detector’s strengths to probe physics that has remained inaccessible to other experiments.
Implications for Fundamental Physics
Validating the core-halo model represents a significant step forward in understanding one of the most enigmatic processes in the Standard Model of particle physics. Hadronization is a fundamental aspect of the strong force, the interaction that binds atomic nuclei together, but its dynamics are notoriously difficult to calculate from first principles. By providing a clear, experimental confirmation of the two-component source, this result gives theorists a solid benchmark against which to test and refine their models.
A more precise understanding of hadronization has far-reaching consequences, as it is a critical component in the interpretation of nearly all data from high-energy particle colliders. Every search for new particles and every precision measurement of known ones depends on accurate models of the particle soup created in collisions. With this discovery, physicists now have a sharper tool to explore the fundamental structure of matter and the forces that governed the birth of our universe.